SCHED_SETSCHEDULER(2) (2017-09-15) SCHED_SETSCHEDULER(2)
NAME
sched_setscheduler, sched_getscheduler - set and get
scheduling policy/parameters
SYNOPSIS
#include <sched.h>
int sched_setscheduler(pid_t pid, int policy,
const struct sched_param *param);
int sched_getscheduler(pid_t pid);
DESCRIPTION
The sched_setscheduler() system call sets both the schedul-
ing policy and parameters for the thread whose ID is speci-
fied in pid. If pid equals zero, the scheduling policy and
parameters of the calling thread will be set.
The scheduling parameters are specified in the param argu-
ment, which is a pointer to a structure of the following
form:
struct sched_param {
...
int sched_priority;
...
};
In the current implementation, the structure contains only
one field, sched_priority. The interpretation of param
depends on the selected policy.
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.
For each of the above policies, param->sched_priority must
be 0.
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 execu-
tion. For the rules governing when a process may use these
policies, see sched(7). The real-time policies that may be
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specified in policy are:
SCHED_FIFO a first-in, first-out policy; and
SCHED_RR a round-robin policy.
For each of the above policies, param->sched_priority speci-
fies a scheduling priority for the thread. This is a number
in the range returned by calling sched_get_priority_min(2)
and sched_get_priority_max(2) with the specified policy. On
Linux, these system calls return, respectively, 1 and 99.
Since Linux 2.6.32, the SCHED_RESET_ON_FORK flag can be ORed
in policy when calling sched_setscheduler(). As a result of
including this flag, children created by fork(2) do not
inherit privileged scheduling policies. See sched(7) for
details.
sched_getscheduler() returns the current scheduling policy
of the thread identified by pid. If pid equals zero, the
policy of the calling thread will be retrieved.
RETURN VALUE
On success, sched_setscheduler() returns zero. On success,
sched_getscheduler() returns the policy for the thread (a
nonnegative integer). On error, both calls return -1, and
errno is set appropriately.
ERRORS
EINVAL
Invalid arguments: pid is negative or param is NULL.
EINVAL
(sched_setscheduler()) policy is not one of the recog-
nized policies.
EINVAL
(sched_setscheduler()) param does not make sense for
the specified policy.
EPERM
The calling thread does not have appropriate privi-
leges.
ESRCH
The thread whose ID is pid could not be found.
CONFORMING TO
POSIX.1-2001, POSIX.1-2008 (but see BUGS below). The
SCHED_BATCH and SCHED_IDLE policies are Linux-specific.
NOTES
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Further details of the semantics of all of the above "nor-
mal" and "real-time" scheduling policies can be found in the
sched(7) manual page. That page also describes an addi-
tional policy, SCHED_DEADLINE, which is settable only via
sched_setattr(2).
POSIX systems on which sched_setscheduler() and
sched_getscheduler() are available define
_POSIX_PRIORITY_SCHEDULING in <unistd.h>.
POSIX.1 does not detail the permissions that an unprivileged
thread requires in order to call sched_setscheduler(), and
details vary across systems. For example, the Solaris 7
manual page says that the real or effective user ID of the
caller must match the real user ID or the save set-user-ID
of the target.
The scheduling policy and parameters are in fact per-thread
attributes on Linux. The value returned from a call to
gettid(2) can be passed in the argument pid. Specifying pid
as 0 will operate on the attributes of the calling thread,
and passing the value returned from a call to getpid(2) will
operate on the attributes of the main thread of the thread
group. (If you are using the POSIX threads API, then use
pthread_setschedparam(3), pthread_getschedparam(3), and
pthread_setschedprio(3), instead of the sched_*(2) system
calls.)
BUGS
POSIX.1 says that on success, sched_setscheduler() should
return the previous scheduling policy. Linux
sched_setscheduler() does not conform to this requirement,
since it always returns 0 on success.
SEE ALSO
chrt(1), nice(2), sched_get_priority_max(2),
sched_get_priority_min(2), sched_getaffinity(2),
sched_getattr(2), sched_getparam(2),
sched_rr_get_interval(2), sched_setaffinity(2),
sched_setattr(2), sched_setparam(2), sched_yield(2),
setpriority(2), capabilities(7), cpuset(7), sched(7)
COLOPHON
This page is part of release 5.10 of the Linux man-pages
project. A description of the project, information about
reporting bugs, and the latest version of this page, can be
found at https://www.kernel.org/doc/man-pages/.
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