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SubjectRe: [PATCH] sched: Remove WAKEUP_PREEMPT feature check in entity_tick
On Fri, Jul 29, 2011 at 09:56:58AM +0200, Peter Zijlstra wrote:
> On Fri, 2011-07-29 at 15:46 +0800, Yong Zhang wrote:
> > On Fri, Jul 29, 2011 at 03:36:15PM +0800, Lin Ming wrote:
> > > >From another point of view, below !sched_feat(WAKEUP_PREEMPT) still
> > > looks like duplicated.
> > >
> > > if (cfs_rq->nr_running > 1 || !sched_feat(WAKEUP_PREEMPT))
> > > check_preempt_tick(cfs_rq, curr);
> > >
> > > if "!sched_feat(WAKEUP_PREEMPT)" is run,
> > > that implies cfs_rq->nr_running == 1.
> >
> > That's true.
> >
> > >
> > > Why do we need to call check_preempt_tick when there is only 1 task
> > > runnable?
> >
> > Just set_tsk_need_resched(p) if p's slice is over, thus:
> >
> > (n tick) ---> (n+1 tick)
> > set_tsk_need_resched(p);
> > another task Q is awaked
> >
> > If we don't have !sched_feat(WAKEUP_PREEMPT), Q maybe will wait
> > for tick coming to get scheduled. If we have
> > !sched_feat(WAKEUP_PREEMPT), Q will get scheduled when some event
> > happen, like IRQ.
>
> Nah, if there is 1 runnable task it will always run, preemption simply
> doesn't matter. There's nothing to preempt it with.

Hmmm, so the newly waked task could be scheduled a little later.
That means schedule tick judge everything.

Thanks,
Yong

>
> I've queued Lin's patch as I don't see the point of this thing either,
> normally WAKEUP_PREEMPT is enabled so it says || 0 which is kinda
> useless :-)
>
> And I'm starting to think we should just kill all of WAKEUP_PREEMPT I
> don't think we ever want to disable it anyway..
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