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

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Date:   Wed, 30 Jan 2019 14:29:42 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
        Sargun Dhillon <sargun@...gun.me>
Subject: Re: [PATCH v2] sched/fair: Fix insertion in rq->leaf_cfs_rq_list

On Wed, 30 Jan 2019 at 14:27, Peter Zijlstra <peterz@...radead.org> wrote:
>
> 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

Yes exactly.
You have been quicker than me to reply

> saved some pain I suppose.