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    SubjectRe: [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@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.


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