linux-kernel - Re: [RFC PATCH v2 1/5] sched/fair: Add ancestors of unthrottled undecayed cfs

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Message-ID: <20210910113544.GB24156@blackbody.suse.cz>
Date:   Fri, 10 Sep 2021 13:35:44 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Phil Auld <pauld@...hat.com>, Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Odin Ugedal <odin@...d.al>, Rik van Riel <riel@...riel.com>,
        Giovanni Gherdovich <ggherdovich@...e.cz>
Subject: Re: [RFC PATCH v2 1/5] sched/fair: Add ancestors of unthrottled
 undecayed cfs_rq

Hello Vincent.

Thank you for looking into this!

On Thu, Sep 09, 2021 at 03:57:37PM +0200, Vincent Guittot <vincent.guittot@...aro.org> wrote:
> > +               /* Nothing to run but something to decay? Complete the branch */
> > +               if (cfs_rq->on_list)
> 
> Could you use !cfs_rq_is decayed(cfs_rq) ?

What needs to be checked here is whether the list was modified by adding
the cfs_rq (and branch needs closing).

It'd appear that the equal condition like in tg_unthrottle_up() would
make do, i.e.
	!cfs_rq_is_decayed(cfs_rq) || cfs_rq->nr_running
but the unthrottle_cfs_rq() can be called under a still throttled
ancestor (i.e. throttle_count not dropping to zero) and in such a case
cfs_rq should not be added to the list yet.

Therefore, mere !cfs_rq_is_decayed(cfs_rq) doesn't seem correct to me.

> > +                       for_each_sched_entity(se) {
> > +                               if (list_add_leaf_cfs_rq(group_cfs_rq(se)))
> > +                                       break;
> > +                       }
> > +               assert_list_leaf_cfs_rq(rq);
> 
> Instead of adding  a loop here you should better jump to unthrottle_throttle ?

Oh, that looks a bit clumsy now (it's an artifact I've left when
reordering the patch in the series to be backport-friendly). 
Jump to unthrottle_throttle seems easier indeed, there would be just the
additional check
	if (rq->curr == rq->idle && rq->cfs.nr_running)
. Besides unnecessary work, it should be harmless.

Is the jump the preferred form?

Michal