linux-kernel - Re: [PATCH 7/9] sched/fair: Optimize cgroup pick_next_task

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Date:	Thu, 30 Jan 2014 13:37:10 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Paul Turner <pjt@...gle.com>,
	Benjamin Segall <bsegall@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mike Galbraith <bitbucket@...ine.de>
Subject: Re: [PATCH 7/9] sched/fair: Optimize cgroup pick_next_task_fair

On Thu, Jan 30, 2014 at 01:18:09PM +0100, Vincent Guittot wrote:
> On 28 January 2014 18:16, Peter Zijlstra <peterz@...radead.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.
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