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SubjectRe: [PATCH 4/4] sched: Idle task shortcut optimization
On Fri, Jan 17, 2014 at 10:04:04AM +0100, Daniel Lezcano wrote:
> @@ -2679,11 +2715,8 @@ need_resched:
>
> pre_schedule(rq, prev);
>
> - if (unlikely(!rq->nr_running))
> - rq->idle_stamp = idle_balance(rq) ? 0 : rq_clock(rq);
> -
> put_prev_task(rq, prev);
> - next = pick_next_task(rq);
> + next = pick_next_task_or_idle(rq);
> clear_tsk_need_resched(prev);
> clear_preempt_need_resched();
> rq->skip_clock_update = 0;

I have vague memories that we need to have idle_balance() before
put_prev_task(), but I can't recollect why this would be so.

That said, if I resurrect these patches:

https://lkml.org/lkml/2012/6/14/271

I suppose we could write something like:

struct task_struct *pick_next_task(struct rq *rq, struct task_struct *prev)
{
const struct sched_class *class;
struct task_struct *p;

again:
if (likely(rq->nr_running)) {

if (likely(rq->nr_running == rq->cfs.h_nr_running))
return fair_sched_class.pick_next_task(rq, prev);

for_each_class(class) {
p = class->pick_next_task(rq, prev);
if (p)
return p;
}
}

if (idle_balance(rq))
goto again;

rq->idle_stamp = rq_clock(rq);

return idle_sched_class.pick_next_task(rq, prev);
}

Which keeps idle_balance() before put_prev_task(), and by using
idle_sched_clas.pick_next_task() doesn't rape the idle class interface
like you did :-)






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