linux-kernel - Re: [PATCH v2] sched/pelt: fix false running accounting

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Date:   Tue, 4 Jul 2017 09:37:54 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>
Subject: Re: [PATCH v2] sched/pelt: fix false running accounting

On 4 July 2017 at 09:27, Peter Zijlstra <peterz@...radead.org> wrote:
> On Sat, Jul 01, 2017 at 07:06:13AM +0200, Vincent Guittot wrote:
>> The running state is a subset of runnable state which means that running
>> can't be set if runnable (weight) is cleared. There are corner cases
>> where the current sched_entity has been already dequeued but cfs_rq->curr
>> has not been updated yet and still points to the dequeued sched_entity.
>> If ___update_load_avg is called at that time, weight will be 0 and running
>> will be set which is not possible.
>>
>> This case happens during pick_next_task_fair() when a cfs_rq becomes idles.
>> The current sched_entity has been dequeued so se->on_rq is cleared and
>> cfs_rq->weight is null. But cfs_rq->curr still points to se (it will be
>> cleared when picking the idle thread). Because the cfs_rq becomes idle,
>> idle_balance() is called and ends up to call update_blocked_averages()
>> with these wrong running and runnable states.
>>
>> Add a test in ___update_load_avg to correct the running state in this case.
>
> Cute, however did you find that ?

In fact, while rebasing and running more tests on my patch "update
scale invariance of PELT" that changes how to scale the load and
utilization, I have seen that sometimes the utilization was increasing
but not the load when CPU was going into idle state because the
stolen_idle time was applied as idle time for load but running time
for utilization. This patch has highlighted the problem.