linux-kernel - Re: [PATCH v3] sched/fair: Fix the decision for load balance

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Message-ID: <20231031153928.GD15024@noisy.programming.kicks-ass.net>
Date:   Tue, 31 Oct 2023 16:39:28 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Keisuke Nishimura <keisuke.nishimura@...ia.fr>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org, Chen Yu <yu.c.chen@...el.com>,
        Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Mel Gorman <mgorman@...e.de>,
        Valentin Schneider <vschneid@...hat.com>,
        Julia Lawall <julia.lawall@...ia.fr>
Subject: Re: [PATCH v3] sched/fair: Fix the decision for load balance

On Tue, Oct 31, 2023 at 02:38:22PM +0100, Keisuke Nishimura 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>
> Reviewed-by: Chen Yu <yu.c.chen@...el.com>
> Reviewed-by: Shrikanth Hegde <sshegde@...ux.vnet.ibm.com>
> Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>

I'll sit on this until after the merge window, but then I'll queue it
for sched/urgent.

Thanks!