linux-kernel - Re: [PATCH] sched/fair: Fix misuse of available_idle

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Date:   Thu, 8 Sep 2022 11:36:32 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Abel Wu <wuyun.abel@...edance.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Rohit Jain <rohit.k.jain@...cle.com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Fix misuse of available_idle_cpu()

On Thu, Sep 08, 2022 at 04:07:02PM +0800, Abel Wu wrote:
> The function available_idle_cpu() was introduced to distinguish
> between the code paths that cares if the vCPU is preempted and
> the ones don't care. In general, available_idle_cpu() is used in
> selecting cpus for immediate use, e.g. ttwu. While idle_cpu() is
> used in the paths that only cares about the cpu is idle or not,
> and __update_idle_core() is one of them.
> 
> Use idle_cpu() instead in the idle path to make has_idle_core
> a better hint.
> 
> Fixes: 943d355d7fee (sched/core: Distinguish between idle_cpu() calls based on desired effect, introduce available_idle_cpu())
> Signed-off-by: Abel Wu <wuyun.abel@...edance.com>

Seems fair. As vCPU preemption is specific to virtualisation, it is very
unlikely that SMT is exposed to the guest so the impact of the patch is
minimal but I still think it's right so;

Acked-by: Mel Gorman <mgorman@...e.de>

-- 
Mel Gorman
SUSE Labs