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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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