linux-kernel - Re: [PATCH 1/8] sched/fair: Clean up active balance nr_balance

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Message-ID: <jhjo8h12dsn.mognet@arm.com>
Date:   Wed, 03 Feb 2021 18:42:48 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Quentin Perret <qperret@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH 1/8] sched/fair: Clean up active balance nr_balance_failed trickery

>> @@ -9805,9 +9810,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>>  					active_load_balance_cpu_stop, busiest,
>>  					&busiest->active_balance_work);
>>  			}
>> -
>> -			/* We've kicked active balancing, force task migration. */
>> -			sd->nr_balance_failed = sd->cache_nice_tries+1;
>
> This has an impact on future calls to need_active_balance() too, no? We enter
> this path because need_active_balance() returned true; one of the conditions it
> checks for is
>
> 	return unlikely(sd->nr_balance_failed > sd->cache_nice_tries+2);
>
> So since we used to reset nr_balanced_failed to cache_nice_tries+1, the above
> condition would be false in the next call or two IIUC. But since we remove
> that, we could end up here again soon.
>
> Was this intentional?
>

Partially, I'd say :-)

If you look at active_load_balance_cpu_stop(), it does

  sd->nr_balance_failed = 0;

when it successfully pulls a task. So we get a reset of the failed counter
on pull, which I've preserved. As for interactions with later
need_active_balance(), the commit that introduced the current counter write
(which is over 15 years old!):  

  3950745131e2 ("[PATCH] sched: fix SMT scheduling problems")

only states the task_hot() issue; thus I'm doubtful whether said
interaction was intentional.