linux-kernel - Re: [PATCH] sched: next buddy hint on sleep and preempt path

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Date:	Tue, 1 Mar 2011 23:08:05 -0800
From:	Paul Turner <pjt@...gle.com>
To:	Mike Galbraith <efault@....de>
Cc:	Venkatesh Pallipadi <venki@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH] sched: next buddy hint on sleep and preempt path

On Tue, Mar 1, 2011 at 10:47 PM, Mike Galbraith <efault@....de> wrote:
> On Tue, 2011-03-01 at 21:43 -0800, Paul Turner wrote:
>> On Tue, Mar 1, 2011 at 3:33 PM, Venkatesh Pallipadi <venki@...gle.com> wrote:
>
>> >        for_each_sched_entity(se) {
>> >                cfs_rq = cfs_rq_of(se);
>> >                dequeue_entity(cfs_rq, se, flags);
>> >
>> >                /* Don't dequeue parent if it has other entities besides us */
>> > -               if (cfs_rq->load.weight)
>> > +               if (cfs_rq->load.weight) {
>> > +                       /*
>> > +                        * Bias pick_next to pick a task from this cfs_rq, as
>> > +                        * p is sleeping when it is within its sched_slice.
>> > +                        */
>> > +                       if (task_flags & DEQUEUE_SLEEP && se->parent)
>> > +                               set_next_buddy(se->parent);
>>
>> re-using the last_buddy would seem like a more natural fit here; also
>> doesn't have a clobber race with a wakeup
>
> Hm, that would break last_buddy no?  A preempted task won't get the CPU
> back after light preempting thread deactivates.  (it's disabled atm
> unless heavily overloaded anyway, but..)

Ommm yeah.. we're actually a little snookered in this case since the
pre-empting thread's sleep will be voluntary which will try to return
time to its hierarchy.

I suppose keeping the last_buddy is preferable to the occasional clobber.

>
> This wants a tweak either way though.
>
> static inline struct task_struct *task_of(struct sched_entity *se)
> {
> #ifdef CONFIG_SCHED_DEBUG
>        WARN_ON_ONCE(!entity_is_task(se));
> #endif
>        return container_of(se, struct task_struct, se);
> }
>
> static void set_next_buddy(struct sched_entity *se)
> {
>        if (likely(task_of(se)->policy != SCHED_IDLE)) {
>                for_each_sched_entity(se)
>                        cfs_rq_of(se)->next = se;
>        }
> }
>

yeah good catch, the if's do need to be broken out so that setting
buddies it works properly with non-task entities

>
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