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perf_event_open(2) System Calls Manual perf_event_open(2)
perf_event_open - set up performance monitoring
Standard C library (libc, -lc)
#include <linux/perf_event.h> /* Definition of PERF_* constants */
#include <linux/hw_breakpoint.h> /* Definition of HW_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
int syscall(SYS_perf_event_open, struct perf_event_attr *attr,
pid_t pid, int cpu, int group_fd, unsigned long flags);
Note: glibc provides no wrapper for perf_event_open(),
necessitating the use of syscall(2).
Given a list of parameters, perf_event_open() returns a file
descriptor, for use in subsequent system calls (read(2), mmap(2),
prctl(2), fcntl(2), etc.).
A call to perf_event_open() creates a file descriptor that allows
measuring performance information. Each file descriptor
corresponds to one event that is measured; these can be grouped
together to measure multiple events simultaneously.
Events can be enabled and disabled in two ways: via ioctl(2) and
via prctl(2). When an event is disabled it does not count or
generate overflows but does continue to exist and maintain its
count value.
Events come in two flavors: counting and sampled. A counting
event is one that is used for counting the aggregate number of
events that occur. In general, counting event results are
gathered with a read(2) call. A sampling event periodically
writes measurements to a buffer that can then be accessed via
mmap(2).
Arguments
The pid and cpu arguments allow specifying which process and CPU
to monitor:
pid == 0 and cpu == -1
This measures the calling process/thread on any CPU.
pid == 0 and cpu >= 0
This measures the calling process/thread only when running
on the specified CPU.
pid > 0 and cpu == -1
This measures the specified process/thread on any CPU.
pid > 0 and cpu >= 0
This measures the specified process/thread only when
running on the specified CPU.
pid == -1 and cpu >= 0
This measures all processes/threads on the specified CPU.
This requires CAP_PERFMON (since Linux 5.8) or
CAP_SYS_ADMIN capability or a
/proc/sys/kernel/perf_event_paranoid value of less than 1.
pid == -1 and cpu == -1
This setting is invalid and will return an error.
When pid is greater than zero, permission to perform this system
call is governed by CAP_PERFMON (since Linux 5.9) and a ptrace
access mode PTRACE_MODE_READ_REALCREDS check on older Linux
versions; see ptrace(2).
The group_fd argument allows event groups to be created. An
event group has one event which is the group leader. The leader
is created first, with group_fd = -1. The rest of the group
members are created with subsequent perf_event_open() calls with
group_fd being set to the file descriptor of the group leader.
(A single event on its own is created with group_fd = -1 and is
considered to be a group with only 1 member.) An event group is
scheduled onto the CPU as a unit: it will be put onto the CPU
only if all of the events in the group can be put onto the CPU.
This means that the values of the member events can be
meaningfully compared —added, divided (to get ratios), and so on—
with each other, since they have counted events for the same set
of executed instructions.
The flags argument is formed by ORing together zero or more of
the following values:
PERF_FLAG_FD_CLOEXEC (since Linux 3.14)
This flag enables the close-on-exec flag for the created
event file descriptor, so that the file descriptor is
automatically closed on execve(2). Setting the close-on-
exec flags at creation time, rather than later with
fcntl(2), avoids potential race conditions where the
calling thread invokes perf_event_open() and fcntl(2) at
the same time as another thread calls fork(2) then
execve(2).
PERF_FLAG_FD_NO_GROUP
This flag tells the event to ignore the group_fd parameter
except for the purpose of setting up output redirection
using the PERF_FLAG_FD_OUTPUT flag.
PERF_FLAG_FD_OUTPUT (broken since Linux 2.6.35)
This flag re-routes the event's sampled output to instead
be included in the mmap buffer of the event specified by
group_fd.
PERF_FLAG_PID_CGROUP (since Linux 2.6.39)
This flag activates per-container system-wide monitoring.
A container is an abstraction that isolates a set of
resources for finer-grained control (CPUs, memory, etc.).
In this mode, the event is measured only if the thread
running on the monitored CPU belongs to the designated
container (cgroup). The cgroup is identified by passing a
file descriptor opened on its directory in the cgroupfs
filesystem. For instance, if the cgroup to monitor is
called test, then a file descriptor opened on
/dev/cgroup/test (assuming cgroupfs is mounted on
/dev/cgroup) must be passed as the pid parameter. cgroup
monitoring is available only for system-wide events and
may therefore require extra permissions.
The perf_event_attr structure provides detailed configuration
information for the event being created.
struct perf_event_attr {
__u32 type; /* Type of event */
__u32 size; /* Size of attribute structure */
__u64 config; /* Type-specific configuration */
union {
__u64 sample_period; /* Period of sampling */
__u64 sample_freq; /* Frequency of sampling */
};
__u64 sample_type; /* Specifies values included in sample */
__u64 read_format; /* Specifies values returned in read */
__u64 disabled : 1, /* off by default */
inherit : 1, /* children inherit it */
pinned : 1, /* must always be on PMU */
exclusive : 1, /* only group on PMU */
exclude_user : 1, /* don't count user */
exclude_kernel : 1, /* don't count kernel */
exclude_hv : 1, /* don't count hypervisor */
exclude_idle : 1, /* don't count when idle */
mmap : 1, /* include mmap data */
comm : 1, /* include comm data */
freq : 1, /* use freq, not period */
inherit_stat : 1, /* per task counts */
enable_on_exec : 1, /* next exec enables */
task : 1, /* trace fork/exit */
watermark : 1, /* wakeup_watermark */
precise_ip : 2, /* skid constraint */
mmap_data : 1, /* non-exec mmap data */
sample_id_all : 1, /* sample_type all events */
exclude_host : 1, /* don't count in host */
exclude_guest : 1, /* don't count in guest */
exclude_callchain_kernel : 1,
/* exclude kernel callchains */
exclude_callchain_user : 1,
/* exclude user callchains */
mmap2 : 1, /* include mmap with inode data */
comm_exec : 1, /* flag comm events that are
due to exec */
use_clockid : 1, /* use clockid for time fields */
context_switch : 1, /* context switch data */
write_backward : 1, /* Write ring buffer from end
to beginning */
namespaces : 1, /* include namespaces data */
ksymbol : 1, /* include ksymbol events */
bpf_event : 1, /* include bpf events */
aux_output : 1, /* generate AUX records
instead of events */
cgroup : 1, /* include cgroup events */
text_poke : 1, /* include text poke events */
build_id : 1, /* use build id in mmap2 events */
inherit_thread : 1, /* children only inherit */
/* if cloned with CLONE_THREAD */
remove_on_exec : 1, /* event is removed from task
on exec */
sigtrap : 1, /* send synchronous SIGTRAP
on event */
__reserved_1 : 26;
union {
__u32 wakeup_events; /* wakeup every n events */
__u32 wakeup_watermark; /* bytes before wakeup */
};
__u32 bp_type; /* breakpoint type */
union {
__u64 bp_addr; /* breakpoint address */
__u64 kprobe_func; /* for perf_kprobe */
__u64 uprobe_path; /* for perf_uprobe */
__u64 config1; /* extension of config */
};
union {
__u64 bp_len; /* breakpoint length */
__u64 kprobe_addr; /* with kprobe_func == NULL */
__u64 probe_offset; /* for perf_[k,u]probe */
__u64 config2; /* extension of config1 */
};
__u64 branch_sample_type; /* enum perf_branch_sample_type */
__u64 sample_regs_user; /* user regs to dump on samples */
__u32 sample_stack_user; /* size of stack to dump on
samples */
__s32 clockid; /* clock to use for time fields */
__u64 sample_regs_intr; /* regs to dump on samples */
__u32 aux_watermark; /* aux bytes before wakeup */
__u16 sample_max_stack; /* max frames in callchain */
__u16 __reserved_2; /* align to u64 */
__u32 aux_sample_size; /* max aux sample size */
__u32 __reserved_3; /* align to u64 */
__u64 sig_data; /* user data for sigtrap */
};
The fields of the perf_event_attr structure are described in more
detail below:
type This field specifies the overall event type. It has one
of the following values:
PERF_TYPE_HARDWARE
This indicates one of the "generalized" hardware
events provided by the kernel. See the config
field definition for more details.
PERF_TYPE_SOFTWARE
This indicates one of the software-defined events
provided by the kernel (even if no hardware support
is available).
PERF_TYPE_TRACEPOINT
This indicates a tracepoint provided by the kernel
tracepoint infrastructure.
PERF_TYPE_HW_CACHE
This indicates a hardware cache event. This has a
special encoding, described in the config field
definition.
PERF_TYPE_RAW
This indicates a "raw" implementation-specific
event in the config field.
PERF_TYPE_BREAKPOINT (since Linux 2.6.33)
This indicates a hardware breakpoint as provided by
the CPU. Breakpoints can be read/write accesses to
an address as well as execution of an instruction
address.
dynamic PMU
Since Linux 2.6.38, perf_event_open() can support
multiple PMUs. To enable this, a value exported by
the kernel can be used in the type field to
indicate which PMU to use. The value to use can be
found in the sysfs filesystem: there is a
subdirectory per PMU instance under
/sys/bus/event_source/devices. In each
subdirectory there is a type file whose content is
an integer that can be used in the type field. For
instance, /sys/bus/event_source/devices/cpu/type
contains the value for the core CPU PMU, which is
usually 4.
kprobe and uprobe (since Linux 4.17)
These two dynamic PMUs create a kprobe/uprobe and
attach it to the file descriptor generated by
perf_event_open. The kprobe/uprobe will be
destroyed on the destruction of the file
descriptor. See fields kprobe_func, uprobe_path,
kprobe_addr, and probe_offset for more details.
size The size of the perf_event_attr structure for
forward/backward compatibility. Set this using
sizeof(struct perf_event_attr) to allow the kernel to see
the struct size at the time of compilation.
The related define PERF_ATTR_SIZE_VER0 is set to 64; this
was the size of the first published struct.
PERF_ATTR_SIZE_VER1 is 72, corresponding to the addition
of breakpoints in Linux 2.6.33. PERF_ATTR_SIZE_VER2 is 80
corresponding to the addition of branch sampling in Linux
3.4. PERF_ATTR_SIZE_VER3 is 96 corresponding to the
addition of sample_regs_user and sample_stack_user in
Linux 3.7. PERF_ATTR_SIZE_VER4 is 104 corresponding to
the addition of sample_regs_intr in Linux 3.19.
PERF_ATTR_SIZE_VER5 is 112 corresponding to the addition
of aux_watermark in Linux 4.1.
config This specifies which event you want, in conjunction with
the type field. The config1 and config2 fields are also
taken into account in cases where 64 bits is not enough to
fully specify the event. The encoding of these fields are
event dependent.
There are various ways to set the config field that are
dependent on the value of the previously described type
field. What follows are various possible settings for
config separated out by type.
If type is PERF_TYPE_HARDWARE, we are measuring one of the
generalized hardware CPU events. Not all of these are
available on all platforms. Set config to one of the
following:
PERF_COUNT_HW_CPU_CYCLES
Total cycles. Be wary of what happens during
CPU frequency scaling.
PERF_COUNT_HW_INSTRUCTIONS
Retired instructions. Be careful, these can
be affected by various issues, most notably
hardware interrupt counts.
PERF_COUNT_HW_CACHE_REFERENCES
Cache accesses. Usually this indicates Last
Level Cache accesses but this may vary
depending on your CPU. This may include
prefetches and coherency messages; again this
depends on the design of your CPU.
PERF_COUNT_HW_CACHE_MISSES
Cache misses. Usually this indicates Last
Level Cache misses; this is intended to be
used in conjunction with the
PERF_COUNT_HW_CACHE_REFERENCES event to
calculate cache miss rates.
PERF_COUNT_HW_BRANCH_INSTRUCTIONS
Retired branch instructions. Prior to Linux
2.6.35, this used the wrong event on AMD
processors.
PERF_COUNT_HW_BRANCH_MISSES
Mispredicted branch instructions.
PERF_COUNT_HW_BUS_CYCLES
Bus cycles, which can be different from total
cycles.
PERF_COUNT_HW_STALLED_CYCLES_FRONTEND (since Linux
3.0)
Stalled cycles during issue.
PERF_COUNT_HW_STALLED_CYCLES_BACKEND (since Linux
3.0)
Stalled cycles during retirement.
PERF_COUNT_HW_REF_CPU_CYCLES (since Linux 3.3)
Total cycles; not affected by CPU frequency
scaling.
If type is PERF_TYPE_SOFTWARE, we are measuring software
events provided by the kernel. Set config to one of the
following:
PERF_COUNT_SW_CPU_CLOCK
This reports the CPU clock, a high-resolution
per-CPU timer.
PERF_COUNT_SW_TASK_CLOCK
This reports a clock count specific to the
task that is running.
PERF_COUNT_SW_PAGE_FAULTS
This reports the number of page faults.
PERF_COUNT_SW_CONTEXT_SWITCHES
This counts context switches. Until Linux
2.6.34, these were all reported as user-space
events, after that they are reported as
happening in the kernel.
PERF_COUNT_SW_CPU_MIGRATIONS
This reports the number of times the process
has migrated to a new CPU.
PERF_COUNT_SW_PAGE_FAULTS_MIN
This counts the number of minor page faults.
These did not require disk I/O to handle.
PERF_COUNT_SW_PAGE_FAULTS_MAJ
This counts the number of major page faults.
These required disk I/O to handle.
PERF_COUNT_SW_ALIGNMENT_FAULTS (since Linux 2.6.33)
This counts the number of alignment faults.
These happen when unaligned memory accesses
happen; the kernel can handle these but it
reduces performance. This happens only on
some architectures (never on x86).
PERF_COUNT_SW_EMULATION_FAULTS (since Linux 2.6.33)
This counts the number of emulation faults.
The kernel sometimes traps on unimplemented
instructions and emulates them for user space.
This can negatively impact performance.
PERF_COUNT_SW_DUMMY (since Linux 3.12)
This is a placeholder event that counts
nothing. Informational sample record types
such as mmap or comm must be associated with
an active event. This dummy event allows
gathering such records without requiring a
counting event.
PERF_COUNT_SW_BPF_OUTPUT (since Linux 4.4)
This is used to generate raw sample data from
BPF. BPF programs can write to this event
using bpf_perf_event_output helper.
PERF_COUNT_SW_CGROUP_SWITCHES (since Linux 5.13)
This counts context switches to a task in a
different cgroup. In other words, if the next
task is in the same cgroup, it won't count the
switch.
If type is PERF_TYPE_TRACEPOINT, then we are measuring
kernel tracepoints. The value to use in config can be
obtained from under debugfs tracing/events/*/*/id if
ftrace is enabled in the kernel.
If type is PERF_TYPE_HW_CACHE, then we are measuring a
hardware CPU cache event. To calculate the appropriate
config value, use the following equation:
config = (perf_hw_cache_id) |
(perf_hw_cache_op_id << 8) |
(perf_hw_cache_op_result_id << 16);
where perf_hw_cache_id is one of:
PERF_COUNT_HW_CACHE_L1D
for measuring Level 1 Data Cache
PERF_COUNT_HW_CACHE_L1I
for measuring Level 1 Instruction Cache
PERF_COUNT_HW_CACHE_LL
for measuring Last-Level Cache
PERF_COUNT_HW_CACHE_DTLB
for measuring the Data TLB
PERF_COUNT_HW_CACHE_ITLB
for measuring the Instruction TLB
PERF_COUNT_HW_CACHE_BPU
for measuring the branch prediction unit
PERF_COUNT_HW_CACHE_NODE (since Linux 3.1)
for measuring local memory accesses
and perf_hw_cache_op_id is one of:
PERF_COUNT_HW_CACHE_OP_READ
for read accesses
PERF_COUNT_HW_CACHE_OP_WRITE
for write accesses
PERF_COUNT_HW_CACHE_OP_PREFETCH
for prefetch accesses
and perf_hw_cache_op_result_id is one of:
PERF_COUNT_HW_CACHE_RESULT_ACCESS
to measure accesses
PERF_COUNT_HW_CACHE_RESULT_MISS
to measure misses
If type is PERF_TYPE_RAW, then a custom "raw" config value
is needed. Most CPUs support events that are not covered
by the "generalized" events. These are implementation
defined; see your CPU manual (for example the Intel Volume
3B documentation or the AMD BIOS and Kernel Developer
Guide). The libpfm4 library can be used to translate from
the name in the architectural manuals to the raw hex value
perf_event_open() expects in this field.
If type is PERF_TYPE_BREAKPOINT, then leave config set to
zero. Its parameters are set in other places.
If type is kprobe or uprobe, set retprobe (bit 0 of
config, see
/sys/bus/event_source/devices/[k,u]probe/format/retprobe)
for kretprobe/uretprobe. See fields kprobe_func,
uprobe_path, kprobe_addr, and probe_offset for more
details.
kprobe_func
uprobe_path
kprobe_addr
probe_offset
These fields describe the kprobe/uprobe for dynamic PMUs
kprobe and uprobe. For kprobe: use kprobe_func and
probe_offset, or use kprobe_addr and leave kprobe_func as
NULL. For uprobe: use uprobe_path and probe_offset.
sample_period
sample_freq
A "sampling" event is one that generates an overflow
notification every N events, where N is given by
sample_period. A sampling event has sample_period > 0.
When an overflow occurs, requested data is recorded in the
mmap buffer. The sample_type field controls what data is
recorded on each overflow.
sample_freq can be used if you wish to use frequency
rather than period. In this case, you set the freq flag.
The kernel will adjust the sampling period to try and
achieve the desired rate. The rate of adjustment is a
timer tick.
sample_type
The various bits in this field specify which values to
include in the sample. They will be recorded in a ring-
buffer, which is available to user space using mmap(2).
The order in which the values are saved in the sample are
documented in the MMAP Layout subsection below; it is not
the enum perf_event_sample_format order.
PERF_SAMPLE_IP
Records instruction pointer.
PERF_SAMPLE_TID
Records the process and thread IDs.
PERF_SAMPLE_TIME
Records a timestamp.
PERF_SAMPLE_ADDR
Records an address, if applicable.
PERF_SAMPLE_READ
Record counter values for all events in a group,
not just the group leader.
PERF_SAMPLE_CALLCHAIN
Records the callchain (stack backtrace).
PERF_SAMPLE_ID
Records a unique ID for the opened event's group
leader.
PERF_SAMPLE_CPU
Records CPU number.
PERF_SAMPLE_PERIOD
Records the current sampling period.
PERF_SAMPLE_STREAM_ID
Records a unique ID for the opened event. Unlike
PERF_SAMPLE_ID the actual ID is returned, not the
group leader. This ID is the same as the one
returned by PERF_FORMAT_ID.
PERF_SAMPLE_RAW
Records additional data, if applicable. Usually
returned by tracepoint events.
PERF_SAMPLE_BRANCH_STACK (since Linux 3.4)
This provides a record of recent branches, as
provided by CPU branch sampling hardware (such as
Intel Last Branch Record). Not all hardware
supports this feature.
See the branch_sample_type field for how to filter
which branches are reported.
PERF_SAMPLE_REGS_USER (since Linux 3.7)
Records the current user-level CPU register state
(the values in the process before the kernel was
called).
PERF_SAMPLE_STACK_USER (since Linux 3.7)
Records the user level stack, allowing stack
unwinding.
PERF_SAMPLE_WEIGHT (since Linux 3.10)
Records a hardware provided weight value that
expresses how costly the sampled event was. This
allows the hardware to highlight expensive events
in a profile.
PERF_SAMPLE_DATA_SRC (since Linux 3.10)
Records the data source: where in the memory
hierarchy the data associated with the sampled
instruction came from. This is available only if
the underlying hardware supports this feature.
PERF_SAMPLE_IDENTIFIER (since Linux 3.12)
Places the SAMPLE_ID value in a fixed position in
the record, either at the beginning (for sample
events) or at the end (if a non-sample event).
This was necessary because a sample stream may have
records from various different event sources with
different sample_type settings. Parsing the event
stream properly was not possible because the format
of the record was needed to find SAMPLE_ID, but the
format could not be found without knowing what
event the sample belonged to (causing a circular
dependency).
The PERF_SAMPLE_IDENTIFIER setting makes the event
stream always parsable by putting SAMPLE_ID in a
fixed location, even though it means having
duplicate SAMPLE_ID values in records.
PERF_SAMPLE_TRANSACTION (since Linux 3.13)
Records reasons for transactional memory abort
events (for example, from Intel TSX transactional
memory support).
The precise_ip setting must be greater than 0 and a
transactional memory abort event must be measured
or no values will be recorded. Also note that some
perf_event measurements, such as sampled cycle
counting, may cause extraneous aborts (by causing
an interrupt during a transaction).
PERF_SAMPLE_REGS_INTR (since Linux 3.19)
Records a subset of the current CPU register state
as specified by sample_regs_intr. Unlike
PERF_SAMPLE_REGS_USER the register values will
return kernel register state if the overflow
happened while kernel code is running. If the CPU
supports hardware sampling of register state (i.e.,
PEBS on Intel x86) and precise_ip is set higher
than zero then the register values returned are
those captured by hardware at the time of the
sampled instruction's retirement.
PERF_SAMPLE_PHYS_ADDR (since Linux 4.13)
Records physical address of data like in
PERF_SAMPLE_ADDR.
PERF_SAMPLE_CGROUP (since Linux 5.7)
Records (perf_event) cgroup ID of the process.
This corresponds to the id field in the
PERF_RECORD_CGROUP event.
PERF_SAMPLE_DATA_PAGE_SIZE (since Linux 5.11)
Records page size of data like in PERF_SAMPLE_ADDR.
PERF_SAMPLE_CODE_PAGE_SIZE (since Linux 5.11)
Records page size of ip like in PERF_SAMPLE_IP.
PERF_SAMPLE_WEIGHT_STRUCT (since Linux 5.12)
Records hardware provided weight values like in
PERF_SAMPLE_WEIGHT, but it can represent multiple
values in a struct. This shares the same space as
PERF_SAMPLE_WEIGHT, so users can apply either of
those, not both. It has the following format and
the meaning of each field is dependent on the
hardware implementation.
union perf_sample_weight {
u64 full; /* PERF_SAMPLE_WEIGHT */
struct { /* PERF_SAMPLE_WEIGHT_STRUCT */
u32 var1_dw;
u16 var2_w;
u16 var3_w;
};
};
read_format
This field specifies the format of the data returned by
read(2) on a perf_event_open() file descriptor.
PERF_FORMAT_TOTAL_TIME_ENABLED
Adds the 64-bit time_enabled field. This can be
used to calculate estimated totals if the PMU is
overcommitted and multiplexing is happening.
PERF_FORMAT_TOTAL_TIME_RUNNING
Adds the 64-bit time_running field. This can be
used to calculate estimated totals if the PMU is
overcommitted and multiplexing is happening.
PERF_FORMAT_ID
Adds a 64-bit unique value that corresponds to the
event group.
PERF_FORMAT_GROUP
Allows all counter values in an event group to be
read with one read.
PERF_FORMAT_LOST (since Linux 6.0)
Adds a 64-bit value that is the number of lost
samples for this event. This would be only
meaningful when sample_period or sample_freq is
set.
disabled
The disabled bit specifies whether the counter starts out
disabled or enabled. If disabled, the event can later be
enabled by ioctl(2), prctl(2), or enable_on_exec.
When creating an event group, typically the group leader
is initialized with disabled set to 1 and any child events
are initialized with disabled set to 0. Despite disabled
being 0, the child events will not start until the group
leader is enabled.
inherit
The inherit bit specifies that this counter should count
events of child tasks as well as the task specified. This
applies only to new children, not to any existing children
at the time the counter is created (nor to any new
children of existing children).
Inherit does not work for some combinations of read_format
values, such as PERF_FORMAT_GROUP.
pinned The pinned bit specifies that the counter should always be
on the CPU if at all possible. It applies only to
hardware counters and only to group leaders. If a pinned
counter cannot be put onto the CPU (e.g., because there
are not enough hardware counters or because of a conflict
with some other event), then the counter goes into an
'error' state, where reads return end-of-file (i.e.,
read(2) returns 0) until the counter is subsequently
enabled or disabled.
exclusive
The exclusive bit specifies that when this counter's group
is on the CPU, it should be the only group using the CPU's
counters. In the future this may allow monitoring
programs to support PMU features that need to run alone so
that they do not disrupt other hardware counters.
Note that many unexpected situations may prevent events
with the exclusive bit set from ever running. This
includes any users running a system-wide measurement as
well as any kernel use of the performance counters
(including the commonly enabled NMI Watchdog Timer
interface).
exclude_user
If this bit is set, the count excludes events that happen
in user space.
exclude_kernel
If this bit is set, the count excludes events that happen
in kernel space.
exclude_hv
If this bit is set, the count excludes events that happen
in the hypervisor. This is mainly for PMUs that have
built-in support for handling this (such as POWER). Extra
support is needed for handling hypervisor measurements on
most machines.
exclude_idle
If set, don't count when the CPU is running the idle task.
While you can currently enable this for any event type, it
is ignored for all but software events.
mmap The mmap bit enables generation of PERF_RECORD_MMAP
samples for every mmap(2) call that has PROT_EXEC set.
This allows tools to notice new executable code being
mapped into a program (dynamic shared libraries for
example) so that addresses can be mapped back to the
original code.
comm The comm bit enables tracking of process command name as
modified by the execve(2) and prctl(PR_SET_NAME) system
calls as well as writing to /proc/self/comm. If the
comm_exec flag is also successfully set (possible since
Linux 3.16), then the misc flag PERF_RECORD_MISC_COMM_EXEC
can be used to differentiate the execve(2) case from the
others.
freq If this bit is set, then sample_frequency not
sample_period is used when setting up the sampling
interval.
inherit_stat
This bit enables saving of event counts on context switch
for inherited tasks. This is meaningful only if the
inherit field is set.
enable_on_exec
If this bit is set, a counter is automatically enabled
after a call to execve(2).
task If this bit is set, then fork/exit notifications are
included in the ring buffer.
watermark
If set, have an overflow notification happen when we cross
the wakeup_watermark boundary. Otherwise, overflow
notifications happen after wakeup_events samples.
precise_ip (since Linux 2.6.35)
This controls the amount of skid. Skid is how many
instructions execute between an event of interest
happening and the kernel being able to stop and record the
event. Smaller skid is better and allows more accurate
reporting of which events correspond to which
instructions, but hardware is often limited with how small
this can be.
The possible values of this field are the following:
0 SAMPLE_IP can have arbitrary skid.
1 SAMPLE_IP must have constant skid.
2 SAMPLE_IP requested to have 0 skid.
3 SAMPLE_IP must have 0 skid. See also the
description of PERF_RECORD_MISC_EXACT_IP.
mmap_data (since Linux 2.6.36)
This is the counterpart of the mmap field. This enables
generation of PERF_RECORD_MMAP samples for mmap(2) calls
that do not have PROT_EXEC set (for example data and SysV
shared memory).
sample_id_all (since Linux 2.6.38)
If set, then TID, TIME, ID, STREAM_ID, and CPU can
additionally be included in non-PERF_RECORD_SAMPLEs if the
corresponding sample_type is selected.
If PERF_SAMPLE_IDENTIFIER is specified, then an additional
ID value is included as the last value to ease parsing the
record stream. This may lead to the id value appearing
twice.
The layout is described by this pseudo-structure:
struct sample_id {
{ u32 pid, tid; } /* if PERF_SAMPLE_TID set */
{ u64 time; } /* if PERF_SAMPLE_TIME set */
{ u64 id; } /* if PERF_SAMPLE_ID set */
{ u64 stream_id;} /* if PERF_SAMPLE_STREAM_ID set */
{ u32 cpu, res; } /* if PERF_SAMPLE_CPU set */
{ u64 id; } /* if PERF_SAMPLE_IDENTIFIER set */
};
exclude_host (since Linux 3.2)
When conducting measurements that include processes
running VM instances (i.e., have executed a KVM_RUN
ioctl(2)), only measure events happening inside a guest
instance. This is only meaningful outside the guests;
this setting does not change counts gathered inside of a
guest. Currently, this functionality is x86 only.
exclude_guest (since Linux 3.2)
When conducting measurements that include processes
running VM instances (i.e., have executed a KVM_RUN
ioctl(2)), do not measure events happening inside guest
instances. This is only meaningful outside the guests;
this setting does not change counts gathered inside of a
guest. Currently, this functionality is x86 only.
exclude_callchain_kernel (since Linux 3.7)
Do not include kernel callchains.
exclude_callchain_user (since Linux 3.7)
Do not include user callchains.
mmap2 (since Linux 3.16)
Generate an extended executable mmap record that contains
enough additional information to uniquely identify shared
mappings. The mmap flag must also be set for this to
work.
comm_exec (since Linux 3.16)
This is purely a feature-detection flag, it does not
change kernel behavior. If this flag can successfully be
set, then, when comm is enabled, the
PERF_RECORD_MISC_COMM_EXEC flag will be set in the misc
field of a comm record header if the rename event being
reported was caused by a call to execve(2). This allows
tools to distinguish between the various types of process
renaming.
use_clockid (since Linux 4.1)
This allows selecting which internal Linux clock to use
when generating timestamps via the clockid field. This
can make it easier to correlate perf sample times with
timestamps generated by other tools.
context_switch (since Linux 4.3)
This enables the generation of PERF_RECORD_SWITCH records
when a context switch occurs. It also enables the
generation of PERF_RECORD_SWITCH_CPU_WIDE records when
sampling in CPU-wide mode. This functionality is in
addition to existing tracepoint and software events for
measuring context switches. The advantage of this method
is that it will give full information even with strict
perf_event_paranoid settings.
write_backward (since Linux 4.6)
This causes the ring buffer to be written from the end to
the beginning. This is to support reading from
overwritable ring buffer.
namespaces (since Linux 4.11)
This enables the generation of PERF_RECORD_NAMESPACES
records when a task enters a new namespace. Each
namespace has a combination of device and inode numbers.
ksymbol (since Linux 5.0)
This enables the generation of PERF_RECORD_KSYMBOL records
when new kernel symbols are registered or unregistered.
This is analyzing dynamic kernel functions like eBPF.
bpf_event (since Linux 5.0)
This enables the generation of PERF_RECORD_BPF_EVENT
records when an eBPF program is loaded or unloaded.
aux_output (since Linux 5.4)
This allows normal (non-AUX) events to generate data for
AUX events if the hardware supports it.
cgroup (since Linux 5.7)
This enables the generation of PERF_RECORD_CGROUP records
when a new cgroup is created (and activated).
text_poke (since Linux 5.8)
This enables the generation of PERF_RECORD_TEXT_POKE
records when there's a change to the kernel text (i.e.,
self-modifying code).
build_id (since Linux 5.12)
This changes the contents in the PERF_RECORD_MMAP2 to have
a build-id instead of device and inode numbers.
inherit_thread (since Linux 5.13)
This disables the inheritance of the event to a child
process. Only new threads in the same process (which is
cloned with CLONE_THREAD) will inherit the event.
remove_on_exec (since Linux 5.13)
This closes the event when it starts a new process image
by execve(2).
sigtrap (since Linux 5.13)
This enables synchronous signal delivery of SIGTRAP on
event overflow.
wakeup_events
wakeup_watermark
This union sets how many samples (wakeup_events) or bytes
(wakeup_watermark) happen before an overflow notification
happens. Which one is used is selected by the watermark
bit flag.
wakeup_events counts only PERF_RECORD_SAMPLE record types.
To receive overflow notification for all PERF_RECORD types
choose watermark and set wakeup_watermark to 1.
Prior to Linux 3.0, setting wakeup_events to 0 resulted in
no overflow notifications; more recent kernels treat 0 the
same as 1.
bp_type (since Linux 2.6.33)
This chooses the breakpoint type. It is one of:
HW_BREAKPOINT_EMPTY
No breakpoint.
HW_BREAKPOINT_R
Count when we read the memory location.
HW_BREAKPOINT_W
Count when we write the memory location.
HW_BREAKPOINT_RW
Count when we read or write the memory location.
HW_BREAKPOINT_X
Count when we execute code at the memory location.
The values can be combined via a bitwise or, but the
combination of HW_BREAKPOINT_R or HW_BREAKPOINT_W with
HW_BREAKPOINT_X is not allowed.
bp_addr (since Linux 2.6.33)
This is the address of the breakpoint. For execution
breakpoints, this is the memory address of the instruction
of interest; for read and write breakpoints, it is the
memory address of the memory location of interest.
config1 (since Linux 2.6.39)
config1 is used for setting events that need an extra
register or otherwise do not fit in the regular config
field. Raw OFFCORE_EVENTS on Nehalem/Westmere/SandyBridge
use this field on Linux 3.3 and later kernels.
bp_len (since Linux 2.6.33)
bp_len is the length of the breakpoint being measured if
type is PERF_TYPE_BREAKPOINT. Options are
HW_BREAKPOINT_LEN_1, HW_BREAKPOINT_LEN_2,
HW_BREAKPOINT_LEN_4, and HW_BREAKPOINT_LEN_8. For an
execution breakpoint, set this to sizeof(long).
config2 (since Linux 2.6.39)
config2 is a further extension of the config1 field.
branch_sample_type (since Linux 3.4)
If PERF_SAMPLE_BRANCH_STACK is enabled, then this
specifies what branches to include in the branch record.
The first part of the value is the privilege level, which
is a combination of one of the values listed below. If
the user does not set privilege level explicitly, the
kernel will use the event's privilege level. Event and
branch privilege levels do not have to match.
PERF_SAMPLE_BRANCH_USER
Branch target is in user space.
PERF_SAMPLE_BRANCH_KERNEL
Branch target is in kernel space.
PERF_SAMPLE_BRANCH_HV
Branch target is in hypervisor.
PERF_SAMPLE_BRANCH_PLM_ALL
A convenience value that is the three preceding
values ORed together.
In addition to the privilege value, at least one or more
of the following bits must be set.
PERF_SAMPLE_BRANCH_ANY
Any branch type.
PERF_SAMPLE_BRANCH_ANY_CALL
Any call branch (includes direct calls, indirect
calls, and far jumps).
PERF_SAMPLE_BRANCH_IND_CALL
Indirect calls.
PERF_SAMPLE_BRANCH_CALL (since Linux 4.4)
Direct calls.
PERF_SAMPLE_BRANCH_ANY_RETURN
Any return branch.
PERF_SAMPLE_BRANCH_IND_JUMP (since Linux 4.2)
Indirect jumps.
PERF_SAMPLE_BRANCH_COND (since Linux 3.16)
Conditional branches.
PERF_SAMPLE_BRANCH_ABORT_TX (since Linux 3.11)
Transactional memory aborts.
PERF_SAMPLE_BRANCH_IN_TX (since Linux 3.11)
Branch in transactional memory transaction.
PERF_SAMPLE_BRANCH_NO_TX (since Linux 3.11)
Branch not in transactional memory transaction.
PERF_SAMPLE_BRANCH_CALL_STACK (since Linux 4.1)
Branch is part of a hardware-generated call stack.
This requires hardware support, currently only
found on Intel x86 Haswell or newer.
sample_regs_user (since Linux 3.7)
This bit mask defines the set of user CPU registers to
dump on samples. The layout of the register mask is
architecture-specific and is described in the kernel
header file arch/ARCH/include/uapi/asm/perf_regs.h.
sample_stack_user (since Linux 3.7)
This defines the size of the user stack to dump if
PERF_SAMPLE_STACK_USER is specified.
clockid (since Linux 4.1)
If use_clockid is set, then this field selects which
internal Linux timer to use for timestamps. The available
timers are defined in linux/time.h, with CLOCK_MONOTONIC,
CLOCK_MONOTONIC_RAW, CLOCK_REALTIME, CLOCK_BOOTTIME, and
CLOCK_TAI currently supported.
aux_watermark (since Linux 4.1)
This specifies how much data is required to trigger a
PERF_RECORD_AUX sample.
sample_max_stack (since Linux 4.8)
When sample_type includes PERF_SAMPLE_CALLCHAIN, this
field specifies how many stack frames to report when
generating the callchain.
aux_sample_size (since Linux 5.5)
When PERF_SAMPLE_AUX flag is set, specify the desired size
of AUX data. Note that it can get smaller data than the
specified size.
sig_data (since Linux 5.13)
This data will be copied to user's signal handler (through
si_perf in the siginfo_t) to disambiguate which event
triggered the signal.
Reading results
Once a perf_event_open() file descriptor has been opened, the
values of the events can be read from the file descriptor. The
values that are there are specified by the read_format field in
the attr structure at open time.
If you attempt to read into a buffer that is not big enough to
hold the data, the error ENOSPC results.
Here is the layout of the data returned by a read:
• If PERF_FORMAT_GROUP was specified to allow reading all events
in a group at once:
struct read_format {
u64 nr; /* The number of events */
u64 time_enabled; /* if PERF_FORMAT_TOTAL_TIME_ENABLED */
u64 time_running; /* if PERF_FORMAT_TOTAL_TIME_RUNNING */
struct {
u64 value; /* The value of the event */
u64 id; /* if PERF_FORMAT_ID */
u64 lost; /* if PERF_FORMAT_LOST */
} values[nr];
};
• If PERF_FORMAT_GROUP was not specified:
struct read_format {
u64 value; /* The value of the event */
u64 time_enabled; /* if PERF_FORMAT_TOTAL_TIME_ENABLED */
u64 time_running; /* if PERF_FORMAT_TOTAL_TIME_RUNNING */
u64 id; /* if PERF_FORMAT_ID */
u64 lost; /* if PERF_FORMAT_LOST */
};
The values read are as follows:
nr The number of events in this file descriptor. Available
only if PERF_FORMAT_GROUP was specified.
time_enabled
time_running
Total time the event was enabled and running. Normally
these values are the same. Multiplexing happens if the
number of events is more than the number of available PMU
counter slots. In that case the events run only part of
the time and the time_enabled and time running values can
be used to scale an estimated value for the count.
value An unsigned 64-bit value containing the counter result.
id A globally unique value for this particular event; only
present if PERF_FORMAT_ID was specified in read_format.
lost The number of lost samples of this event; only present if
PERF_FORMAT_LOST was specified in read_format.
MMAP layout
When using perf_event_open() in sampled mode, asynchronous events
(like counter overflow or PROT_EXEC mmap tracking) are logged
into a ring-buffer. This ring-buffer is created and accessed
through mmap(2).
The mmap size should be 1+2^n pages, where the first page is a
metadata page (struct perf_event_mmap_page) that contains various
bits of information such as where the ring-buffer head is.
Before Linux 2.6.39, there is a bug that means you must allocate
an mmap ring buffer when sampling even if you do not plan to
access it.
The structure of the first metadata mmap page is as follows:
struct perf_event_mmap_page {
__u32 version; /* version number of this structure */
__u32 compat_version; /* lowest version this is compat with */
__u32 lock; /* seqlock for synchronization */
__u32 index; /* hardware counter identifier */
__s64 offset; /* add to hardware counter value */
__u64 time_enabled; /* time event active */
__u64 time_running; /* time event on CPU */
union {
__u64 capabilities;
struct {
__u64 cap_usr_time / cap_usr_rdpmc / cap_bit0 : 1,
cap_bit0_is_deprecated : 1,
cap_user_rdpmc : 1,
cap_user_time : 1,
cap_user_time_zero : 1,
};
};
__u16 pmc_width;
__u16 time_shift;
__u32 time_mult;
__u64 time_offset;
__u64 __reserved[120]; /* Pad to 1 k */
__u64 data_head; /* head in the data section */
__u64 data_tail; /* user-space written tail */
__u64 data_offset; /* where the buffer starts */
__u64 data_size; /* data buffer size */
__u64 aux_head;
__u64 aux_tail;
__u64 aux_offset;
__u64 aux_size;
}
The following list describes the fields in the
perf_event_mmap_page structure in more detail:
version
Version number of this structure.
compat_version
The lowest version this is compatible with.
lock A seqlock for synchronization.
index A unique hardware counter identifier.
offset When using rdpmc for reads this offset value must be added
to the one returned by rdpmc to get the current total
event count.
time_enabled
Time the event was active.
time_running
Time the event was running.
cap_usr_time / cap_usr_rdpmc / cap_bit0 (since Linux 3.4)
There was a bug in the definition of cap_usr_time and
cap_usr_rdpmc from Linux 3.4 until Linux 3.11. Both bits
were defined to point to the same location, so it was
impossible to know if cap_usr_time or cap_usr_rdpmc were
actually set.
Starting with Linux 3.12, these are renamed to cap_bit0
and you should use the cap_user_time and cap_user_rdpmc
fields instead.
cap_bit0_is_deprecated (since Linux 3.12)
If set, this bit indicates that the kernel supports the
properly separated cap_user_time and cap_user_rdpmc bits.
If not-set, it indicates an older kernel where
cap_usr_time and cap_usr_rdpmc map to the same bit and
thus both features should be used with caution.
cap_user_rdpmc (since Linux 3.12)
If the hardware supports user-space read of performance
counters without syscall (this is the "rdpmc" instruction
on x86), then the following code can be used to do a read:
u32 seq, time_mult, time_shift, idx, width;
u64 count, enabled, running;
u64 cyc, time_offset;
do {
seq = pc->lock;
barrier();
enabled = pc->time_enabled;
running = pc->time_running;
if (pc->cap_usr_time && enabled != running) {
cyc = rdtsc();
time_offset = pc->time_offset;
time_mult = pc->time_mult;
time_shift = pc->time_shift;
}
idx = pc->index;
count = pc->offset;
if (pc->cap_usr_rdpmc && idx) {
width = pc->pmc_width;
count += rdpmc(idx - 1);
}
barrier();
} while (pc->lock != seq);
cap_user_time (since Linux 3.12)
This bit indicates the hardware has a constant, nonstop
timestamp counter (TSC on x86).
cap_user_time_zero (since Linux 3.12)
Indicates the presence of time_zero which allows mapping
timestamp values to the hardware clock.
pmc_width
If cap_usr_rdpmc, this field provides the bit-width of the
value read using the rdpmc or equivalent instruction.
This can be used to sign extend the result like:
pmc <<= 64 - pmc_width;
pmc >>= 64 - pmc_width; // signed shift right
count += pmc;
time_shift
time_mult
time_offset
If cap_usr_time, these fields can be used to compute the
time delta since time_enabled (in nanoseconds) using rdtsc
or similar.
u64 quot, rem;
u64 delta;
quot = cyc >> time_shift;
rem = cyc & (((u64)1 << time_shift) - 1);
delta = time_offset + quot * time_mult +
((rem * time_mult) >> time_shift);
Where time_offset, time_mult, time_shift, and cyc are read
in the seqcount loop described above. This delta can then
be added to enabled and possible running (if idx),
improving the scaling:
enabled += delta;
if (idx)
running += delta;
quot = count / running;
rem = count % running;
count = quot * enabled + (rem * enabled) / running;
time_zero (since Linux 3.12)
If cap_usr_time_zero is set, then the hardware clock (the
TSC timestamp counter on x86) can be calculated from the
time_zero, time_mult, and time_shift values:
time = timestamp - time_zero;
quot = time / time_mult;
rem = time % time_mult;
cyc = (quot << time_shift) + (rem << time_shift) / time_mult;
And vice versa:
quot = cyc >> time_shift;
rem = cyc & (((u64)1 << time_shift) - 1);
timestamp = time_zero + quot * time_mult +
((rem * time_mult) >> time_shift);
data_head
This points to the head of the data section. The value
continuously increases, it does not wrap. The value needs
to be manually wrapped by the size of the mmap buffer
before accessing the samples.
On SMP-capable platforms, after reading the data_head
value, user space should issue an rmb().
data_tail
When the mapping is PROT_WRITE, the data_tail value should
be written by user space to reflect the last read data.
In this case, the kernel will not overwrite unread data.
data_offset (since Linux 4.1)
Contains the offset of the location in the mmap buffer
where perf sample data begins.
data_size (since Linux 4.1)
Contains the size of the perf sample region within the
mmap buffer.
aux_head
aux_tail
aux_offset
aux_size (since Linux 4.1)
The AUX region allows mmap(2)-ing a separate sample buffer
for high-bandwidth data streams (separate from the main
perf sample buffer). An example of a high-bandwidth
stream is instruction tracing support, as is found in
newer Intel processors.
To set up an AUX area, first aux_offset needs to be set
with an offset greater than data_offset+data_size and
aux_size needs to be set to the desired buffer size. The
desired offset and size must be page aligned, and the size
must be a power of two. These values are then passed to
mmap in order to map the AUX buffer. Pages in the AUX
buffer are included as part of the RLIMIT_MEMLOCK resource
limit (see setrlimit(2)), and also as part of the
perf_event_mlock_kb allowance.
By default, the AUX buffer will be truncated if it will
not fit in the available space in the ring buffer. If the
AUX buffer is mapped as a read only buffer, then it will
operate in ring buffer mode where old data will be
overwritten by new. In overwrite mode, it might not be
possible to infer where the new data began, and it is the
consumer's job to disable measurement while reading to
avoid possible data races.
The aux_head and aux_tail ring buffer pointers have the
same behavior and ordering rules as the previous described
data_head and data_tail.
The following 2^n ring-buffer pages have the layout described
below.
If perf_event_attr.sample_id_all is set, then all event types
will have the sample_type selected fields related to where/when
(identity) an event took place (TID, TIME, ID, CPU, STREAM_ID)
described in PERF_RECORD_SAMPLE below, it will be stashed just
after the perf_event_header and the fields already present for
the existing fields, that is, at the end of the payload. This
allows a newer perf.data file to be supported by older perf
tools, with the new optional fields being ignored.
The mmap values start with a header:
struct perf_event_header {
__u32 type;
__u16 misc;
__u16 size;
};
Below, we describe the perf_event_header fields in more detail.
For ease of reading, the fields with shorter descriptions are
presented first.
size This indicates the size of the record.
misc The misc field contains additional information about the
sample.
The CPU mode can be determined from this value by masking
with PERF_RECORD_MISC_CPUMODE_MASK and looking for one of
the following (note these are not bit masks, only one can
be set at a time):
PERF_RECORD_MISC_CPUMODE_UNKNOWN
Unknown CPU mode.
PERF_RECORD_MISC_KERNEL
Sample happened in the kernel.
PERF_RECORD_MISC_USER
Sample happened in user code.
PERF_RECORD_MISC_HYPERVISOR
Sample happened in the hypervisor.
PERF_RECORD_MISC_GUEST_KERNEL (since Linux 2.6.35)
Sample happened in the guest kernel.
PERF_RECORD_MISC_GUEST_USER (since Linux 2.6.35)
Sample happened in guest user code.
Since the following three statuses are generated by
different record types, they alias to the same bit:
PERF_RECORD_MISC_MMAP_DATA (since Linux 3.10)
This is set when the mapping is not executable;
otherwise the mapping is executable.
PERF_RECORD_MISC_COMM_EXEC (since Linux 3.16)
This is set for a PERF_RECORD_COMM record on
kernels more recent than Linux 3.16 if a process
name change was caused by an execve(2) system call.
PERF_RECORD_MISC_SWITCH_OUT (since Linux 4.3)
When a PERF_RECORD_SWITCH or
PERF_RECORD_SWITCH_CPU_WIDE record is generated,
this bit indicates that the context switch is away
from the current process (instead of into the
current process).
In addition, the following bits can be set:
PERF_RECORD_MISC_EXACT_IP
This indicates that the content of PERF_SAMPLE_IP
points to the actual instruction that triggered the
event. See also perf_event_attr.precise_ip.
PERF_RECORD_MISC_SWITCH_OUT_PREEMPT (since Linux 4.17)
When a PERF_RECORD_SWITCH or
PERF_RECORD_SWITCH_CPU_WIDE record is generated,
this indicates the context switch was a preemption.
PERF_RECORD_MISC_MMAP_BUILD_ID (since Linux 5.12)
This indicates that the content of
PERF_SAMPLE_MMAP2 contains build-ID data instead of
device major and minor numbers as well as the inode
number.
PERF_RECORD_MISC_EXT_RESERVED (since Linux 2.6.35)
This indicates there is extended data available
(currently not used).
PERF_RECORD_MISC_PROC_MAP_PARSE_TIMEOUT
This bit is not set by the kernel. It is reserved
for the user-space perf utility to indicate that
/proc/pid/maps parsing was taking too long and was
stopped, and thus the mmap records may be
truncated.
type The type value is one of the below. The values in the
corresponding record (that follows the header) depend on
the type selected as shown.
PERF_RECORD_MMAP
The MMAP events record the PROT_EXEC mappings so that
we can correlate user-space IPs to code. They have
the following structure:
struct {
struct perf_event_header header;
u32 pid, tid;
u64 addr;
u64 len;
u64 pgoff;
char filename[];
};
pid is the process ID.
tid is the thread ID.
addr is the address of the allocated memory. len is
the length of the allocated memory. pgoff is
the page offset of the allocated memory.
filename is a string describing the backing of
the allocated memory.
PERF_RECORD_LOST
This record indicates when events are lost.
struct {
struct perf_event_header header;
u64 id;
u64 lost;
struct sample_id sample_id;
};
id is the unique event ID for the samples that
were lost.
lost is the number of events that were lost.
PERF_RECORD_COMM
This record indicates a change in the process name.
struct {
struct perf_event_header header;
u32 pid;
u32 tid;
char comm[];
struct sample_id sample_id;
};
pid is the process ID.
tid is the thread ID.
comm is a string containing the new name of the
process.
PERF_RECORD_EXIT
This record indicates a process exit event.
struct {
struct perf_event_header header;
u32 pid, ppid;
u32 tid, ptid;
u64 time;
struct sample_id sample_id;
};
PERF_RECORD_THROTTLE
PERF_RECORD_UNTHROTTLE
This record indicates a throttle/unthrottle event.
struct {
struct perf_event_header header;
u64 time;
u64 id;
u64 stream_id;
struct sample_id sample_id;
};
PERF_RECORD_FORK
This record indicates a fork event.
struct {
struct perf_event_header header;
u32 pid, ppid;
u32 tid, ptid;
u64 time;
struct sample_id sample_id;
};
PERF_RECORD_READ
This record indicates a read event.
struct {
struct perf_event_header header;
u32 pid, tid;
struct read_format values;
struct sample_id sample_id;
};
PERF_RECORD_SAMPLE
This record indicates a sample.
struct {
struct perf_event_header header;
u64 sample_id; /* if PERF_SAMPLE_IDENTIFIER */
u64 ip; /* if PERF_SAMPLE_IP */
u32 pid, tid; /* if PERF_SAMPLE_TID */
u64 time; /* if PERF_SAMPLE_TIME */
u64 addr; /* if PERF_SAMPLE_ADDR */
u64 id; /* if PERF_SAMPLE_ID */
u64 stream_id; /* if PERF_SAMPLE_STREAM_ID */
u32 cpu, res; /* if PERF_SAMPLE_CPU */
u64 period; /* if PERF_SAMPLE_PERIOD */
struct read_format v;
/* if PERF_SAMPLE_READ */
u64 nr; /* if PERF_SAMPLE_CALLCHAIN */
u64 ips[nr]; /* if PERF_SAMPLE_CALLCHAIN */
u32 size; /* if PERF_SAMPLE_RAW */
char data[size]; /* if PERF_SAMPLE_RAW */
u64 bnr; /* if PERF_SAMPLE_BRANCH_STACK */
struct perf_branch_entry lbr[bnr];
/* if PERF_SAMPLE_BRANCH_STACK */
u64 abi; /* if PERF_SAMPLE_REGS_USER */
u64 regs[weight(mask)];
/* if PERF_SAMPLE_REGS_USER */
u64 size; /* if PERF_SAMPLE_STACK_USER */
char data[size]; /* if PERF_SAMPLE_STACK_USER */
u64 dyn_size; /* if PERF_SAMPLE_STACK_USER &&
size != 0 */
union perf_sample_weight weight;
/* if PERF_SAMPLE_WEIGHT */
/* || PERF_SAMPLE_WEIGHT_STRUCT */
u64 data_src; /* if PERF_SAMPLE_DATA_SRC */
u64 transaction; /* if PERF_SAMPLE_TRANSACTION */
u64 abi; /* if PERF_SAMPLE_REGS_INTR */
u64 regs[weight(mask)];
/* if PERF_SAMPLE_REGS_INTR */
u64 phys_addr; /* if PERF_SAMPLE_PHYS_ADDR */
u64 cgroup; /* if PERF_SAMPLE_CGROUP */
u64 data_page_size;
/* if PERF_SAMPLE_DATA_PAGE_SIZE */
u64 code_page_size;
/* if PERF_SAMPLE_CODE_PAGE_SIZE */
u64 size; /* if PERF_SAMPLE_AUX */
char data[size]; /* if PERF_SAMPLE_AUX */
};
sample_id
If PERF_SAMPLE_IDENTIFIER is enabled, a 64-bit
unique ID is included. This is a duplication of
the PERF_SAMPLE_ID id value, but included at the
beginning of the sample so parsers can easily
obtain the value.
ip If PERF_SAMPLE_IP is enabled, then a 64-bit
instruction pointer value is included.
pid
tid If PERF_SAMPLE_TID is enabled, then a 32-bit
process ID and 32-bit thread ID are included.
time
If PERF_SAMPLE_TIME is enabled, then a 64-bit
timestamp is included. This is obtained via
local_clock() which is a hardware timestamp if
available and the jiffies value if not.
addr
If PERF_SAMPLE_ADDR is enabled, then a 64-bit
address is included. This is usually the address
of a tracepoint, breakpoint, or software event;
otherwise the value is 0.
id If PERF_SAMPLE_ID is enabled, a 64-bit unique ID
is included. If the event is a member of an event
group, the group leader ID is returned. This ID
is the same as the one returned by PERF_FORMAT_ID.
stream_id
If PERF_SAMPLE_STREAM_ID is enabled, a 64-bit
unique ID is included. Unlike PERF_SAMPLE_ID the
actual ID is returned, not the group leader. This
ID is the same as the one returned by
PERF_FORMAT_ID.
cpu
res If PERF_SAMPLE_CPU is enabled, this is a 32-bit
value indicating which CPU was being used, in
addition to a reserved (unused) 32-bit value.
period
If PERF_SAMPLE_PERIOD is enabled, a 64-bit value
indicating the current sampling period is written.
v If PERF_SAMPLE_READ is enabled, a structure of
type read_format is included which has values for
all events in the event group. The values
included depend on the read_format value used at
perf_event_open() time.
nr
ips[nr]
If PERF_SAMPLE_CALLCHAIN is enabled, then a 64-bit
number is included which indicates how many
following 64-bit instruction pointers will follow.
This is the current callchain.
size
data[size]
If PERF_SAMPLE_RAW is enabled, then a 32-bit value
indicating size is included followed by an array
of 8-bit values of length size. The values are
padded with 0 to have 64-bit alignment.
This RAW record data is opaque with respect to the
ABI. The ABI doesn't make any promises with
respect to the stability of its content, it may
vary depending on event, hardware, and kernel
version.
bnr
lbr[bnr]
If PERF_SAMPLE_BRANCH_STACK is enabled, then a
64-bit value indicating the number of records is
included, followed by bnr perf_branch_entry
structures which each include the fields:
from This indicates the source instruction (may
not be a branch).
to The branch target.
mispred
The branch target was mispredicted.
predicted
The branch target was predicted.
in_tx (since Linux 3.11)
The branch was in a transactional memory
transaction.
abort (since Linux 3.11)
The branch was in an aborted transactional
memory transaction.
cycles (since Linux 4.3)
This reports the number of cycles elapsed
since the previous branch stack update.
The entries are from most to least recent, so the
first entry has the most recent branch.
Support for mispred, predicted, and cycles is
optional; if not supported, those values will be
0.
The type of branches recorded is specified by the
branch_sample_type field.
abi
regs[weight(mask)]
If PERF_SAMPLE_REGS_USER is enabled, then the user
CPU registers are recorded.
The abi field is one of PERF_SAMPLE_REGS_ABI_NONE,
PERF_SAMPLE_REGS_ABI_32, or
PERF_SAMPLE_REGS_ABI_64.
The regs field is an array of the CPU registers
that were specified by the sample_regs_user attr
field. The number of values is the number of bits
set in the sample_regs_user bit mask.
size
data[size]
dyn_size
If PERF_SAMPLE_STACK_USER is enabled, then the
user stack is recorded. This can be used to
generate stack backtraces. size is the size
requested by the user in sample_stack_user or else
the maximum record size. data is the stack data
(a raw dump of the memory pointed to by the stack
pointer at the time of sampling). dyn_size is the
amount of data actually dumped (can be less than
size). Note that dyn_size is omitted if size is
0.
weight
If PERF_SAMPLE_WEIGHT or PERF_SAMPLE_WEIGHT_STRUCT
is enabled, then a 64-bit value provided by the
hardware is recorded that indicates how costly the
event was. This allows expensive events to stand
out more clearly in profiles.
data_src
If PERF_SAMPLE_DATA_SRC is enabled, then a 64-bit
value is recorded that is made up of the following
fields:
mem_op
Type of opcode, a bitwise combination of:
PERF_MEM_OP_NA
Not available
PERF_MEM_OP_LOAD
Load instruction
PERF_MEM_OP_STORE
Store instruction
PERF_MEM_OP_PFETCH
Prefetch
PERF_MEM_OP_EXEC
Executable code
mem_lvl
Memory hierarchy level hit or miss, a bitwise
combination of the following, shifted left by
PERF_MEM_LVL_SHIFT:
PERF_MEM_LVL_NA
Not available
PERF_MEM_LVL_HIT
Hit
PERF_MEM_LVL_MISS
Miss
PERF_MEM_LVL_L1
Level 1 cache
PERF_MEM_LVL_LFB
Line fill buffer
PERF_MEM_LVL_L2
Level 2 cache
PERF_MEM_LVL_L3
Level 3 cache
PERF_MEM_LVL_LOC_RAM
Local DRAM
PERF_MEM_LVL_REM_RAM1
Remote DRAM 1 hop
PERF_MEM_LVL_REM_RAM2
Remote DRAM 2 hops
PERF_MEM_LVL_REM_CCE1
Remote cache 1 hop
PERF_MEM_LVL_REM_CCE2
Remote cache 2 hops
PERF_MEM_LVL_IO
I/O memory
PERF_MEM_LVL_UNC
Uncached memory
mem_snoop
Snoop mode, a bitwise combination of the
following, shifted left by
PERF_MEM_SNOOP_SHIFT:
PERF_MEM_SNOOP_NA
Not available
PERF_MEM_SNOOP_NONE
No snoop
PERF_MEM_SNOOP_HIT
Snoop hit
PERF_MEM_SNOOP_MISS
Snoop miss
PERF_MEM_SNOOP_HITM
Snoop hit modified
mem_lock
Lock instruction, a bitwise combination of the
following, shifted left by
PERF_MEM_LOCK_SHIFT:
PERF_MEM_LOCK_NA
Not available
PERF_MEM_LOCK_LOCKED
Locked transaction
mem_dtlb
TLB access hit or miss, a bitwise combination
of the following, shifted left by
PERF_MEM_TLB_SHIFT:
PERF_MEM_TLB_NA
Not available
PERF_MEM_TLB_HIT
Hit
PERF_MEM_TLB_MISS
Miss
PERF_MEM_TLB_L1
Level 1 TLB
PERF_MEM_TLB_L2
Level 2 TLB
PERF_MEM_TLB_WK
Hardware walker
PERF_MEM_TLB_OS
OS fault handler
transaction
If the PERF_SAMPLE_TRANSACTION flag is set, then a
64-bit field is recorded describing the sources of
any transactional memory aborts.
The field is a bitwise combination of the
following values:
PERF_TXN_ELISION
Abort from an elision type transaction
(Intel-CPU-specific).
PERF_TXN_TRANSACTION
Abort from a generic transaction.
PERF_TXN_SYNC
Synchronous abort (related to the reported
instruction).
PERF_TXN_ASYNC
Asynchronous abort (not related to the
reported instruction).
PERF_TXN_RETRY
Retryable abort (retrying the transaction
may have succeeded).
PERF_TXN_CONFLICT
Abort due to memory conflicts with other
threads.
PERF_TXN_CAPACITY_WRITE
Abort due to write capacity overflow.
PERF_TXN_CAPACITY_READ
Abort due to read capacity overflow.
In addition, a user-specified abort code can be
obtained from the high 32 bits of the field by
shifting right by PERF_TXN_ABORT_SHIFT and masking
with the value PERF_TXN_ABORT_MASK.
abi
regs[weight(mask)]
If PERF_SAMPLE_REGS_INTR is enabled, then the user
CPU registers are recorded.
The abi field is one of PERF_SAMPLE_REGS_ABI_NONE,
PERF_SAMPLE_REGS_ABI_32, or
PERF_SAMPLE_REGS_ABI_64.
The regs field is an array of the CPU registers
that were specified by the sample_regs_intr attr
field. The number of values is the number of bits
set in the sample_regs_intr bit mask.
phys_addr
If the PERF_SAMPLE_PHYS_ADDR flag is set, then the
64-bit physical address is recorded.
cgroup
If the PERF_SAMPLE_CGROUP flag is set, then the
64-bit cgroup ID (for the perf_event subsystem) is
recorded. To get the pathname of the cgroup, the
ID should match to one in a PERF_RECORD_CGROUP.
data_page_size
If the PERF_SAMPLE_DATA_PAGE_SIZE flag is set,
then the 64-bit page size value of the data
address is recorded.
code_page_size
If the PERF_SAMPLE_CODE_PAGE_SIZE flag is set,
then the 64-bit page size value of the ip address
is recorded.
size
data[size]
If PERF_SAMPLE_AUX is enabled, a snapshot of the
aux buffer is recorded.
PERF_RECORD_MMAP2
This record includes extended information on mmap(2)
calls returning executable mappings. The format is
similar to that of the PERF_RECORD_MMAP record, but
includes extra values that allow uniquely identifying
shared mappings. Depending on the
PERF_RECORD_MISC_MMAP_BUILD_ID bit in the header, the
extra values have different layout and meanings.
struct {
struct perf_event_header header;
u32 pid;
u32 tid;
u64 addr;
u64 len;
u64 pgoff;
union {
struct {
u32 maj;
u32 min;
u64 ino;
u64 ino_generation;
};
struct { /* if PERF_RECORD_MISC_MMAP_BUILD_ID */
u8 build_id_size;
u8 __reserved_1;
u16 __reserved_2;
u8 build_id[20];
};
};
u32 prot;
u32 flags;
char filename[];
struct sample_id sample_id;
};
pid is the process ID.
tid is the thread ID.
addr is the address of the allocated memory.
len is the length of the allocated memory.
pgoff is the page offset of the allocated memory.
maj is the major ID of the underlying device.
min is the minor ID of the underlying device.
ino is the inode number.
ino_generation
is the inode generation.
build_id_size
is the actual size of build_id field (up to
20).
build_id
is a raw data to identify a binary.
prot is the protection information.
flags is the flags information.
filename
is a string describing the backing of the
allocated memory.
PERF_RECORD_AUX (since Linux 4.1)
This record reports that new data is available in the
separate AUX buffer region.
struct {
struct perf_event_header header;
u64 aux_offset;
u64 aux_size;
u64 flags;
struct sample_id sample_id;
};
aux_offset
offset in the AUX mmap region where the new
data begins.
aux_size
size of the data made available.
flags describes the AUX update.
PERF_AUX_FLAG_TRUNCATED
if set, then the data returned was
truncated to fit the available buffer
size.
PERF_AUX_FLAG_OVERWRITE
if set, then the data returned has
overwritten previous data.
PERF_RECORD_ITRACE_START (since Linux 4.1)
This record indicates which process has initiated an
instruction trace event, allowing tools to properly
correlate the instruction addresses in the AUX buffer
with the proper executable.
struct {
struct perf_event_header header;
u32 pid;
u32 tid;
};
pid process ID of the thread starting an
instruction trace.
tid thread ID of the thread starting an instruction
trace.
PERF_RECORD_LOST_SAMPLES (since Linux 4.2)
When using hardware sampling (such as Intel PEBS) this
record indicates some number of samples that may have
been lost.
struct {
struct perf_event_header header;
u64 lost;
struct sample_id sample_id;
};
lost the number of potentially lost samples.
PERF_RECORD_SWITCH (since Linux 4.3)
This record indicates a context switch has happened.
The PERF_RECORD_MISC_SWITCH_OUT bit in the misc field
indicates whether it was a context switch into or away
from the current process.
struct {
struct perf_event_header header;
struct sample_id sample_id;
};
PERF_RECORD_SWITCH_CPU_WIDE (since Linux 4.3)
As with PERF_RECORD_SWITCH this record indicates a
context switch has happened, but it only occurs when
sampling in CPU-wide mode and provides additional
information on the process being switched to/from.
The PERF_RECORD_MISC_SWITCH_OUT bit in the misc field
indicates whether it was a context switch into or away
from the current process.
struct {
struct perf_event_header header;
u32 next_prev_pid;
u32 next_prev_tid;
struct sample_id sample_id;
};
next_prev_pid
The process ID of the previous (if switching
in) or next (if switching out) process on the
CPU.
next_prev_tid
The thread ID of the previous (if switching in)
or next (if switching out) thread on the CPU.
PERF_RECORD_NAMESPACES (since Linux 4.11)
This record includes various namespace information of
a process.
struct {
struct perf_event_header header;
u32 pid;
u32 tid;
u64 nr_namespaces;
struct { u64 dev, inode } [nr_namespaces];
struct sample_id sample_id;
};
pid is the process ID
tid is the thread ID
nr_namespace
is the number of namespaces in this record
Each namespace has dev and inode fields and is
recorded in the fixed position like below:
NET_NS_INDEX=0
Network namespace
UTS_NS_INDEX=1
UTS namespace
IPC_NS_INDEX=2
IPC namespace
PID_NS_INDEX=3
PID namespace
USER_NS_INDEX=4
User namespace
MNT_NS_INDEX=5
Mount namespace
CGROUP_NS_INDEX=6
Cgroup namespace
PERF_RECORD_KSYMBOL (since Linux 5.0)
This record indicates kernel symbol
register/unregister events.
struct {
struct perf_event_header header;
u64 addr;
u32 len;
u16 ksym_type;
u16 flags;
char name[];
struct sample_id sample_id;
};
addr is the address of the kernel symbol.
len is the length of the kernel symbol.
ksym_type
is the type of the kernel symbol. Currently
the following types are available:
PERF_RECORD_KSYMBOL_TYPE_BPF
The kernel symbol is a BPF function.
flags If the PERF_RECORD_KSYMBOL_FLAGS_UNREGISTER is
set, then this event is for unregistering the
kernel symbol.
PERF_RECORD_BPF_EVENT (since Linux 5.0)
This record indicates BPF program is loaded or
unloaded.
struct {
struct perf_event_header header;
u16 type;
u16 flags;
u32 id;
u8 tag[BPF_TAG_SIZE];
struct sample_id sample_id;
};
type is one of the following values:
PERF_BPF_EVENT_PROG_LOAD
A BPF program is loaded
PERF_BPF_EVENT_PROG_UNLOAD
A BPF program is unloaded
id is the ID of the BPF program.
tag is the tag of the BPF program. Currently,
BPF_TAG_SIZE is defined as 8.
PERF_RECORD_CGROUP (since Linux 5.7)
This record indicates a new cgroup is created and
activated.
struct {
struct perf_event_header header;
u64 id;
char path[];
struct sample_id sample_id;
};
id is the cgroup identifier. This can be also
retrieved by name_to_handle_at(2) on the cgroup
path (as a file handle).
path is the path of the cgroup from the root.
PERF_RECORD_TEXT_POKE (since Linux 5.8)
This record indicates a change in the kernel text.
This includes addition and removal of the text and the
corresponding length is zero in this case.
struct {
struct perf_event_header header;
u64 addr;
u16 old_len;
u16 new_len;
u8 bytes[];
struct sample_id sample_id;
};
addr is the address of the change
old_len
is the old length
new_len
is the new length
bytes contains old bytes immediately followed by new
bytes.
Overflow handling
Events can be set to notify when a threshold is crossed,
indicating an overflow. Overflow conditions can be captured by
monitoring the event file descriptor with poll(2), select(2), or
epoll(7). Alternatively, the overflow events can be captured via
sa signal handler, by enabling I/O signaling on the file
descriptor; see the discussion of the F_SETOWN and F_SETSIG
operations in fcntl(2).
Overflows are generated only by sampling events (sample_period
must have a nonzero value).
There are two ways to generate overflow notifications.
The first is to set a wakeup_events or wakeup_watermark value
that will trigger if a certain number of samples or bytes have
been written to the mmap ring buffer. In this case, POLL_IN is
indicated.
The other way is by use of the PERF_EVENT_IOC_REFRESH ioctl.
This ioctl adds to a counter that decrements each time the event
overflows. When nonzero, POLL_IN is indicated, but once the
counter reaches 0 POLL_HUP is indicated and the underlying event
is disabled.
Refreshing an event group leader refreshes all siblings and
refreshing with a parameter of 0 currently enables infinite
refreshes; these behaviors are unsupported and should not be
relied on.
Starting with Linux 3.18, POLL_HUP is indicated if the event
being monitored is attached to a different process and that
process exits.
rdpmc instruction
Starting with Linux 3.4 on x86, you can use the rdpmc instruction
to get low-latency reads without having to enter the kernel.
Note that using rdpmc is not necessarily faster than other
methods for reading event values.
Support for this can be detected with the cap_usr_rdpmc field in
the mmap page; documentation on how to calculate event values can
be found in that section.
Originally, when rdpmc support was enabled, any process (not just
ones with an active perf event) could use the rdpmc instruction
to access the counters. Starting with Linux 4.0, rdpmc support
is only allowed if an event is currently enabled in a process's
context. To restore the old behavior, write the value 2 to
/sys/devices/cpu/rdpmc.
perf_event ioctl calls
Various ioctls act on perf_event_open() file descriptors:
PERF_EVENT_IOC_ENABLE
This enables the individual event or event group specified
by the file descriptor argument.
If the PERF_IOC_FLAG_GROUP bit is set in the ioctl
argument, then all events in a group are enabled, even if
the event specified is not the group leader (but see
BUGS).
PERF_EVENT_IOC_DISABLE
This disables the individual counter or event group
specified by the file descriptor argument.
Enabling or disabling the leader of a group enables or
disables the entire group; that is, while the group leader
is disabled, none of the counters in the group will count.
Enabling or disabling a member of a group other than the
leader affects only that counter; disabling a non-leader
stops that counter from counting but doesn't affect any
other counter.
If the PERF_IOC_FLAG_GROUP bit is set in the ioctl
argument, then all events in a group are disabled, even if
the event specified is not the group leader (but see
BUGS).
PERF_EVENT_IOC_REFRESH
Non-inherited overflow counters can use this to enable a
counter for a number of overflows specified by the
argument, after which it is disabled. Subsequent calls of
this ioctl add the argument value to the current count.
An overflow notification with POLL_IN set will happen on
each overflow until the count reaches 0; when that happens
a notification with POLL_HUP set is sent and the event is
disabled. Using an argument of 0 is considered undefined
behavior.
PERF_EVENT_IOC_RESET
Reset the event count specified by the file descriptor
argument to zero. This resets only the counts; there is
no way to reset the multiplexing time_enabled or
time_running values.
If the PERF_IOC_FLAG_GROUP bit is set in the ioctl
argument, then all events in a group are reset, even if
the event specified is not the group leader (but see
BUGS).
PERF_EVENT_IOC_PERIOD
This updates the overflow period for the event.
Since Linux 3.7 (on ARM) and Linux 3.14 (all other
architectures), the new period takes effect immediately.
On older kernels, the new period did not take effect until
after the next overflow.
The argument is a pointer to a 64-bit value containing the
desired new period.
Prior to Linux 2.6.36, this ioctl always failed due to a
bug in the kernel.
PERF_EVENT_IOC_SET_OUTPUT
This tells the kernel to report event notifications to the
specified file descriptor rather than the default one.
The file descriptors must all be on the same CPU.
The argument specifies the desired file descriptor, or -1
if output should be ignored.
PERF_EVENT_IOC_SET_FILTER (since Linux 2.6.33)
This adds an ftrace filter to this event.
The argument is a pointer to the desired ftrace filter.
PERF_EVENT_IOC_ID (since Linux 3.12)
This returns the event ID value for the given event file
descriptor.
The argument is a pointer to a 64-bit unsigned integer to
hold the result.
PERF_EVENT_IOC_SET_BPF (since Linux 4.1)
This allows attaching a Berkeley Packet Filter (BPF)
program to an existing kprobe tracepoint event. You need
CAP_PERFMON (since Linux 5.8) or CAP_SYS_ADMIN privileges
to use this ioctl.
The argument is a BPF program file descriptor that was
created by a previous bpf(2) system call.
PERF_EVENT_IOC_PAUSE_OUTPUT (since Linux 4.7)
This allows pausing and resuming the event's ring-buffer.
A paused ring-buffer does not prevent generation of
samples, but simply discards them. The discarded samples
are considered lost, and cause a PERF_RECORD_LOST sample
to be generated when possible. An overflow signal may
still be triggered by the discarded sample even though the
ring-buffer remains empty.
The argument is an unsigned 32-bit integer. A nonzero
value pauses the ring-buffer, while a zero value resumes
the ring-buffer.
PERF_EVENT_MODIFY_ATTRIBUTES (since Linux 4.17)
This allows modifying an existing event without the
overhead of closing and reopening a new event. Currently
this is supported only for breakpoint events.
The argument is a pointer to a perf_event_attr structure
containing the updated event settings.
PERF_EVENT_IOC_QUERY_BPF (since Linux 4.16)
This allows querying which Berkeley Packet Filter (BPF)
programs are attached to an existing kprobe tracepoint.
You can only attach one BPF program per event, but you can
have multiple events attached to a tracepoint. Querying
this value on one tracepoint event returns the ID of all
BPF programs in all events attached to the tracepoint.
You need CAP_PERFMON (since Linux 5.8) or CAP_SYS_ADMIN
privileges to use this ioctl.
The argument is a pointer to a structure
struct perf_event_query_bpf {
__u32 ids_len;
__u32 prog_cnt;
__u32 ids[0];
};
The ids_len field indicates the number of ids that can fit
in the provided ids array. The prog_cnt value is filled
in by the kernel with the number of attached BPF programs.
The ids array is filled with the ID of each attached BPF
program. If there are more programs than will fit in the
array, then the kernel will return ENOSPC and ids_len will
indicate the number of program IDs that were successfully
copied.
Using prctl(2)
A process can enable or disable all currently open event groups
using the prctl(2) PR_TASK_PERF_EVENTS_ENABLE and
PR_TASK_PERF_EVENTS_DISABLE operations. This applies only to
events created locally by the calling process. This does not
apply to events created by other processes attached to the
calling process or inherited events from a parent process. Only
group leaders are enabled and disabled, not any other members of
the groups.
perf_event related configuration files
Files in /proc/sys/kernel/
/proc/sys/kernel/perf_event_paranoid
The perf_event_paranoid file can be set to restrict
access to the performance counters.
2 allow only user-space measurements (default
since Linux 4.6).
1 allow both kernel and user measurements
(default before Linux 4.6).
0 allow access to CPU-specific data but not raw
tracepoint samples.
-1 no restrictions.
The existence of the perf_event_paranoid file is the
official method for determining if a kernel supports
perf_event_open().
/proc/sys/kernel/perf_event_max_sample_rate
This sets the maximum sample rate. Setting this too
high can allow users to sample at a rate that impacts
overall machine performance and potentially lock up
the machine. The default value is 100000 (samples per
second).
/proc/sys/kernel/perf_event_max_stack
This file sets the maximum depth of stack frame
entries reported when generating a call trace.
/proc/sys/kernel/perf_event_mlock_kb
Maximum number of pages an unprivileged user can
mlock(2). The default is 516 (kB).
Files in /sys/bus/event_source/devices/
Since Linux 2.6.34, the kernel supports having multiple PMUs
available for monitoring. Information on how to program
these PMUs can be found under /sys/bus/event_source/devices/.
Each subdirectory corresponds to a different PMU.
/sys/bus/event_source/devices/*/type (since Linux 2.6.38)
This contains an integer that can be used in the type
field of perf_event_attr to indicate that you wish to
use this PMU.
/sys/bus/event_source/devices/cpu/rdpmc (since Linux 3.4)
If this file is 1, then direct user-space access to
the performance counter registers is allowed via the
rdpmc instruction. This can be disabled by echoing 0
to the file.
As of Linux 4.0 the behavior has changed, so that 1
now means only allow access to processes with active
perf events, with 2 indicating the old allow-anyone-
access behavior.
/sys/bus/event_source/devices/*/format/ (since Linux 3.4)
This subdirectory contains information on the
architecture-specific subfields available for
programming the various config fields in the
perf_event_attr struct.
The content of each file is the name of the config
field, followed by a colon, followed by a series of
integer bit ranges separated by commas. For example,
the file event may contain the value config1:1,6-10,44
which indicates that event is an attribute that
occupies bits 1,6–10, and 44 of
perf_event_attr::config1.
/sys/bus/event_source/devices/*/events/ (since Linux 3.4)
This subdirectory contains files with predefined
events. The contents are strings describing the event
settings expressed in terms of the fields found in the
previously mentioned ./format/ directory. These are
not necessarily complete lists of all events supported
by a PMU, but usually a subset of events deemed useful
or interesting.
The content of each file is a list of attribute names
separated by commas. Each entry has an optional value
(either hex or decimal). If no value is specified,
then it is assumed to be a single-bit field with a
value of 1. An example entry may look like this:
event=0x2,inv,ldlat=3.
/sys/bus/event_source/devices/*/uevent
This file is the standard kernel device interface for
injecting hotplug events.
/sys/bus/event_source/devices/*/cpumask (since Linux 3.7)
The cpumask file contains a comma-separated list of
integers that indicate a representative CPU number for
each socket (package) on the motherboard. This is
needed when setting up uncore or northbridge events,
as those PMUs present socket-wide events.
On success, perf_event_open() returns the new file descriptor.
On error, -1 is returned and errno is set to indicate the error.
The errors returned by perf_event_open() can be inconsistent, and
may vary across processor architectures and performance
monitoring units.
E2BIG Returned if the perf_event_attr size value is too small
(smaller than PERF_ATTR_SIZE_VER0), too big (larger than
the page size), or larger than the kernel supports and the
extra bytes are not zero. When E2BIG is returned, the
perf_event_attr size field is overwritten by the kernel to
be the size of the structure it was expecting.
EACCES Returned when the requested event requires CAP_PERFMON
(since Linux 5.8) or CAP_SYS_ADMIN permissions (or a more
permissive perf_event paranoid setting). Some common
cases where an unprivileged process may encounter this
error: attaching to a process owned by a different user;
monitoring all processes on a given CPU (i.e., specifying
the pid argument as -1); and not setting exclude_kernel
when the paranoid setting requires it.
EBADF Returned if the group_fd file descriptor is not valid, or,
if PERF_FLAG_PID_CGROUP is set, the cgroup file descriptor
in pid is not valid.
EBUSY (since Linux 4.1)
Returned if another event already has exclusive access to
the PMU.
EFAULT Returned if the attr pointer points at an invalid memory
address.
EINTR Returned when trying to mix perf and ftrace handling for a
uprobe.
EINVAL Returned if the specified event is invalid. There are
many possible reasons for this. A not-exhaustive list:
sample_freq is higher than the maximum setting; the cpu to
monitor does not exist; read_format is out of range;
sample_type is out of range; the flags value is out of
range; exclusive or pinned set and the event is not a
group leader; the event config values are out of range or
set reserved bits; the generic event selected is not
supported; or there is not enough room to add the selected
event.
EMFILE Each opened event uses one file descriptor. If a large
number of events are opened, the per-process limit on the
number of open file descriptors will be reached, and no
more events can be created.
ENODEV Returned when the event involves a feature not supported
by the current CPU.
ENOENT Returned if the type setting is not valid. This error is
also returned for some unsupported generic events.
ENOSPC Prior to Linux 3.3, if there was not enough room for the
event, ENOSPC was returned. In Linux 3.3, this was
changed to EINVAL. ENOSPC is still returned if you try to
add more breakpoint events than supported by the hardware.
ENOSYS Returned if PERF_SAMPLE_STACK_USER is set in sample_type
and it is not supported by hardware.
EOPNOTSUPP
Returned if an event requiring a specific hardware feature
is requested but there is no hardware support. This
includes requesting low-skid events if not supported,
branch tracing if it is not available, sampling if no PMU
interrupt is available, and branch stacks for software
events.
EOVERFLOW (since Linux 4.8)
Returned if PERF_SAMPLE_CALLCHAIN is requested and
sample_max_stack is larger than the maximum specified in
/proc/sys/kernel/perf_event_max_stack.
EPERM Returned on many (but not all) architectures when an
unsupported exclude_hv, exclude_idle, exclude_user, or
exclude_kernel setting is specified.
It can also happen, as with EACCES, when the requested
event requires CAP_PERFMON (since Linux 5.8) or
CAP_SYS_ADMIN permissions (or a more permissive perf_event
paranoid setting). This includes setting a breakpoint on
a kernel address, and (since Linux 3.13) setting a kernel
function-trace tracepoint.
ESRCH Returned if attempting to attach to a process that does
not exist.
Linux.
perf_event_open() was introduced in Linux 2.6.31 but was called
perf_counter_open(). It was renamed in Linux 2.6.32.
The official way of knowing if perf_event_open() support is
enabled is checking for the existence of the file
/proc/sys/kernel/perf_event_paranoid.
CAP_PERFMON capability (since Linux 5.8) provides secure approach
to performance monitoring and observability operations in a
system according to the principal of least privilege (POSIX IEEE
1003.1e). Accessing system performance monitoring and
observability operations using CAP_PERFMON rather than the much
more powerful CAP_SYS_ADMIN excludes chances to misuse
credentials and makes operations more secure. CAP_SYS_ADMIN
usage for secure system performance monitoring and observability
is discouraged in favor of the CAP_PERFMON capability.
The F_SETOWN_EX option to fcntl(2) is needed to properly get
overflow signals in threads. This was introduced in Linux
2.6.32.
Prior to Linux 2.6.33 (at least for x86), the kernel did not
check if events could be scheduled together until read time. The
same happens on all known kernels if the NMI watchdog is enabled.
This means to see if a given set of events works you have to
perf_event_open(), start, then read before you know for sure you
can get valid measurements.
Prior to Linux 2.6.34, event constraints were not enforced by the
kernel. In that case, some events would silently return "0" if
the kernel scheduled them in an improper counter slot.
Prior to Linux 2.6.34, there was a bug when multiplexing where
the wrong results could be returned.
Kernels from Linux 2.6.35 to Linux 2.6.39 can quickly crash the
kernel if "inherit" is enabled and many threads are started.
Prior to Linux 2.6.35, PERF_FORMAT_GROUP did not work with
attached processes.
There is a bug in the kernel code between Linux 2.6.36 and Linux
3.0 that ignores the "watermark" field and acts as if a
wakeup_event was chosen if the union has a nonzero value in it.
From Linux 2.6.31 to Linux 3.4, the PERF_IOC_FLAG_GROUP ioctl
argument was broken and would repeatedly operate on the event
specified rather than iterating across all sibling events in a
group.
From Linux 3.4 to Linux 3.11, the mmap cap_usr_rdpmc and
cap_usr_time bits mapped to the same location. Code should
migrate to the new cap_user_rdpmc and cap_user_time fields
instead.
Always double-check your results! Various generalized events
have had wrong values. For example, retired branches measured
the wrong thing on AMD machines until Linux 2.6.35.
The following is a short example that measures the total
instruction count of a call to printf(3).
#include <linux/perf_event.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <unistd.h>
static long
perf_event_open(struct perf_event_attr *hw_event, pid_t pid,
int cpu, int group_fd, unsigned long flags)
{
int ret;
ret = syscall(SYS_perf_event_open, hw_event, pid, cpu,
group_fd, flags);
return ret;
}
int
main(void)
{
int fd;
long long count;
struct perf_event_attr pe;
memset(&pe, 0, sizeof(pe));
pe.type = PERF_TYPE_HARDWARE;
pe.size = sizeof(pe);
pe.config = PERF_COUNT_HW_INSTRUCTIONS;
pe.disabled = 1;
pe.exclude_kernel = 1;
pe.exclude_hv = 1;
fd = perf_event_open(&pe, 0, -1, -1, 0);
if (fd == -1) {
fprintf(stderr, "Error opening leader %llx\n", pe.config);
exit(EXIT_FAILURE);
}
ioctl(fd, PERF_EVENT_IOC_RESET, 0);
ioctl(fd, PERF_EVENT_IOC_ENABLE, 0);
printf("Measuring instruction count for this printf\n");
ioctl(fd, PERF_EVENT_IOC_DISABLE, 0);
read(fd, &count, sizeof(count));
printf("Used %lld instructions\n", count);
close(fd);
}
perf(1), fcntl(2), mmap(2), open(2), prctl(2), read(2)
Documentation/admin-guide/perf-security.rst in the kernel source
tree
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Linux man-pages 6.9.1 2024-06-15 perf_event_open(2)
Pages that refer to this page: bpf(2), gettid(2), mount_setattr(2), openat2(2), syscalls(2), stapprobes(3stap), proc_pid_fd(5), systemd.exec(5), bpf-helpers(7), capabilities(7), cgroups(7)
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