On Fri, 2009-03-20 at 10:43 +0100, Miklos Szeredi wrote:
> Ingo,
>
> I tested this one, and I think it makes sense in any case as an
> optimization. It should also be good for -stable kernels.
>
> Does it look OK?
The idea is good, but there is a risk of preemption latencies here. Some
code paths aren't real quick between setting ->state != TASK_RUNNING and
calling schedule.
[ Both quick: as in O(1) and few instructions ]
So if we're going to do this, we'd need to audit all such code paths --
and there be lots.
The first line of attack for this problem is making wait_task_inactive()
sucks less, which shouldn't be too hard, that unconditional 1 jiffy
sleep is simply retarded.
> Index: linux.git/kernel/sched.c
> ===================================================================
> --- linux.git.orig/kernel/sched.c 2009-03-20 09:40:47.000000000 +0100
> +++ linux.git/kernel/sched.c 2009-03-20 10:28:56.000000000 +0100
> @@ -4632,6 +4632,10 @@ asmlinkage void __sched preempt_schedule
> if (likely(ti->preempt_count || irqs_disabled()))
> return;
>
> + /* No point in preempting we are just about to go to sleep. */
> + if (current->state != TASK_RUNNING)
> + return;
> +
> do {
> add_preempt_count(PREEMPT_ACTIVE);
> schedule();
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