linux-kernel - Re: [PATCH v4 2/3] sched/fair: Clean up active balance nr_balance

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Date:   Tue, 6 Apr 2021 17:36:38 +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>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....com>,
        Quentin Perret <qperret@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Rik van Riel <riel@...riel.com>,
        Lingutla Chandrasekhar <clingutla@...eaurora.org>
Subject: Re: [PATCH v4 2/3] sched/fair: Clean up active balance
 nr_balance_failed trickery

On 01/04/2021 21:30, Valentin Schneider wrote:
> When triggering an active load balance, sd->nr_balance_failed is set to
> such a value that any further can_migrate_task() using said sd will ignore
> the output of task_hot().
> 
> This behaviour makes sense, as active load balance intentionally preempts a
> rq's running task to migrate it right away, but this asynchronous write is
> a bit shoddy, as the stopper thread might run active_load_balance_cpu_stop
> before the sd->nr_balance_failed write either becomes visible to the
> stopper's CPU or even happens on the CPU that appended the stopper work.
> 
> Add a struct lb_env flag to denote active balancing, and use it in
> can_migrate_task(). Remove the sd->nr_balance_failed write that served the
> same purpose. Cleanup the LBF_DST_PINNED active balance special case.
> 
> Signed-off-by: Valentin Schneider <valentin.schneider@....com>

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>

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