linux-kernel - Re: [PATCH v2] sched/fair: Fix insertion in rq->leaf_cfs_rq

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Date:   Wed, 30 Jan 2019 14:27:17 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     linux-kernel@...r.kernel.org, mingo@...hat.com, tj@...nel.org,
        sargun@...gun.me
Subject: Re: [PATCH v2] sched/fair: Fix insertion in rq->leaf_cfs_rq_list

On Wed, Jan 30, 2019 at 02:06:20PM +0100, Peter Zijlstra wrote:
> On Wed, Jan 30, 2019 at 02:04:10PM +0100, Peter Zijlstra wrote:

> > So I don't much like this; at all. But maybe I misunderstand, this is
> > somewhat tricky stuff and I've not looked at it in a while.
> > 
> > So per normal we do:
> > 
> > 	enqueue_task_fair()
> > 	  for_each_sched_entity() {
> > 	    if (se->on_rq)
> > 	      break;
> > 	    enqueue_entity()
> > 	      list_add_leaf_cfs_rq();
> > 	  }
> > 
> > This ensures that all parents are already enqueued, right? because this
> > is what enqueues those parents.
> > 
> > And in this case you add an unconditional second
> > for_each_sched_entity(); even though it is completely redundant, afaict.
> 
> Ah, it doesn't do a second iteration; it continues where the previous
> two left off.
> 
> Still, why isn't this in unthrottle?

Aah, I see, because we need:

  rq->tmp_alone_branch == &rq->lead_cfs_rq_list

at the end of enqueue_task_fair(); having had that assertion would've
saved some pain I suppose.