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From:   Valentin Schneider <valentin.schneider@arm.com>
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     Peng Liu <iwtbavbm@gmail.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] sched/fair: Fix nohz.next_balance update
In-reply-to: <CAKfTPtCM7mE7a63rXB4cG5gHn03ArjTB1ZBje=qEWOGR9mj67g@mail.gmail.com>
Date:   Wed, 06 May 2020 17:02:56 +0100
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On 06/05/20 14:45, Vincent Guittot wrote:
>> But then we may skip an update if we goto abort, no? Imagine we have just
>> NOHZ_STATS_KICK, so we don't call any rebalance_domains(), and then as we
>> go through the last NOHZ CPU in the loop we hit need_resched(). We would
>> end in the abort part without any update to nohz.next_balance, despite
>> having accumulated relevant data in the local next_balance variable.
>
> Yes but on the other end, the last CPU has not been able to run the
> rebalance_domain so we must not move  nohz.next_balance otherwise it
> will have to wait for at least another full period
> In fact, I think that we have a problem with current implementation
> because if we abort because  local cpu because busy we might end up
> skipping idle load balance for a lot of idle CPUs
>
> As an example, imagine that we have 10 idle CPUs with the same
> rq->next_balance which equal nohz.next_balance.  _nohz_idle_balance
> starts on CPU0, it processes idle lb for CPU1 but then has to abort
> because of need_resched. If we update nohz.next_balance like
> currently, the next idle load balance  will happen after a full
> balance interval whereas we still have 8 CPUs waiting for running an
> idle load balance.
>
> My proposal also fixes this problem
>

That's a very good point; so with NOHZ_BALANCE_KICK we can reduce
nohz.next_balance via rebalance_domains(), and otherwise we would only
increase it if we go through a complete for_each_cpu() loop in
_nohz_idle_balance().

That said, if for some reason we keep bailing out of the loop, we won't
push nohz.next_balance forward and thus may repeatedly nohz-balance only
the first few CPUs in the NOHZ mask. I think that can happen if we have
say 2 tasks pinned to a single rq, in that case nohz_balancer_kick() will
kick a NOHZ balance whenever now >= nohz.next_balance.