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

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Message-ID: <YxnrTdwA493gIHS8@hirez.programming.kicks-ass.net>
Date:   Thu, 8 Sep 2022 15:17:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...e.de>
Cc:     Abel Wu <wuyun.abel@...edance.com>,
        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 11:36:32AM +0100, Mel Gorman wrote:
> 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

Right; only pinned guests typically expose such topology information
(anything else would be quite broken).

> minimal but I still think it's right so;

I'm not convinced; all of select_idle_sibling() seems to use
available_idle_cpu(), and that's the only consumer of
__update_idle_core(), so in that respect the current state makes sense.