linux-kernel - Re: [PATCH] sched/fair: Remove setting task's se->runnable

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Message-ID: <0aba9bb6-eec7-130e-373f-5f87f192f2d4@arm.com>
Date:   Wed, 11 Jul 2018 10:43:28 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Quentin Perret <quentin.perret@....com>,
        kernel-team@...roid.com, Joel Fernandes <joelaf@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Remove setting task's se->runnable_weight
 during PELT update

On 07/11/2018 01:09 AM, Joel Fernandes wrote:
> On Mon, Jul 09, 2018 at 05:47:53PM +0100, Dietmar Eggemann wrote:
>> A CFS (SCHED_OTHER, SCHED_BATCH or SCHED_IDLE policy) task's
>> se->runnable_weight must always be in sync with its se->load.weight.
>>
>> se->runnable_weight is set to se->load.weight when the task is
>> forked (init_entity_runnable_average()) or reniced (reweight_entity()).
>>
>> There are two cases in set_load_weight() which since they currently only
>> set se->load.weight could lead to a situation in which se->load.weight
>> is different to se->runnable_weight for a CFS task:
>>
>> (1) A task switches to SCHED_IDLE.
>>
>> (2) A SCHED_FIFO, SCHED_RR or SCHED_DEADLINE task which has been reniced
>>      (during which only its static priority gets set) switches to
>>      SCHED_OTHER or SCHED_BATCH.
>>
>> Set se->runnable_weight to se->load.weight in these two cases to prevent
>> this. This eliminates the need to explicitly set it to se->load.weight
>> during PELT updates in the CFS scheduler fastpath.
> 
> Looks good to me. By the way just asking, is there a chance where
> se_weight(se) and se_runnable(se) can ever be different?

Yes they can be different, not for a task though but for se's 
representing task groups. It got introduced to be able to propagate load 
and runnable load independently through the task groups hierarchies.