linux-kernel - Re: [PATCH v6 4/4] sched/fair: Don't double balance_interval for migrate

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Date: Fri, 23 Feb 2024 10:32:13 +0100
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Qais Yousef <qyousef@...alina.io>
Cc: Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Dietmar Eggemann <dietmar.eggemann@....com>, linux-kernel@...r.kernel.org, 
	Pierre Gondois <Pierre.Gondois@....com>
Subject: Re: [PATCH v6 4/4] sched/fair: Don't double balance_interval for migrate_misfit

On Tue, 20 Feb 2024 at 23:56, Qais Yousef <qyousef@...alina.io> wrote:
>
> It is not necessarily an indication of the system being busy and
> requires a backoff of the load balancer activities. But pushing it high
> could mean generally delaying other misfit activities or other type of
> imbalances.
>
> Also don't pollute nr_balance_failed because of misfit failures. The
> value is used for enabling cache hot migration and in migrate_util/load
> types. None of which should be impacted (skewed) by misfit failures.
>
> Signed-off-by: Qais Yousef <qyousef@...alina.io>

Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>

> ---
>  kernel/sched/fair.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 20006fcf7df2..4c1235a5dd60 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11467,8 +11467,12 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>                  * We do not want newidle balance, which can be very
>                  * frequent, pollute the failure counter causing
>                  * excessive cache_hot migrations and active balances.
> +                *
> +                * Similarly for migration_misfit which is not related to
> +                * load/util migration, don't pollute nr_balance_failed.
>                  */
> -               if (idle != CPU_NEWLY_IDLE)
> +               if (idle != CPU_NEWLY_IDLE &&
> +                   env.migration_type != migrate_misfit)
>                         sd->nr_balance_failed++;
>
>                 if (need_active_balance(&env)) {
> @@ -11551,8 +11555,13 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>          * repeatedly reach this code, which would lead to balance_interval
>          * skyrocketing in a short amount of time. Skip the balance_interval
>          * increase logic to avoid that.
> +        *
> +        * Similarly misfit migration which is not necessarily an indication of
> +        * the system being busy and requires lb to backoff to let it settle
> +        * down.
>          */
> -       if (env.idle == CPU_NEWLY_IDLE)
> +       if (env.idle == CPU_NEWLY_IDLE ||
> +           env.migration_type == migrate_misfit)
>                 goto out;
>
>         /* tune up the balancing interval */
> --
> 2.34.1
>