linux-kernel - Re: [PATCH] sched/fair: Fix nohz.next

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Date:   Wed, 06 May 2020 11:28:27 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Peng Liu <iwtbavbm@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        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>
Subject: Re: [PATCH] sched/fair: Fix nohz.next_balance update

On 04/05/20 16:17, Vincent Guittot wrote:
>> Since we can gather all the updated rq->next_balance, including this_cpu,
>> in _nohz_idle_balance(), it's safe to remove the extra lines in
>> rebalance_domains() which are originally intended for this_cpu. And
>> finally the updating only happen in _nohz_idle_balance().
>
> I'm not sure that's always true. Nothing prevents nohz_idle_balance()
> to return false . Then run_rebalance_domains() calls
> rebalance_domains(this_rq ,SCHED_IDLE) outside _nohz_idle_balance().
> In this case we must keep the code in rebalance_domains().
>
> For example when the tick is not stopped when entering idle. Or when
> need_resched() returns true.
>

Going back to this; nohz_idle_balance() will return true regardless of the
return value of _nohz_idle_balance(), so AFAICT we won't fall through to
the rebalance_domains() in run_rebalance_domains() in case we had
need_resched() in _nohz_idle_balance().

This was changed in b7031a02ec75 ("sched/fair: Add NOHZ_STATS_KICK");
before then we would always have the local rebalance_domains(). Now, since
the bail out is caused by need_resched(), I think it's not such a crazy
thing *not* to do the local rebalance_domains(), but I wasn't super clear
on all of this.