Re: [PATCH] sched: leave sched_setscheduler earlier if possible.

Steven Rostedt
2011-03-24T14:22:04Z

On Thu, 2011-03-24 at 14:00 +0100, Dario Faggioli wrote:
> sched_setscheduler (in sched.c) is called in order of changing the
> scheduling policy and/or the real-time priority of a task. Thus,
> if we find out that neither of those are actually being modified, it
> is possible to return earlier and save the overhead of a full
> deactivate+activate cycle of the task in question.
>
> Beside that, if we have more than one SCHED_FIFO task with the same
> priority on the same rq (which means they share the same priority queue)
> having one of them changing its position in the priority queue because of
> a sched_setscheduler (as it happens by means of the deactivate+activate)
> that does not actually change the priority violates POSIX which states,
> for SCHED_FIFO:
>
> "If a thread whose policy or priority has been modified by
> pthread_setschedprio() is a running thread or is runnable, the effect on
> its position in the thread list depends on the direction of the
> modification, as follows: a. <...> b. If the priority is unchanged, the
> thread does not change position in the thread list. c. <...>"
>
> (http://pubs.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_08.html)
>
> Signed-off-by: Dario Faggioli <raistlin@linux.it>

Acked-by: Steven Rostedt <rostedt@goodmis.org>

> ---
> kernel/sched.c | 10 ++++++++++
> 1 files changed, 10 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index c5ae6bc..d73bbc5 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c

> @@ -4998,6 +4998,16 @@ recheck:

Peter, this is why I prefer:

recheck:
over
recheck:

-- Steve

> return -EINVAL;
> }
>
> + /*
> + * If not changing anything there's no need to proceed further
> + */
> + if (unlikely(policy == p->policy && (!rt_policy(policy) ||
> + param->sched_priority == p->rt_priority))) {
> + __task_rq_unlock(rq);
> + raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> + return 0;
> + }
> +
> #ifdef CONFIG_RT_GROUP_SCHED
> if (user) {
> /*
> --
> 1.7.4.1
>