Re: [PATCH 7/9] sched/fair: Optimize cgroup pick_next_task_fair

Vincent Guittot
2014-01-30T12:56:49Z

On 30 January 2014 13:37, Peter Zijlstra <peterz@infradead.org> wrote:
> On Thu, Jan 30, 2014 at 01:18:09PM +0100, Vincent Guittot wrote:
>> On 28 January 2014 18:16, Peter Zijlstra <peterz@infradead.org> wrote:
>>
>> [snip]
>>
>> >
>> > @@ -4662,9 +4682,86 @@ static void check_preempt_wakeup(struct
>> > static struct task_struct *
>> > pick_next_task_fair(struct rq *rq, struct task_struct *prev)
>> > {
>> > - struct task_struct *p;
>> > struct cfs_rq *cfs_rq = &rq->cfs;
>> > struct sched_entity *se;
>> > + struct task_struct *p;
>> > +
>> > +#ifdef CONFIG_FAIR_GROUP_SCHED
>> > + if (!cfs_rq->nr_running)
>> > + return NULL;
>>
>> Couldn't you move the test above out of the CONFIG_FAIR_GROUP_SCHED
>> and remove the same test that is done after the simple label
>
> No, we have to check it twice because..
>>
>> > +
>> > + if (prev->sched_class != &fair_sched_class)
>> > + goto simple;
>> > +
>> > + /*
>> > + * Because of the set_next_buddy() in dequeue_task_fair() it is rather
>> > + * likely that a next task is from the same cgroup as the current.
>> > + *
>> > + * Therefore attempt to avoid putting and setting the entire cgroup
>> > + * hierarchy, only change the part that actually changes.
>> > + */
>> > +
>> > + do {
>> > + struct sched_entity *curr = cfs_rq->curr;
>> > +
>> > + /*
>> > + * Since we got here without doing put_prev_entity() we also
>> > + * have to consider cfs_rq->curr. If it is still a runnable
>> > + * entity, update_curr() will update its vruntime, otherwise
>> > + * forget we've ever seen it.
>> > + */
>> > + if (curr && curr->on_rq)
>> > + update_curr(cfs_rq);
>> > + else
>> > + curr = NULL;
>> > +
>> > + /*
>> > + * This call to check_cfs_rq_runtime() will do the throttle and
>> > + * dequeue its entity in the parent(s). Therefore the 'simple'
>> > + * nr_running test will indeed be correct.
>> > + */
>> > + if (unlikely(check_cfs_rq_runtime(cfs_rq)))
>> > + goto simple;
>
> ... here if you read the comment above, we could have modified the
> nr_running.

ok, i missed this point

>
>> > + se = pick_next_entity(cfs_rq, curr);
>> > + cfs_rq = group_cfs_rq(se);
>> > + } while (cfs_rq);
>> > +
>> > + p = task_of(se);
>> > +
>> > + /*
>> > + * Since we haven't yet done put_prev_entity and if the selected task
>> > + * is a different task than we started out with, try and touch the
>> > + * least amount of cfs_rqs.
>> > + */
>> > + if (prev != p) {
>> > + struct sched_entity *pse = &prev->se;
>> > +
>> > + while (!(cfs_rq = is_same_group(se, pse))) {
>> > + int se_depth = se->depth;
>> > + int pse_depth = pse->depth;
>> > +
>> > + if (se_depth <= pse_depth) {
>> > + put_prev_entity(cfs_rq_of(pse), pse);
>> > + pse = parent_entity(pse);
>> > + }
>> > + if (se_depth >= pse_depth) {
>> > + set_next_entity(cfs_rq_of(se), se);
>> > + se = parent_entity(se);
>> > + }
>> > + }
>> > +
>> > + put_prev_entity(cfs_rq, pse);
>> > + set_next_entity(cfs_rq, se);
>> > + }
>> > +
>> > + if (hrtick_enabled(rq))
>> > + hrtick_start_fair(rq, p);
>> > +
>> > + return p;
>> > +simple:
>> > + cfs_rq = &rq->cfs;
>> > +#endif
>> >
>> > if (!cfs_rq->nr_running)
>> > return NULL;
>
> And therefore this test needs to stay.