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Date:   Mon, 26 Oct 2020 10:01:31 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Vineeth Pillai <viremana@linux.microsoft.com>
Cc:     "Li, Aubrey" <aubrey.li@linux.intel.com>,
        Joel Fernandes <joel@joelfernandes.org>,
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Subject: Re: [PATCH v8 -tip 02/26] sched: Introduce sched_class::pick_task()
Message-ID: <20201026090131.GE2628@hirez.programming.kicks-ass.net>
References: <20201020014336.2076526-1-joel@joelfernandes.org>
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 <a2b66294-6a66-b5c1-7706-29bc92f416f5@linux.intel.com>
 <20201023214702.GA3603399@google.com>
 <d9a70e58-f424-3957-9957-b89071480005@linux.intel.com>
 <4241e5ac-ecdf-8634-fa0d-dd6759e477e1@linux.microsoft.com>
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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 :-(

--- 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);