.\" Copyright (C) 2014 Michael Kerrisk <mtk.manpages@gmail.com>
.\" and Copyright (C) 2014 Peter Zijlstra <peterz@infradead.org>
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.TH SCHED_SETATTR 2 2016-10-08 "Linux" "Linux Programmer's Manual"
.SH NAME
sched_setattr, sched_getattr \-
set and get scheduling policy and attributes
.SH SYNOPSIS
.nf
.B #include <sched.h>

.BI "int sched_setattr(pid_t " pid ", struct sched_attr *" attr ,
.BI "                  unsigned int " flags );

.BI "int sched_getattr(pid_t " pid ", struct sched_attr *" attr ,
.BI "                  unsigned int " size ", unsigned int " flags );
.fi
.\" FIXME . Add feature test macro requirements
.SH DESCRIPTION
.SS sched_setattr()
The
.BR sched_setattr ()
system call sets the scheduling policy and
associated attributes for the thread whose ID is specified in
.IR pid .
If
.I 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
.IR policy :
.TP 14
.BR SCHED_OTHER
the standard round-robin time-sharing policy;
.\" In the 2.6 kernel sources, SCHED_OTHER is actually called
.\" SCHED_NORMAL.
.TP
.BR SCHED_BATCH
for "batch" style execution of processes; and
.TP
.BR SCHED_IDLE
for running
.I very
low priority background jobs.
.PP
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
.BR sched (7).
The real-time policies that may be specified in
.IR policy
are:
.TP 14
.BR SCHED_FIFO
a first-in, first-out policy; and
.TP
.BR SCHED_RR
a round-robin policy.
.PP
Linux also provides the following policy:
.TP 14
.B SCHED_DEADLINE
a deadline scheduling policy; see
.BR sched (7)
for details.
.PP
The
.I 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:

.in +4n
.nf
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) */
    /* Remaining fields are for SCHED_DEADLINE */
    u64 sched_runtime;
    u64 sched_deadline;
    u64 sched_period;
};
.fi
.in

The fields of this structure are as follows:
.TP
.B size
This field should be set to the size of the structure in bytes, as in
.IR "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,
.BR sched_setattr ()
fails with the error
.BR E2BIG
and updates
.I size
to contain the size of the kernel structure.
.IP
The above behavior when the size of the user-space
.I 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
.I sched_attr
structure is increased.
In the future,
it could also allow applications that know about a larger user-space
.I sched_attr
structure to determine whether they are running on an older kernel
that does not support the larger structure.
.TP
.I sched_policy
This field specifies the scheduling policy, as one of the
.BR SCHED_*
values listed above.
.TP
.I sched_flags
This field contains flags controlling scheduling behavior.
Only one such flag is currently defined:
.BR SCHED_FLAG_RESET_ON_FORK .
As a result of including this flag, children created by
.BR fork (2)
do not inherit privileged scheduling policies.
See
.BR sched (7)
for details.
.TP
.I sched_nice
This field specifies the nice value to be set when specifying
.IR sched_policy
as
.BR SCHED_OTHER
or
.BR SCHED_BATCH .
The nice value is a number in the range \-20 (high priority)
to +19 (low priority); see
.BR sched (7).
.TP
.I sched_priority
This field specifies the static priority to be set when specifying
.IR sched_policy
as
.BR SCHED_FIFO
or
.BR SCHED_RR .
The allowed range of priorities for these policies can be determined using
.BR sched_get_priority_min (2)
and
.BR sched_get_priority_max (2).
For other policies, this field must be specified as 0.
.TP
.I 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
.BR SCHED_DEADLINE
scheduling; for further details, see
.BR sched (7).
.TP
.I sched_deadline
This field specifies the "Deadline" parameter for deadline scheduling.
The value is expressed in nanoseconds.
.TP
.I sched_period
This field specifies the "Period" parameter for deadline scheduling.
The value is expressed in nanoseconds.
.PP
The
.I flags
argument is provided to allow for future extensions to the interface;
in the current implementation it must be specified as 0.
.\"
.\"
.SS sched_getattr()
The
.BR sched_getattr ()
system call fetches the scheduling policy and the
associated attributes for the thread whose ID is specified in
.IR pid .
If
.I pid
equals zero,
the scheduling policy and attributes of the calling thread
will be retrieved.

The
.I size
argument should be set to the size of the
.I sched_attr
structure as known to user space.
The value must be at least as large as the size of the initially published
.I sched_attr
structure, or the call fails with the error
.BR EINVAL .

The retrieved scheduling attributes are placed in the fields of the
.I sched_attr
structure pointed to by
.IR attr .
The kernel sets
.I attr.size
to the size of its
.I sched_attr
structure.

If the caller-provided
.I attr
buffer is larger than the kernel's
.I sched_attr
structure,
the additional bytes in the user-space structure are not touched.
If the caller-provided structure is smaller than the kernel
.I sched_attr
structure and the kernel needs to return values outside the provided space,
.BR sched_getattr ()
fails with the error
.BR E2BIG .
As with
.BR sched_setattr (),
these semantics allow for future extensibility of the interface.

The
.I flags
argument is provided to allow for future extensions to the interface;
in the current implementation it must be specified as 0.
.SH RETURN VALUE
On success,
.BR sched_setattr ()
and
.BR sched_getattr ()
return 0.
On error, \-1 is returned, and
.I errno
is set to indicate the cause of the error.
.SH ERRORS
.BR sched_getattr ()
and
.BR sched_setattr ()
can both fail for the following reasons:
.TP
.B EINVAL
.I attr
is NULL; or
.I pid
is negative; or
.I flags
is not zero.
.TP
.B ESRCH
The thread whose ID is
.I pid
could not be found.
.PP
In addition,
.BR sched_getattr ()
can fail for the following reasons:
.TP
.B E2BIG
The buffer specified by
.I size
and
.I attr
is too small.
.TP
.B EINVAL
.I size
is invalid; that is, it is smaller than the initial version of the
.I sched_attr
structure (48 bytes) or larger than the system page size.
.PP
In addition,
.BR sched_setattr ()
can fail for the following reasons:
.TP
.B E2BIG
The buffer specified by
.I size
and
.I attr
is larger than the kernel structure,
and one or more of the excess bytes is nonzero.
.TP
.B EBUSY
.B SCHED_DEADLINE
admission control failure, see
.BR sched (7).
.TP
.B EINVAL
.I attr.sched_policy
is not one of the recognized policies;
.I attr.sched_flags
contains a flag other than
.BR SCHED_FLAG_RESET_ON_FORK ;
or
.I attr.sched_priority
is invalid; or
.I attr.sched_policy
is
.BR SCHED_DEADLINE
and the deadline scheduling parameters in
.I attr
are invalid.
.TP
.B EPERM
The caller does not have appropriate privileges.
.TP
.B EPERM
The CPU affinity mask of the thread specified by
.I pid
does not include all CPUs in the system
(see
.BR sched_setaffinity (2)).
.SH VERSIONS
These system calls first appeared in Linux 3.14.
.\" FIXME . Add glibc version
.SH CONFORMING TO
These system calls are nonstandard Linux extensions.
.SH NOTES
.BR sched_setattr ()
provides a superset of the functionality of
.BR sched_setscheduler (2),
.BR sched_setparam (2),
.BR 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)
.BR setpriority (2).
Analogously,
.BR sched_getattr ()
provides a superset of the functionality of
.BR sched_getscheduler (2),
.BR sched_getparam (2),
and (partially)
.BR getpriority (2).
.SH BUGS
In Linux versions up to
.\" FIXME . patch sent to Peter Zijlstra
3.15,
.BR sched_settattr ()
failed with the error
.BR EFAULT
instead of
.BR E2BIG
for the case described in ERRORS.
.\" In Linux versions up to up 3.15,
.\" FIXME . patch from Peter Zijlstra pending
.\" .BR sched_setattr ()
.\" allowed a negative
.\" .I attr.sched_policy
.\" value.
.SH SEE ALSO
.ad l
.nh
.BR chrt (1),
.BR nice (2),
.BR sched_get_priority_max (2),
.BR sched_get_priority_min (2),
.BR sched_getaffinity (2),
.BR sched_getparam (2),
.BR sched_getscheduler (2),
.BR sched_rr_get_interval (2),
.BR sched_setaffinity (2),
.BR sched_setparam (2),
.BR sched_setscheduler (2),
.BR sched_yield (2),
.BR setpriority (2),
.BR pthread_getschedparam (3),
.BR pthread_setschedparam (3),
.BR pthread_setschedprio (3),
.BR capabilities (7),
.BR cpuset (7),
.BR sched (7)
.ad
.SH COLOPHON
This page is part of release 4.10 of the Linux
.I 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/.