linux-kernel - Re: [PATCH v2] sched/fair: Fix enqueue_task

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Date:   Mon, 11 May 2020 16:44:10 -0400
From:   Phil Auld <pauld@...hat.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH v2] sched/fair: Fix enqueue_task_fair warning some more

On Mon, May 11, 2020 at 09:25:43PM +0200 Vincent Guittot wrote:
> On Thu, 7 May 2020 at 22:36, Phil Auld <pauld@...hat.com> wrote:
> >
> > sched/fair: Fix enqueue_task_fair warning some more
> >
> > The recent patch, fe61468b2cb (sched/fair: Fix enqueue_task_fair warning)
> > did not fully resolve the issues with the rq->tmp_alone_branch !=
> > &rq->leaf_cfs_rq_list warning in enqueue_task_fair. There is a case where
> > the first for_each_sched_entity loop exits due to on_rq, having incompletely
> > updated the list.  In this case the second for_each_sched_entity loop can
> > further modify se. The later code to fix up the list management fails to do
> > what is needed because se no longer points to the sched_entity which broke
> > out of the first loop.
> >
> > Address this by calling leaf_add_rq_list if there are throttled parents while
> > doing the second for_each_sched_entity loop.
> >
> 
> Fixes: fe61468b2cb (sched/fair: Fix enqueue_task_fair warning)
> 
> > Suggested-by: Vincent Guittot <vincent.guittot@...aro.org>
> > Signed-off-by: Phil Auld <pauld@...hat.com>
> > Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Cc: Vincent Guittot <vincent.guittot@...aro.org>
> > Cc: Ingo Molnar <mingo@...nel.org>
> > Cc: Juri Lelli <juri.lelli@...hat.com>
> 
> With the Fixes tag and the typo mentioned by Tao
>

Right, that last line of the commit message should read "list_add_leaf_cfs_rq"


> Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>

Thanks Vincent.

Peter/Ingo, do you want me to resend or can you fix when applying?


Thanks,
Phil

> 
> > ---
> >  kernel/sched/fair.c | 7 +++++++
> >  1 file changed, 7 insertions(+)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 02f323b85b6d..c6d57c334d51 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -5479,6 +5479,13 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> >                 /* end evaluation on encountering a throttled cfs_rq */
> >                 if (cfs_rq_throttled(cfs_rq))
> >                         goto enqueue_throttle;
> > +
> > +               /*
> > +                * One parent has been throttled and cfs_rq removed from the
> > +                * list. Add it back to not break the leaf list.
> > +                */
> > +               if (throttled_hierarchy(cfs_rq))
> > +                       list_add_leaf_cfs_rq(cfs_rq);
> >         }
> >
> >  enqueue_throttle:
> > --
> > 2.18.0
> >
> > V2 rework the fix based on Vincent's suggestion. Thanks Vincent.
> >
> >
> > Cheers,
> > Phil
> >
> > --
> >
> 

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