Re: [PATCH] sched: Remove WAKEUP_PREEMPT feature check in entity_tick

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Fri, 2011-07-29 at 16:46 +0800, Yong Zhang wrote:
> Let's take UP for example, we have cpu-hug task A and threadirq B.
> 
>         n tick          --->            n+1 tick
> set_tsk_need_resched(A);
>                 B comes in and
>                 wake up thread-B;
> 
> So for system on which we disable WAKEUP_PREEMPT,
> if we don't have that check, thread-B will wait until n+1 tick comes
> to get to run.
> But if we have that check, thread-B will get to run after IRQ-B returns.

But that's exactly what wakeup preemption is about, waking tasks don't
get to preempt running tasks. So no doing that preemption is exactly
right for !WAKEUP_PREEMPT.

Anyway, I've queued the removal patch since that removes all
confusion ;-)