Re: [PATCH 4/4] sched: Idle task shortcut optimization

From: Daniel Lezcano
Date: Fri Jan 17 2014 - 10:06:56 EST

On 01/17/2014 03:26 PM, Peter Zijlstra wrote:
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);
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:

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;

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 :-)

But put_prev_task is called before pick_next_task, so idle_balance() is called after now, no ?

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