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

Peter Zijlstra
2014-01-30T12:37:20Z

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.

> > + 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.