Re: [PATCH v8 -tip 02/26] sched: Introduce sched_class::pick_task()

[Joel Fernandes]

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);