linux-kernel - Re: [PATCH] sched/fair: fix runnable

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Date:   Thu, 27 Feb 2020 11:20:05 +0000
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Ben Segall <bsegall@...gle.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Phil Auld <pauld@...hat.com>, Parth Shah <parth@...ux.ibm.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Hillf Danton <hdanton@...a.com>, zhout@...aldi.net
Subject: Re: [PATCH] sched/fair: fix runnable_avg for throttled cfs

On 26.02.20 21:01, Vincent Guittot wrote:
> On Wed, 26 Feb 2020 at 20:04, <bsegall@...gle.com> wrote:
>>
>> Vincent Guittot <vincent.guittot@...aro.org> writes:
>>
>>> When a cfs_rq is throttled, its group entity is dequeued and its running
>>> tasks are removed. We must update runnable_avg with current h_nr_running
>>> and update group_se->runnable_weight with new h_nr_running at each level

                                              ^^^

Shouldn't this be 'current' rather 'new' h_nr_running for
group_se->runnable_weight? IMHO, you want to cache the current value
before you add/subtract task_delta.

>>> of the hierarchy.
>>
>> You'll also need to do this for task enqueue/dequeue inside of a
>> throttled hierarchy, I'm pretty sure.
> 
> AFAICT, this is already done with patch "sched/pelt: Add a new
> runnable average signal" when task is enqueued/dequeued inside a
> throttled hierarchy
> 
>>
>>>
>>> Fixes: 9f68395333ad ("sched/pelt: Add a new runnable average signal")
>>> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
>>> ---
>>> This patch applies on top of tip/sched/core
>>>
>>>  kernel/sched/fair.c | 10 ++++++++++
>>>  1 file changed, 10 insertions(+)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index fcc968669aea..6d46974e9be7 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -4703,6 +4703,11 @@ static void throttle_cfs_rq(struct cfs_rq *cfs_rq)
>>>
>>>               if (dequeue)
>>>                       dequeue_entity(qcfs_rq, se, DEQUEUE_SLEEP);
>>> +             else {
>>> +                     update_load_avg(qcfs_rq, se, 0);
>>> +                     se_update_runnable(se);
>>> +             }
>>> +
>>>               qcfs_rq->h_nr_running -= task_delta;
>>>               qcfs_rq->idle_h_nr_running -= idle_task_delta;
>>>
>>> @@ -4772,6 +4777,11 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
>>>               cfs_rq = cfs_rq_of(se);
>>>               if (enqueue)
>>>                       enqueue_entity(cfs_rq, se, ENQUEUE_WAKEUP);
>>> +             else {
>>> +                     update_load_avg(cfs_rq, se, 0);
>>
>>
>>> +                     se_update_runnable(se);
>>> +             }
>>> +
>>>               cfs_rq->h_nr_running += task_delta;
>>>               cfs_rq->idle_h_nr_running += idle_task_delta;