linux-kernel - Re: [PATCH 5/8] sched/fair: Make check_misfit_status() only compare dynamic capacities

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Message-ID: <f1ea5b53-5953-15dc-6b67-9b6d520c61fc@arm.com>
Date:   Thu, 4 Feb 2021 11:49:40 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Qais Yousef <qais.yousef@....com>,
        Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Quentin Perret <qperret@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH 5/8] sched/fair: Make check_misfit_status() only compare
 dynamic capacities

On 03/02/2021 16:15, Qais Yousef wrote:
> On 01/28/21 18:31, Valentin Schneider wrote:

[...]

>> @@ -10238,7 +10236,7 @@ static void nohz_balancer_kick(struct rq *rq)
>>  		 * When ASYM_CPUCAPACITY; see if there's a higher capacity CPU
>>  		 * to run the misfit task on.
>>  		 */
>> -		if (check_misfit_status(rq, sd)) {
>> +		if (check_misfit_status(rq)) {

Since check_misfit_status() doesn't need sd anymore it looks like that
rcu_dereference(per_cpu(sd_asym_cpucapacity, cpu)) could be replaced by
static_branch_unlikely(&sched_asym_cpucapacity)) in nohz_balancer_kick().

But as you mentioned in an earlier conversation we do need to check sd
because of asymmetric CPU capacity systems w/ exclusive cpusets which
could create symmetric islands (unique capacity_orig among CPUs).

Maybe worth putting a comment here (similar to the one in sis()) so
people don't try to optimize?