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Subjectsched_{set,get}attr() manpage
Hi Michael,

find below an updated manpage, I did not apply the comments on parts
that are identical to SCHED_SETSCHEDULER(2) in order to keep these texts
in alignment. I feel that if we change one we should also change the
other, and such a 'patch' is best done separate from the new manpage

I did add the missing EBUSY error, and amended the text where it said
we'd return EINVAL in that case.

I added a paragraph stating that SCHED_DEADLINE preempted anything else
userspace can do (with the explicit mention of userspace to leave me
wriggle room for the kernel's stop task :-).

I also did a short paragraph on the deadline sched_yield(). For further
deadline yield details we should maybe add to the SCHED_YIELD(2)

Re juri/claudio; no I think sched_yield() as implemented for deadline
makes sense, no other yield semantics other than NOP makes sense for it,
and since we have the syscall already might as well make it do something


sched_setattr, sched_getattr - set and get scheduling policy/attributes

#include <sched.h>

struct sched_attr {
u32 size;
u32 sched_policy;
u64 sched_flags;

s32 sched_nice;
u32 sched_priority;
u64 sched_runtime;
u64 sched_deadline;
u64 sched_period;
int sched_setattr(pid_t pid, const struct sched_attr *attr, unsigned int flags);

int sched_getattr(pid_t pid, const struct sched_attr *attr, unsigned int size, unsigned int flags);

sched_setattr() sets both the scheduling policy and the
associated attributes for the process whose ID is specified in
pid. If pid equals zero, the scheduling policy and attributes
of the calling process will be set. The interpretation of the
argument attr depends on the selected policy. Currently, Linux
supports the following "normal" (i.e., non-real-time) scheduling

SCHED_OTHER the standard "fair" time-sharing policy;

SCHED_BATCH for "batch" style execution of processes; and

SCHED_IDLE for running very low priority background jobs.

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

SCHED_FIFO a first-in, first-out policy;

SCHED_RR a round-robin policy; and

SCHED_DEADLINE a deadline policy.

The semantics of each of these policies are detailed below.

sched_attr::size must be set to the size of the structure, as in
sizeof(struct sched_attr), if the provided structure is smaller
than the kernel structure, any additional fields are assumed
'0'. If the provided structure is larger than the kernel
structure, the kernel verifies all additional fields are '0' if
not the syscall will fail with -E2BIG.

sched_attr::sched_policy the desired scheduling policy.

sched_attr::sched_flags additional flags that can influence
scheduling behaviour. Currently as per Linux kernel 3.14:

SCHED_FLAG_RESET_ON_FORK - resets the scheduling policy
to: (struct sched_attr){ .sched_policy = SCHED_OTHER, }
on fork().

is the only supported flag.

sched_attr::sched_nice should only be set for SCHED_OTHER,
SCHED_BATCH, the desired nice value [-20,19], see NICE(2).

sched_attr::sched_priority should only be set for SCHED_FIFO,
SCHED_RR, the desired static priority [1,99].

sched_attr::sched_period should only be set for SCHED_DEADLINE
and are the traditional sporadic task model parameters.

The flags argument should be 0.

sched_getattr() queries the scheduling policy currently applied
to the process identified by pid. If pid equals zero, the
policy of the calling process will be retrieved.

The size argument should reflect the size of struct sched_attr
as known to userspace. The kernel fills out sched_attr::size to
the size of its sched_attr structure. If the user provided
structure is larger, additional fields are not touched. If the
user provided structure is smaller, but the kernel needs to
return values outside the provided space, the syscall will fail
with -E2BIG.

The flags argument should be 0.

The other sched_attr fields are filled out as described in

Scheduling Policies
The scheduler is the kernel component that decides which runnable
process will be executed by the CPU next. Each process has an associ‐
ated scheduling policy and a static scheduling priority, sched_prior‐
ity; these are the settings that are modified by 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
(SCHED_OTHER, SCHED_IDLE, SCHED_BATCH), sched_priority is not used in
scheduling decisions (it must be specified as 0).

Processes scheduled under one of the real-time policies (SCHED_FIFO,
SCHED_RR) have a sched_priority 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 sched_get_priority_min(2) and
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 sched_priority 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

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 prior‐
ity becomes ready to run, the currently running process will be pre‐
empted 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.

SCHED_DEADLINE: Sporadic task model deadline scheduling
SCHED_DEADLINE is an implementation of GEDF (Global Earliest
Deadline First) with additional CBS (Constant Bandwidth Server).
The CBS guarantees that tasks that over-run their specified
budget are throttled and do not affect the correct performance
of other SCHED_DEADLINE tasks.

SCHED_DEADLINE tasks will fail FORK(2) with -EAGAIN

Setting SCHED_DEADLINE can fail with -EBUSY when admission
control tests fail.

Because of the nature of (G)EDF, SCHED_DEADLINE tasks are the
highest priority (user controllable) tasks in the system, if any
SCHED_DEADLINE task is runnable it will preempt anything
FIFO/RR/OTHER/BATCH/IDLE task out there.

A SCHED_DEADLINE task calling sched_yield() will 'yield' the
current job and wait for a new period to begin.

SCHED_FIFO: First In-First Out scheduling
SCHED_FIFO can only be used with static priorities higher than 0, which
means that when a SCHED_FIFO processes becomes runnable, it will always
immediately preempt any currently running SCHED_OTHER, SCHED_BATCH, or
SCHED_IDLE process. SCHED_FIFO is a simple scheduling algorithm with‐
out time slicing. For processes scheduled under the SCHED_FIFO policy,
the following rules apply:

* A SCHED_FIFO 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 prior‐
ity are blocked again.

* When a SCHED_FIFO process becomes runnable, it will be inserted at
the end of the list for its priority.

* A call to sched_setscheduler() or sched_setparam(2) will put the
SCHED_FIFO (or SCHED_RR) process identified by pid 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

* A process calling sched_yield(2) will be put at the end of the list.

No other events will move a process scheduled under the SCHED_FIFO pol‐
icy in the wait list of runnable processes with equal static priority.

A SCHED_FIFO process runs until either it is blocked by an I/O request,
it is preempted by a higher priority process, or it calls

SCHED_RR: Round Robin scheduling
SCHED_RR is a simple enhancement of SCHED_FIFO. Everything described
above for SCHED_FIFO also applies to SCHED_RR, except that each process
is only allowed to run for a maximum time quantum. If a SCHED_RR
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 SCHED_RR 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 sched_rr_get_interval(2).

SCHED_OTHER: Default Linux time-sharing scheduling
SCHED_OTHER can only be used at static priority 0. SCHED_OTHER is the
standard Linux time-sharing scheduler that is intended for all pro‐
cesses that do not require the special real-time mechanisms. The
process to run is chosen from the static priority 0 list based on a
dynamic priority that is determined only inside this list. The dynamic
priority is based on the nice value (set by nice(2) or 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
SCHED_OTHER processes.

SCHED_BATCH: Scheduling batch processes
(Since Linux 2.6.16.) SCHED_BATCH can only be used at static priority
0. This policy is similar to SCHED_OTHER 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.

This policy is useful for workloads that are noninteractive, but do not
want to lower their nice value, and for workloads that want a determin‐
istic scheduling policy without interactivity causing extra preemptions
(between the workload's tasks).

SCHED_IDLE: Scheduling very low priority jobs
(Since Linux 2.6.23.) SCHED_IDLE 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 SCHED_OTHER or SCHED_BATCH

On success, sched_setattr() and sched_getattr() return 0. On
error, -1 is returned, and errno is set appropriately.

EINVAL The scheduling policy is not one of the recognized policies,
param is NULL, or param does not make sense for the policy.

EPERM The calling process does not have appropriate privileges.

ESRCH The process whose ID is pid could not be found.

E2BIG The provided storage for struct sched_attr is either too
big, see sched_setattr(), or too small, see sched_getattr().

EBUSY SCHED_DEADLINE admission control failure

While the text above (and in SCHED_SETSCHEDULER(2)) talks about
processes, in actual fact these system calls are thread specific.
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