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    SubjectRe: [PATCH v8 -tip 02/26] sched: Introduce sched_class::pick_task()
    On Mon, Oct 26, 2020 at 10:01:31AM +0100, Peter Zijlstra wrote:
    > On Sat, Oct 24, 2020 at 08:27:16AM -0400, Vineeth Pillai wrote:
    > >
    > >
    > > On 10/24/20 7:10 AM, Vineeth Pillai wrote:
    > > >
    > > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    > > > index 93a3b874077d..4cae5ac48b60 100644
    > > > --- a/kernel/sched/fair.c
    > > > +++ b/kernel/sched/fair.c
    > > > @@ -4428,12 +4428,14 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct
    > > > sched_entity *curr)
    > > >                         se = second;
    > > >         }
    > > >
    > > > -       if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) <
    > > > 1) {
    > > > +       if (left && cfs_rq->next &&
    > > > +                       wakeup_preempt_entity(cfs_rq->next, left) < 1) {
    > > >                 /*
    > > >                  * Someone really wants this to run. If it's not unfair,
    > > > run it.
    > > >                  */
    > > >                 se = cfs_rq->next;
    > > > -       } else if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last,
    > > > left) < 1) {
    > > > +       } else if (left && cfs_rq->last &&
    > > > +                       wakeup_preempt_entity(cfs_rq->last, left) < 1) {
    > > >                 /*
    > > >                  * Prefer last buddy, try to return the CPU to a
    > > > preempted task.
    > > >
    > > >
    > > > There reason for left being NULL needs to be investigated. This was
    > > > there from v1 and we did not yet get to it. I shall try to debug later
    > > > this week.
    > >
    > > Thinking more about it and looking at the crash, I think that
    > > 'left == NULL' can happen in pick_next_entity for core scheduling.
    > > If a cfs_rq has only one task that is running, then it will be
    > > dequeued and 'left = __pick_first_entity()' will be NULL as the
    > > cfs_rq will be empty. This would not happen outside of coresched
    > > because we never call pick_tack() before put_prev_task() which
    > > will enqueue the task back.
    > >
    > > With core scheduling, a cpu can call pick_task() for its sibling while
    > > the sibling is still running the active task and put_prev_task has yet
    > > not been called. This can result in 'left == NULL'.
    >
    > Quite correct. Hurmph.. the reason we do this is because... we do the
    > update_curr() the wrong way around. And I can't seem to remember why we
    > do that (it was in my original patches).
    >
    > Something like so seems the obvious thing to do, but I can't seem to
    > remember why we're not doing it :-(

    The code below is just a refactor and not a functional change though, right?

    i.e. pick_next_entity() is already returning se = curr, if se == NULL.

    But the advantage of your refactor is it doesn't crash the kernel.

    So your change appears safe to me unless I missed something.

    thanks,

    - Joel


    > --- a/kernel/sched/fair.c
    > +++ b/kernel/sched/fair.c
    > @@ -6950,15 +6950,10 @@ static struct task_struct *pick_task_fai
    > do {
    > struct sched_entity *curr = cfs_rq->curr;
    >
    > - se = pick_next_entity(cfs_rq, NULL);
    > + if (curr && curr->on_rq)
    > + update_curr(cfs_rq);
    >
    > - if (curr) {
    > - if (se && curr->on_rq)
    > - update_curr(cfs_rq);
    > -
    > - if (!se || entity_before(curr, se))
    > - se = curr;
    > - }
    > + se = pick_next_entity(cfs_rq, curr);
    >
    > cfs_rq = group_cfs_rq(se);
    > } while (cfs_rq);

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