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SubjectRe: sched_{set,get}attr() manpage
On Tue, Apr 29, 2014 at 03:08:55PM +0200, Michael Kerrisk (man-pages) wrote:

Juri, Dario, Can you have a look at the 2nd part; I'm not at all sure I
got the activate/release the right way around.

My current thinking was that we activate first, and then release it to
go run. But googling the terms only confused me more. I suppose its one
of those things that's not actually _that_ well defined. And I hope the
ASCII art actually clarifies things better than the terms used.

> [1] A page describing the sched_setattr() and sched_getattr() APIs

NAME
sched_setattr, sched_getattr - set and get scheduling policy/attributes

SYNOPSIS
#include <sched.h>

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

/* SCHED_NORMAL, SCHED_BATCH */
s32 sched_nice;

/* SCHED_FIFO, SCHED_RR */
u32 sched_priority;

/* SCHED_DEADLINE */
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);

DESCRIPTION
sched_setattr() sets both the scheduling policy and the
associated attributes for the process whose ID is specified in
pid.

sched_setattr() replaces sched_setscheduler(), sched_setparam(),
nice() and some of setpriority().

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
policies:

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
execution:

SCHED_FIFO a static priority first-in, first-out policy;

SCHED_RR a static priority round-robin policy; and

SCHED_DEADLINE a dynamic priority deadline policy.

The semantics of each of these policies are detailed in
sched(7).

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 sched(7).

sched_attr::sched_priority should only be set for SCHED_FIFO,
SCHED_RR, the desired static priority [1,99], see sched(7).

sched_attr::sched_runtime
sched_attr::sched_deadline
sched_attr::sched_period should only be set for SCHED_DEADLINE
and are the traditional sporadic task model parameters, see
sched(7).

The flags argument should be 0.

sched_getattr() queries the scheduling policy currently applied
to the process identified by pid.

Similar to sched_setattr(), sched_getattr() replaces
sched_getscheduler(), sched_getparam() and some of
getpriority().

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
sched_setattr().

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

ERRORS
EINVAL The scheduling policy is not one of the recognized policies,
param is NULL, or param does not make sense for the selected
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, see sched(7).

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

> [2] A piece of text describing the SCHED_DEADLINE policy, which I can
> drop into sched(7).

SCHED_DEADLINE: Sporadic task model deadline scheduling
SCHED_DEADLINE is an implementation of GEDF (Global Earliest
Deadline First) with additional CBS (Constant Bandwidth Server).

A sporadic task is on that has a sequence of jobs, where each job
is activated at most once per period [us]. Each job will have an
absolute deadline relative to its activation before which it must
finish its execution, and it shall at no time run longer
than runtime [us] after its release.

activation/wakeup absolute deadline
| release |
v v v
-------x--------x--------------x--------x-------
|<- Runtime -->|
|<---------- Deadline ->|
|<---------- Period ----------->|

This gives: runtime <= (rel) deadline <= period.

The CBS guarantees that tasks that over-run their specified
runtime are throttled and do not affect the correct performance
of other SCHED_DEADLINE tasks.

In general a task set of such tasks it not feasible/schedulable
within the given constraints. Therefore we must do an admittance
test on setting/changing SCHED_DEADLINE policy/attributes.

This admission test calculates that the task set is
feasible/schedulable, failing this, sched_setattr() will return
-EBUSY.

For example, it is required (but not sufficient) for the total
utilization to be less or equal to the total amount of cpu time
available. That is, since each job can maximally run for runtime
[us] per period [us], that task's utilization is runtime/period.
Summing this over all tasks must be less than the total amount of
CPUs present.

SCHED_DEADLINE tasks will fail fork(2) with -EAGAIN.

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.



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