Messages in this thread | | | Subject | Re: [patch] don't preempt not TASK_RUNNING tasks | From | Peter Zijlstra <> | Date | Fri, 20 Mar 2009 11:03:12 +0100 |
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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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