linux-kernel - Re: [PATCH 13/15] sched,fair: propagate sum_exec

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Date:   Wed, 28 Aug 2019 09:14:54 -0400
From:   Rik van Riel <riel@...riel.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel@...r.kernel.org
Cc:     kernel-team@...com, pjt@...gle.com, peterz@...radead.org,
        mingo@...hat.com, morten.rasmussen@....com, tglx@...utronix.de,
        mgorman@...hsingularity.net, vincent.guittot@...aro.org
Subject: Re: [PATCH 13/15] sched,fair: propagate sum_exec_runtime up the
 hierarchy

On Wed, 2019-08-28 at 09:51 +0200, Dietmar Eggemann wrote:
> On 22/08/2019 04:17, Rik van Riel wrote:
> > Now that enqueue_task_fair and dequeue_task_fair no longer iterate
> > up
> > the hierarchy all the time, a method to lazily propagate
> > sum_exec_runtime
> > up the hierarchy is necessary.
> > 
> > Once a tick, propagate the newly accumulated exec_runtime up the
> > hierarchy,
> > and feed it into CFS bandwidth control.
> > 
> > Remove the pointless call to account_cfs_rq_runtime from
> > update_curr,
> > which is always called with a root cfs_rq.
> 
> But what about the call to account_cfs_rq_runtime() in
> set_curr_task_fair()? Here you always call it with the root cfs_rq.
> Shouldn't this be called also in a loop over all se's until !se-
> >parent
> (like in propagate_exec_runtime() further below).

I believe that call should be only on the cgroup
cfs_rq, with account_cfs_rq_runtime figuring out
whether more runtime needs to be obtained from
further up in the hierarchy.

By default we should probably work under the assumption
that account_cfs_rq_runtime() will succeed at the current
level, and no gymnastics are required to obtain CPU time.

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