Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.35 as permitted sender) client-ip=23.128.96.35;
MIME-Version: 1.0
References: <20231027171742.1426070-1-keisuke.nishimura@inria.fr>
In-Reply-To: <20231027171742.1426070-1-keisuke.nishimura@inria.fr>
From:   Vincent Guittot <vincent.guittot@linaro.org>
Date:   Mon, 30 Oct 2023 09:02:40 +0100
Message-ID: <CAKfTPtCsmR13LO9T1ApdyxvXyFSq78RNEC+xmbakK+GNQUVXsA@mail.gmail.com>
Subject: Re: [PATCH] sched/fair: Fix the decision for load balance
To:     Keisuke Nishimura <keisuke.nishimura@inria.fr>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Mel Gorman <mgorman@suse.de>,
        Valentin Schneider <vschneid@redhat.com>,
        Julia Lawall <julia.lawall@inria.fr>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

On Fri, 27 Oct 2023 at 19:25, Keisuke Nishimura
<keisuke.nishimura@inria.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@inria.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@linaro.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
>