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

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Message-ID: <20180712081722.GB154647@joelaf.mtv.corp.google.com>
Date:   Thu, 12 Jul 2018 01:17:22 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
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 Wed, Jul 11, 2018 at 10:43:28AM +0200, Dietmar Eggemann wrote:
> 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.

I know that task-group se has different values. I was saying for task ses,
the extra division can be skipped possibly improving performance (if at all).

thanks,

- Joel