linux-kernel - Re: [PATCH] sched/fair: Fix the decision for load balance

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Date:   Mon, 30 Oct 2023 09:02:40 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Keisuke Nishimura <keisuke.nishimura@...ia.fr>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Mel Gorman <mgorman@...e.de>,
        Valentin Schneider <vschneid@...hat.com>,
        Julia Lawall <julia.lawall@...ia.fr>
Subject: Re: [PATCH] sched/fair: Fix the decision for load balance

On Fri, 27 Oct 2023 at 19:25, Keisuke Nishimura
<keisuke.nishimura@...ia.fr> wrote:
>
> should_we_balance is called for the decision to do load-balancing.
> When sched ticks invoke this function, only one CPU should return
> true. However, in the current code, two CPUs can return true. The
> following situation, where b means busy and i means idle, is an
> example because CPU 0 and CPU 2 return true.
>
>         [0, 1] [2, 3]
>          b  b   i  b
>
> This fix checks if there exists an idle CPU with busy sibling(s)
> after looking for a CPU on an idle core. If some idle CPUs with busy
> siblings are found, just the first one should do load-balancing.
>
> Fixes: b1bfeab9b002 ("sched/fair: Consider the idle state of the whole core for load balance")
> Signed-off-by: Keisuke Nishimura <keisuke.nishimura@...ia.fr>
> ---
>  kernel/sched/fair.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 2048138ce54b..eff0316d6c7d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11083,8 +11083,9 @@ static int should_we_balance(struct lb_env *env)
>                 return cpu == env->dst_cpu;
>         }
>
> -       if (idle_smt == env->dst_cpu)
> -               return true;
> +       /* Is there an idle CPU with busy siblings? */

Nit. I agree with Shrikanth that we should keep using similar comment :

/* Are we the first idle CPU with busy siblings */

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

> +       if (idle_smt != -1)
> +               return idle_smt == env->dst_cpu;
>
>         /* Are we the first CPU of this group ? */
>         return group_balance_cpu(sg) == env->dst_cpu;
> --
> 2.34.1
>