linux-kernel - Re: [PATCH 2/3] sched/fair: don't set LBF_ALL

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Date:   Wed, 6 Jan 2021 16:07:11 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...hat.com, juri.lelli@...hat.com, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] sched/fair: don't set LBF_ALL_PINNED unnecessarily

On Wed, Jan 06, 2021 at 02:34:18PM +0100, Vincent Guittot wrote:
> Setting LBF_ALL_PINNED during active load balance is only valid when there
> is only 1 running task on the rq otherwise this ends up increasing the
> balance interval whereas other tasks could migrate after the next interval
> once they become cache-cold as an example.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> ---
>  kernel/sched/fair.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 5428b8723e61..69a455113b10 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9759,7 +9759,8 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>  			if (!cpumask_test_cpu(this_cpu, busiest->curr->cpus_ptr)) {
>  				raw_spin_unlock_irqrestore(&busiest->lock,
>  							    flags);
> -				env.flags |= LBF_ALL_PINNED;
> +				if (busiest->nr_running == 1)
> +					env.flags |= LBF_ALL_PINNED;
>  				goto out_one_pinned;
>  			}

Hmm.. that wasn't the intention. It's entirely possible to have multiple
tasks pinned.

ISTR we set all-pinned and then clear it in can_migrate_task() when we
actually find a task that can be moved to the destination CPU. That's a
construct that works perfectly fine with multiple tasks.