linux-kernel - Re: [PATCH v2 2/2] sched/fair: Trigger nohz.next_balance updates when a CPU goes NOHZ-idle

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Date:   Tue, 20 Jul 2021 12:47:21 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH v2 2/2] sched/fair: Trigger nohz.next_balance updates when
 a CPU goes NOHZ-idle

On 19/07/2021 18:28, Valentin Schneider wrote:
> On 19/07/21 17:24, Dietmar Eggemann wrote:
>> On 19/07/2021 12:31, Valentin Schneider wrote:

[...]

>>>       * Ensures that if we miss the CPU, we must see the has_blocked
>>> @@ -10531,6 +10540,8 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags,
>>>              if (need_resched()) {
>>>                      if (flags & NOHZ_STATS_KICK)
>>>                              has_blocked_load = true;
>>
>> This looks weird now? 'has_blocked_load = true' vs
>> 'WRITE_ONCE(nohz.needs_update, 1)'.
>>
> 
> Well, has_blocked_load lets us factorize the nohz.has_blocked write
> (one is needed either when aborting or at the tail of the cpumask
> iteration), whereas there is just a single write for nohz.needs_update
> (when aborting).

You're right. Looks good then.

[...]