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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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sched_setattr(2) System Calls Manual sched_setattr(2)
sched_setattr, sched_getattr - set and get scheduling policy and
attributes
Standard C library (libc, -lc)
#include <sched.h> /* Definition of SCHED_* constants */
#include <sys/syscall.h> /* Definition of SYS_* constants */
#include <unistd.h>
int syscall(SYS_sched_setattr, pid_t pid, struct sched_attr *attr,
unsigned int flags);
int syscall(SYS_sched_getattr, pid_t pid, struct sched_attr *attr,
unsigned int size, unsigned int flags);
Note: glibc provides no wrappers for these system calls,
necessitating the use of syscall(2).
sched_setattr()
The sched_setattr() system call sets the scheduling policy and
associated attributes for the thread whose ID is specified in
pid. If pid equals zero, the scheduling policy and attributes of
the calling thread will be set.
Currently, Linux supports the following "normal" (i.e., non-real-
time) scheduling policies as values that may be specified in
policy:
SCHED_OTHER
the standard round-robin time-sharing policy;
SCHED_BATCH
for "batch" style execution of processes; and
SCHED_IDLE
for running very low priority background jobs.
Various "real-time" policies are also supported, for special
time-critical applications that need precise control over the way
in which runnable threads are selected for execution. For the
rules governing when a process may use these policies, see
sched(7). The real-time policies that may be specified in policy
are:
SCHED_FIFO
a first-in, first-out policy; and
SCHED_RR
a round-robin policy.
Linux also provides the following policy:
SCHED_DEADLINE
a deadline scheduling policy; see sched(7) for details.
The attr argument is a pointer to a structure that defines the
new scheduling policy and attributes for the specified thread.
This structure has the following form:
struct sched_attr {
u32 size; /* Size of this structure */
u32 sched_policy; /* Policy (SCHED_*) */
u64 sched_flags; /* Flags */
s32 sched_nice; /* Nice value (SCHED_OTHER,
SCHED_BATCH) */
u32 sched_priority; /* Static priority (SCHED_FIFO,
SCHED_RR) */
/* For SCHED_DEADLINE */
u64 sched_runtime;
u64 sched_deadline;
u64 sched_period;
/* Utilization hints */
u32 sched_util_min;
u32 sched_util_max;
};
The fields of the sched_attr structure are as follows:
size This field should be set to the size of the structure in
bytes, as in sizeof(struct sched_attr). If the provided
structure is smaller than the kernel structure, any
additional fields are assumed to be '0'. If the provided
structure is larger than the kernel structure, the kernel
verifies that all additional fields are 0; if they are
not, sched_setattr() fails with the error E2BIG and
updates size to contain the size of the kernel structure.
The above behavior when the size of the user-space
sched_attr structure does not match the size of the kernel
structure allows for future extensibility of the
interface. Malformed applications that pass oversize
structures won't break in the future if the size of the
kernel sched_attr structure is increased. In the future,
it could also allow applications that know about a larger
user-space sched_attr structure to determine whether they
are running on an older kernel that does not support the
larger structure.
sched_policy
This field specifies the scheduling policy, as one of the
SCHED_* values listed above.
sched_flags
This field contains zero or more of the following flags
that are ORed together to control scheduling behavior:
SCHED_FLAG_RESET_ON_FORK
Children created by fork(2) do not inherit
privileged scheduling policies. See sched(7) for
details.
SCHED_FLAG_RECLAIM (since Linux 4.13)
This flag allows a SCHED_DEADLINE thread to reclaim
bandwidth unused by other real-time threads.
SCHED_FLAG_DL_OVERRUN (since Linux 4.16)
This flag allows an application to get informed
about run-time overruns in SCHED_DEADLINE threads.
Such overruns may be caused by (for example) coarse
execution time accounting or incorrect parameter
assignment. Notification takes the form of a
SIGXCPU signal which is generated on each overrun.
This SIGXCPU signal is process-directed (see
signal(7)) rather than thread-directed. This is
probably a bug. On the one hand, sched_setattr()
is being used to set a per-thread attribute. On
the other hand, if the process-directed signal is
delivered to a thread inside the process other than
the one that had a run-time overrun, the
application has no way of knowing which thread
overran.
SCHED_FLAG_UTIL_CLAMP_MIN
SCHED_FLAG_UTIL_CLAMP_MAX (both since Linux 5.3)
These flags indicate that the sched_util_min or
sched_util_max fields, respectively, are present,
representing the expected minimum and maximum
utilization of the thread.
The utilization attributes provide the scheduler
with boundaries within which it should schedule the
thread, potentially informing its decisions
regarding task placement and frequency selection.
sched_nice
This field specifies the nice value to be set when
specifying sched_policy as SCHED_OTHER or SCHED_BATCH.
The nice value is a number in the range -20 (high
priority) to +19 (low priority); see sched(7).
sched_priority
This field specifies the static priority to be set when
specifying sched_policy as SCHED_FIFO or SCHED_RR. The
allowed range of priorities for these policies can be
determined using sched_get_priority_min(2) and
sched_get_priority_max(2). For other policies, this field
must be specified as 0.
sched_runtime
This field specifies the "Runtime" parameter for deadline
scheduling. The value is expressed in nanoseconds. This
field, and the next two fields, are used only for
SCHED_DEADLINE scheduling; for further details, see
sched(7).
sched_deadline
This field specifies the "Deadline" parameter for deadline
scheduling. The value is expressed in nanoseconds.
sched_period
This field specifies the "Period" parameter for deadline
scheduling. The value is expressed in nanoseconds.
sched_util_min
sched_util_max (both since Linux 5.3)
These fields specify the expected minimum and maximum
utilization, respectively. They are ignored unless their
corresponding SCHED_FLAG_UTIL_CLAMP_MIN or
SCHED_FLAG_UTIL_CLAMP_MAX is set in sched_flags.
Utilization is a value in the range [0, 1024],
representing the percentage of CPU time used by a task
when running at the maximum frequency on the highest
capacity CPU of the system. This is a fixed point
representation, where 1024 corresponds to 100%, and 0
corresponds to 0%. For example, a 20% utilization task is
a task running for 2ms every 10ms at maximum frequency and
is represented by a utilization value of 0.2 * 1024 = 205.
A task with a minimum utilization value larger than 0 is
more likely scheduled on a CPU with a capacity big enough
to fit the specified value. A task with a maximum
utilization value smaller than 1024 is more likely
scheduled on a CPU with no more capacity than the
specified value.
A task utilization boundary can be reset by setting its
field to UINT32_MAX (since Linux 5.11).
The flags argument is provided to allow for future extensions to
the interface; in the current implementation it must be specified
as 0.
sched_getattr()
The sched_getattr() system call fetches the scheduling policy and
the associated attributes for the thread whose ID is specified in
pid. If pid equals zero, the scheduling policy and attributes of
the calling thread will be retrieved.
The size argument should be set to the size of the sched_attr
structure as known to user space. The value must be at least as
large as the size of the initially published sched_attr
structure, or the call fails with the error EINVAL.
The retrieved scheduling attributes are placed in the fields of
the sched_attr structure pointed to by attr. The kernel sets
attr.size to the size of its sched_attr structure.
If the caller-provided attr buffer is larger than the kernel's
sched_attr structure, the additional bytes in the user-space
structure are not touched. If the caller-provided structure is
smaller than the kernel sched_attr structure, the kernel will
silently not return any values which would be stored outside the
provided space. As with sched_setattr(), these semantics allow
for future extensibility of the interface.
The flags argument is provided to allow for future extensions to
the interface; in the current implementation it must be specified
as 0.
On success, sched_setattr() and sched_getattr() return 0. On
error, -1 is returned, and errno is set to indicate the error.
sched_getattr() and sched_setattr() can both fail for the
following reasons:
EINVAL attr is NULL; or pid is negative; or flags is not zero.
ESRCH The thread whose ID is pid could not be found.
In addition, sched_getattr() can fail for the following reasons:
E2BIG The buffer specified by size and attr is too small.
EINVAL size is invalid; that is, it is smaller than the initial
version of the sched_attr structure (48 bytes) or larger
than the system page size.
In addition, sched_setattr() can fail for the following reasons:
E2BIG The buffer specified by size and attr is larger than the
kernel structure, and one or more of the excess bytes is
nonzero.
EBUSY SCHED_DEADLINE admission control failure, see sched(7).
EINVAL attr.sched_policy is not one of the recognized policies.
EINVAL attr.sched_flags contains a flag other than
SCHED_FLAG_RESET_ON_FORK.
EINVAL attr.sched_priority is invalid.
EINVAL attr.sched_policy is SCHED_DEADLINE, and the deadline
scheduling parameters in attr are invalid.
EINVAL attr.sched_flags contains SCHED_FLAG_UTIL_CLAMP_MIN or
SCHED_FLAG_UTIL_CLAMP_MAX, and attr.sched_util_min or
attr.sched_util_max are out of bounds.
EOPNOTSUPP
SCHED_FLAG_UTIL_CLAMP was provided, but the kernel was not
built with CONFIG_UCLAMP_TASK support.
EPERM The caller does not have appropriate privileges.
EPERM The CPU affinity mask of the thread specified by pid does
not include all CPUs in the system (see
sched_setaffinity(2)).
Linux.
Linux 3.14.
glibc does not provide wrappers for these system calls; call them
using syscall(2).
sched_setattr() provides a superset of the functionality of
sched_setscheduler(2), sched_setparam(2), nice(2), and (other
than the ability to set the priority of all processes belonging
to a specified user or all processes in a specified group)
setpriority(2). Analogously, sched_getattr() provides a superset
of the functionality of sched_getscheduler(2), sched_getparam(2),
and (partially) getpriority(2).
In Linux versions up to 3.15, sched_setattr() failed with the
error EFAULT instead of E2BIG for the case described in ERRORS.
Up to Linux 5.3, sched_getattr() failed with the error EFBIG if
the in-kernel sched_attr structure was larger than the size
passed by user space.
chrt(1), nice(2), sched_get_priority_max(2),
sched_get_priority_min(2), sched_getaffinity(2),
sched_getparam(2), sched_getscheduler(2),
sched_rr_get_interval(2), sched_setaffinity(2),
sched_setparam(2), sched_setscheduler(2), sched_yield(2),
setpriority(2), pthread_getschedparam(3),
pthread_setschedparam(3), pthread_setschedprio(3),
capabilities(7), cpuset(7), sched(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.9.1.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2024-06-26. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
[email protected]
Linux man-pages 6.9.1 2024-06-13 sched_setattr(2)
Pages that refer to this page: uclampset(1), openat2(2), sched_setparam(2), sched_setscheduler(2), syscalls(2), capabilities(7), credentials(7), sched(7)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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