Re: [PATCH 01/13] sched: Add 3 new scheduler syscalls to support anextended scheduling parameters ABI

From: Peter Zijlstra
Date: Tue Jan 21 2014 - 10:39:38 EST

On Tue, Jan 21, 2014 at 03:36:37PM +0100, Michael Kerrisk wrote:
> Peter, Dario,

> Is someone (e.g., one of you) planning to write man pages for the new
> sched_setattr() and sched_getattr() system calls? (Also, for the
> future, please CC linux-api@xxxxxxxxxxxxxxx on patches that change the
> API, then those of us who don't follow LKML get a heads up about
> upcoming API changes.)

first draft, shamelessly stolen from SCHED_SETSCHEDULER(2).

One note on both the original as well as the below: process is
ambiguous, the syscalls actually apply to a single thread of a process,
not the entire process.


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);

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

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.

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 other sched_attr fields are filled out as described in

${insert SCHED_* descriptions}

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 -EINVAL when admission
control tests fail.

${NOTE: should we change that to -EBUSY ? }

Other than that its pretty much the same as the existing
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at