Messages in this thread | | | Date | Tue, 27 Oct 2020 10:19:11 -0400 | From | Joel Fernandes <> | Subject | Re: [PATCH v8 -tip 02/26] sched: Introduce sched_class::pick_task() |
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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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