man-pages/man2/sched_setscheduler.2

.\" Copyright (C) Tom Bjorkholm, Markus Kuhn & David A. Wheeler 1996-1999
.\" and Copyright (C) 2007 Carsten Emde <Carsten.Emde@osadl.org>
.\" and Copyright (C) 2008 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
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.\"
.\" 1996-04-01 Tom Bjorkholm <tomb@mydata.se>
.\"            First version written
.\" 1996-04-10 Markus Kuhn <mskuhn@cip.informatik.uni-erlangen.de>
.\"            revision
.\" 1999-08-18 David A. Wheeler <dwheeler@ida.org> added Note.
.\" Modified, 25 Jun 2002, Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Corrected description of queue placement by sched_setparam() and
.\"		sched_setscheduler()
.\"	A couple of grammar clean-ups
.\" Modified 2004-05-27 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" 2005-03-23, mtk, Added description of SCHED_BATCH.
.\" 2007-07-10, Carsten Emde <Carsten.Emde@osadl.org>
.\"     Add text on real-time features that are currently being
.\"     added to the mainline kernel.
.\" 2008-05-07, mtk; Rewrote and restructured various parts of the page to
.\"     improve readability.
.\" 2010-06-19, mtk, documented SCHED_RESET_ON_FORK
.\"
.\" Worth looking at: http://rt.wiki.kernel.org/index.php
.\"
.TH SCHED_SETSCHEDULER 2 2013-02-12 "Linux" "Linux Programmer's Manual"
.SH NAME
sched_setscheduler, sched_getscheduler \-
set and get scheduling policy/parameters
.SH SYNOPSIS
.nf
.B #include <sched.h>
.sp
.BI "int sched_setscheduler(pid_t " pid ", int " policy ,
.br
.BI "                       const struct sched_param *" param );
.sp
.BI "int sched_getscheduler(pid_t " pid );
.sp
\fBstruct sched_param {
    ...
    int \fIsched_priority\fB;
    ...
};
.fi
.SH DESCRIPTION
.BR sched_setscheduler ()
sets both the scheduling policy and the associated parameters for the
process whose ID is specified in \fIpid\fP.
If \fIpid\fP equals zero, the
scheduling policy and parameters of the calling process will be set.
The interpretation of
the argument \fIparam\fP depends on the selected policy.
Currently, Linux supports the following "normal"
(i.e., non-real-time) scheduling policies:
.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
The following "real-time" policies are also supported,
for special time-critical applications that need precise control over
the way in which runnable processes are selected for execution:
.TP 14
.BR SCHED_FIFO
a first-in, first-out policy; and
.TP
.BR SCHED_RR
a round-robin policy.
.PP
The semantics of each of these policies are detailed below.

.BR sched_getscheduler ()
queries the scheduling policy currently applied to the process
identified by \fIpid\fP.
If \fIpid\fP equals zero, the policy of the
calling process will be retrieved.
.\"
.SS Scheduling policies
The scheduler is the kernel component that decides which runnable process
will be executed by the CPU next.
Each process has an associated scheduling policy and a \fIstatic\fP
scheduling priority, \fIsched_priority\fP; these are the settings
that are modified by
.BR sched_setscheduler ().
The scheduler makes it decisions based on knowledge of the scheduling
policy and static priority of all processes on the system.

For processes scheduled under one of the normal scheduling policies
(\fBSCHED_OTHER\fP, \fBSCHED_IDLE\fP, \fBSCHED_BATCH\fP),
\fIsched_priority\fP is not used in scheduling
decisions (it must be specified as 0).

Processes scheduled under one of the real-time policies
(\fBSCHED_FIFO\fP, \fBSCHED_RR\fP) have a
\fIsched_priority\fP value in the range 1 (low) to 99 (high).
(As the numbers imply, real-time processes always have higher priority
than normal processes.)
Note well: POSIX.1-2001 only requires an implementation to support a
minimum 32 distinct priority levels for the real-time policies,
and some systems supply just this minimum.
Portable programs should use
.BR sched_get_priority_min (2)
and
.BR sched_get_priority_max (2)
to find the range of priorities supported for a particular policy.

Conceptually, the scheduler maintains a list of runnable
processes for each possible \fIsched_priority\fP value.
In order to determine which process runs next, the scheduler looks for
the nonempty list with the highest static priority and selects the
process at the head of this list.

A process's scheduling policy determines
where it will be inserted into the list of processes
with equal static priority and how it will move inside this list.

All scheduling is preemptive: if a process with a higher static
priority becomes ready to run, the currently running process
will be preempted and
returned to the wait list for its static priority level.
The scheduling policy only determines the
ordering within the list of runnable processes with equal static
priority.
.SS SCHED_FIFO: First in-first out scheduling
\fBSCHED_FIFO\fP can only be used with static priorities higher than
0, which means that when a \fBSCHED_FIFO\fP processes becomes runnable,
it will always immediately preempt any currently running
\fBSCHED_OTHER\fP, \fBSCHED_BATCH\fP, or \fBSCHED_IDLE\fP process.
\fBSCHED_FIFO\fP is a simple scheduling
algorithm without time slicing.
For processes scheduled under the
\fBSCHED_FIFO\fP policy, the following rules apply:
.IP * 3
A \fBSCHED_FIFO\fP process that has been preempted by another process of
higher priority will stay at the head of the list for its priority and
will resume execution as soon as all processes of higher priority are
blocked again.
.IP *
When a \fBSCHED_FIFO\fP process becomes runnable, it
will be inserted at the end of the list for its priority.
.IP *
A call to
.BR sched_setscheduler ()
or
.BR sched_setparam (2)
will put the
\fBSCHED_FIFO\fP (or \fBSCHED_RR\fP) process identified by
\fIpid\fP at the start of the list if it was runnable.
As a consequence, it may preempt the currently running process if
it has the same priority.
(POSIX.1-2001 specifies that the process should go to the end
of the list.)
.\" In 2.2.x and 2.4.x, the process is placed at the front of the queue
.\" In 2.0.x, the Right Thing happened: the process went to the back -- MTK
.IP *
A process calling
.BR sched_yield (2)
will be put at the end of the list.
.PP
No other events will move a process
scheduled under the \fBSCHED_FIFO\fP policy in the wait list of
runnable processes with equal static priority.

A \fBSCHED_FIFO\fP
process runs until either it is blocked by an I/O request, it is
preempted by a higher priority process, or it calls
.BR sched_yield (2).
.SS SCHED_RR: Round-robin scheduling
\fBSCHED_RR\fP is a simple enhancement of \fBSCHED_FIFO\fP.
Everything
described above for \fBSCHED_FIFO\fP also applies to \fBSCHED_RR\fP,
except that each process is only allowed to run for a maximum time
quantum.
If a \fBSCHED_RR\fP process has been running for a time
period equal to or longer than the time quantum, it will be put at the
end of the list for its priority.
A \fBSCHED_RR\fP process that has
been preempted by a higher priority process and subsequently resumes
execution as a running process will complete the unexpired portion of
its round-robin time quantum.
The length of the time quantum can be
retrieved using
.BR sched_rr_get_interval (2).
.\" On Linux 2.4, the length of the RR interval is influenced
.\" by the process nice value -- MTK
.\"
.SS SCHED_OTHER: Default Linux time-sharing scheduling
\fBSCHED_OTHER\fP can only be used at static priority 0.
\fBSCHED_OTHER\fP is the standard Linux time-sharing scheduler that is
intended for all processes that do not require the special
real-time mechanisms.
The process to run is chosen from the static
priority 0 list based on a \fIdynamic\fP priority that is determined only
inside this list.
The dynamic priority is based on the nice value (set by
.BR nice (2)
or
.BR setpriority (2))
and increased for each time quantum the process is ready to run,
but denied to run by the scheduler.
This ensures fair progress among all \fBSCHED_OTHER\fP processes.
.\"
.SS SCHED_BATCH: Scheduling batch processes
(Since Linux 2.6.16.)
\fBSCHED_BATCH\fP can only be used at static priority 0.
This policy is similar to \fBSCHED_OTHER\fP in that it schedules
the process according to its dynamic priority
(based on the nice value).
The difference is that this policy
will cause the scheduler to always assume
that the process is CPU-intensive.
Consequently, the scheduler will apply a small scheduling
penalty with respect to wakeup behaviour,
so that this process is mildly disfavored in scheduling decisions.

.\" The following paragraph is drawn largely from the text that
.\" accompanied Ingo Molnar's patch for the implementation of
.\" SCHED_BATCH.
This policy is useful for workloads that are noninteractive,
but do not want to lower their nice value,
and for workloads that want a deterministic scheduling policy without
interactivity causing extra preemptions (between the workload's tasks).
.\"
.SS SCHED_IDLE: Scheduling very low priority jobs
(Since Linux 2.6.23.)
\fBSCHED_IDLE\fP can only be used at static priority 0;
the process nice value has no influence for this policy.

This policy is intended for running jobs at extremely low
priority (lower even than a +19 nice value with the
.B SCHED_OTHER
or
.B SCHED_BATCH
policies).
.\"
.SS Resetting scheduling policy for child processes
Since Linux 2.6.32, the
.B SCHED_RESET_ON_FORK
flag can be ORed in
.I policy
when calling
.BR sched_setscheduler ().
As a result of including this flag, children created by
.BR fork (2)
do not inherit privileged scheduling policies.
This feature is intended for media-playback applications,
and can be used to prevent applications evading the
.BR RLIMIT_RTTIME
resource limit (see
.BR getrlimit (2))
by creating multiple child processes.

More precisely, if the
.BR SCHED_RESET_ON_FORK
flag is specified,
the following rules apply for subsequently created children:
.IP * 3
If the calling process has a scheduling policy of
.B SCHED_FIFO
or
.BR SCHED_RR ,
the policy is reset to
.BR SCHED_OTHER
in child processes.
.IP *
If the calling process has a negative nice value,
the nice value is reset to zero in child processes.
.PP
After the
.BR SCHED_RESET_ON_FORK
flag has been enabled,
it can only be reset if the process has the
.BR CAP_SYS_NICE
capability.
This flag is disabled in child processes created by
.BR fork (2).

The
.B SCHED_RESET_ON_FORK
flag is visible in the policy value returned by
.BR sched_getscheduler ()
.\"
.SS Privileges and resource limits
In Linux kernels before 2.6.12, only privileged
.RB ( CAP_SYS_NICE )
processes can set a nonzero static priority (i.e., set a real-time
scheduling policy).
The only change that an unprivileged process can make is to set the
.B SCHED_OTHER
policy, and this can only be done if the effective user ID of the caller of
.BR sched_setscheduler ()
matches the real or effective user ID of the target process
(i.e., the process specified by
.IR pid )
whose policy is being changed.

Since Linux 2.6.12, the
.B RLIMIT_RTPRIO
resource limit defines a ceiling on an unprivileged process's
static priority for the
.B SCHED_RR
and
.B SCHED_FIFO
policies.
The rules for changing scheduling policy and priority are as follows:
.IP * 3
If an unprivileged process has a nonzero
.B RLIMIT_RTPRIO
soft limit, then it can change its scheduling policy and priority,
subject to the restriction that the priority cannot be set to a
value higher than the maximum of its current priority and its
.B RLIMIT_RTPRIO
soft limit.
.IP *
If the
.B RLIMIT_RTPRIO
soft limit is 0, then the only permitted changes are to lower the priority,
or to switch to a non-real-time policy.
.IP *
Subject to the same rules,
another unprivileged process can also make these changes,
as long as the effective user ID of the process making the change
matches the real or effective user ID of the target process.
.IP *
Special rules apply for the
.BR SCHED_IDLE .
In Linux kernels before 2.6.39,
an unprivileged process operating under this policy cannot
change its policy, regardless of the value of its
.BR RLIMIT_RTPRIO
resource limit.
In Linux kernels since 2.6.39,
.\" commit c02aa73b1d18e43cfd79c2f193b225e84ca497c8
an unprivileged process can switch to either the
.BR SCHED_BATCH
or the
.BR SCHED_NORMAL
policy so long as its nice value falls within the range permitted by its
.BR RLIMIT_NICE
resource limit (see
.BR getrlimit (2)).
.PP
Privileged
.RB ( CAP_SYS_NICE )
processes ignore the
.B RLIMIT_RTPRIO
limit; as with older kernels,
they can make arbitrary changes to scheduling policy and priority.
See
.BR getrlimit (2)
for further information on
.BR RLIMIT_RTPRIO .
.SS Response time
A blocked high priority process waiting for the I/O has a certain
response time before it is scheduled again.
The device driver writer
can greatly reduce this response time by using a "slow interrupt"
interrupt handler.
.\" as described in
.\" .BR request_irq (9).
.SS Miscellaneous
Child processes inherit the scheduling policy and parameters across a
.BR fork (2).
The scheduling policy and parameters are preserved across
.BR execve (2).

Memory locking is usually needed for real-time processes to avoid
paging delays; this can be done with
.BR mlock (2)
or
.BR mlockall (2).

Since a nonblocking infinite loop in a process scheduled under
\fBSCHED_FIFO\fP or \fBSCHED_RR\fP will block all processes with lower
priority forever, a software developer should always keep available on
the console a shell scheduled under a higher static priority than the
tested application.
This will allow an emergency kill of tested
real-time applications that do not block or terminate as expected.
See also the description of the
.BR RLIMIT_RTTIME
resource limit in
.BR getrlimit (2).

POSIX systems on which
.BR sched_setscheduler ()
and
.BR sched_getscheduler ()
are available define
.B _POSIX_PRIORITY_SCHEDULING
in \fI<unistd.h>\fP.
.SH RETURN VALUE
On success,
.BR sched_setscheduler ()
returns zero.
On success,
.BR sched_getscheduler ()
returns the policy for the process (a nonnegative integer).
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.B EINVAL
The scheduling \fIpolicy\fP is not one of the recognized policies,
\fIparam\fP is NULL,
or \fIparam\fP does not make sense for the \fIpolicy\fP.
.TP
.B EPERM
The calling process does not have appropriate privileges.
.TP
.B ESRCH
The process whose ID is \fIpid\fP could not be found.
.SH CONFORMING TO
POSIX.1-2001 (but see BUGS below).
The \fBSCHED_BATCH\fP and \fBSCHED_IDLE\fP policies are Linux-specific.
.SH NOTES
POSIX.1 does not detail the permissions that an unprivileged
process requires in order to call
.BR sched_setscheduler (),
and details vary across systems.
For example, the Solaris 7 manual page says that
the real or effective user ID of the calling process must
match the real user ID or the save set-user-ID of the target process.
.PP
The scheduling policy and parameters are in fact per-thread
attributes on Linux.
The value returned from a call to
.BR gettid (2)
can be passed in the argument
.IR pid .
Specifying
.I pid
as 0 will operate on the attribute for the calling thread,
and passing the value returned from a call to
.BR getpid (2)
will operate on the attribute for the main thread of the thread group.
(If you are using the POSIX threads API, then use
.BR pthread_setschedparam (3),
.BR pthread_getschedparam (3),
and
.BR pthread_setschedprio (3),
instead of the
.BR sched_* (2)
system calls.)
.PP
Originally, Standard Linux was intended as a general-purpose operating
system being able to handle background processes, interactive
applications, and less demanding real-time applications (applications that
need to usually meet timing deadlines).
Although the Linux kernel 2.6
allowed for kernel preemption and the newly introduced O(1) scheduler
ensures that the time needed to schedule is fixed and deterministic
irrespective of the number of active tasks, true real-time computing
was not possible up to kernel version 2.6.17.
.SS Real-time features in the mainline Linux kernel
.\" FIXME . Probably this text will need some minor tweaking
.\" by about the time of 2.6.30; ask Carsten Emde about this then.
From kernel version 2.6.18 onward, however, Linux is gradually
becoming equipped with real-time capabilities,
most of which are derived from the former
.I realtime-preempt
patches developed by Ingo Molnar, Thomas Gleixner,
Steven Rostedt, and others.
Until the patches have been completely merged into the
mainline kernel
(this is expected to be around kernel version 2.6.30),
they must be installed to achieve the best real-time performance.
These patches are named:
.in +4n
.nf

patch-\fIkernelversion\fP-rt\fIpatchversion\fP
.fi
.in
.PP
and can be downloaded from
.UR http://www.kernel.org\:/pub\:/linux\:/kernel\:/projects\:/rt/
.UE .

Without the patches and prior to their full inclusion into the mainline
kernel, the kernel configuration offers only the three preemption classes
.BR CONFIG_PREEMPT_NONE ,
.BR CONFIG_PREEMPT_VOLUNTARY ,
and
.B CONFIG_PREEMPT_DESKTOP
which respectively provide no, some, and considerable
reduction of the worst-case scheduling latency.

With the patches applied or after their full inclusion into the mainline
kernel, the additional configuration item
.B CONFIG_PREEMPT_RT
becomes available.
If this is selected, Linux is transformed into a regular
real-time operating system.
The FIFO and RR scheduling policies that can be selected using
.BR sched_setscheduler ()
are then used to run a process
with true real-time priority and a minimum worst-case scheduling latency.
.SH BUGS
POSIX says that on success,
.BR sched_setscheduler ()
should return the previous scheduling policy.
Linux
.BR sched_setscheduler ()
does not conform to this requirement,
since it always returns 0 on success.
.SH SEE ALSO
.ad l
.nh
.BR chrt (1),
.BR getpriority (2),
.BR mlock (2),
.BR mlockall (2),
.BR munlock (2),
.BR munlockall (2),
.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_rr_get_interval (2),
.BR sched_setaffinity (2),
.BR sched_setparam (2),
.BR sched_yield (2),
.BR setpriority (2),
.BR capabilities (7),
.BR cpuset (7)
.ad j
.PP
.I Programming for the real world \- POSIX.4
by Bill O. Gallmeister, O'Reilly & Associates, Inc., ISBN 1-56592-074-0.
.PP
.I Documentation/scheduler/sched-rt-group.txt
in the Linux kernel source tree
(since kernel 2.6.25).