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

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Date:   Tue, 12 May 2020 09:37:55 -0400
From:   Phil Auld <pauld@...hat.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        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

Hi Dietmar,

On Tue, May 12, 2020 at 11:00:16AM +0200 Dietmar Eggemann wrote:
> On 11/05/2020 22:44, Phil Auld wrote:
> > 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?
> 
> 
> Maybe you could add that 'the throttled parent was already added back to
> the list by a task enqueue in a parallel child hierarchy'.
> 
> IMHO, this is part of the description because otherwise the throttled
> parent would have connected the branch.
> 
> And the not-adding of the intermediate child cfs_rq would have gone
> unnoticed.

Okay, I'll add that statement. For those curious here are the lines from about
70ms earlier in the trace where the throttled parent (0xffffa085e48ce000) is added
to the list.

bz1738415-test-6264  [005]  1271.315046: sched_waking:         comm=bz1738415-test pid=6269 prio=120 target_cpu=005
bz1738415-test-6264  [005]  1271.315048: sched_migrate_task:   comm=bz1738415-test pid=6269 prio=120 orig_cpu=5 dest_cpu=17
bz1738415-test-6264  [005]  1271.315050: bprint:               enqueue_task_fair: se 0xffffa081e6d7de80 on_rq 0 cfs_rq = 0xffffa085e48ce000
bz1738415-test-6264  [005]  1271.315051: bprint:               enqueue_entity: Add_leaf_rq: cpu 17: nr_r 2; cfs 0xffffa085e48ce000 onlist 0 tmp_a_b = 0xffffa085ef92c868 &rq->l_c_r_l = 0xffffa085ef92c868
bz1738415-test-6264  [005]  1271.315053: bprint:               enqueue_entity: Add_leaf_rq: cpu 17: nr_r 2: parent onlist  Set tmp_alone_branch to 0xffffa085ef92c868
bz1738415-test-6264  [005]  1271.315053: bprint:               enqueue_task_fair: current se = 0xffffa081e6d7de80, orig_se = 0xffffa081e6d7de80
bz1738415-test-6264  [005]  1271.315055: bprint:               enqueue_task_fair: Add_leaf_rq: cpu 17: nr_r 2; cfs 0xffffa085e48ce000 onlist 1 tmp_a_b = 0xffffa085ef92c868 &rq->l_c_r_l = 0xffffa085ef92c868
bz1738415-test-6264  [005]  1271.315056: sched_wake_idle_without_ipi: cpu=17

> 
> Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>

Thanks,

Phil


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