linux-kernel - Re: [PATCH 06/10] sched/fair: Clear the target CPU from the cpumask of CPUs searched

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Message-ID: <20201204113030.GZ3371@techsingularity.net>
Date:   Fri, 4 Dec 2020 11:30:30 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Ziljstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Linux-ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 06/10] sched/fair: Clear the target CPU from the cpumask
 of CPUs searched

On Fri, Dec 04, 2020 at 11:56:36AM +0100, Vincent Guittot wrote:
> > The intent was that the sibling might still be an idle candidate. In
> > the current draft of the series, I do not even clear this so that the
> > SMT sibling is considered as an idle candidate. The reasoning is that if
> > there are no idle cores then an SMT sibling of the target is as good an
> > idle CPU to select as any.
> 
> Isn't the purpose of select_idle_smt ?
> 

Only in part.

> select_idle_core() looks for an idle core and opportunistically saves
> an idle CPU candidate to skip select_idle_cpu. In this case this is
> useless loops for select_idle_core() because we are sure that the core
> is not idle
> 

If select_idle_core() finds an idle candidate other than the sibling,
it'll use it if there is no idle core -- it picks a busy sibling based
on a linear walk of the cpumask. Similarly, select_idle_cpu() is not
guaranteed to scan the sibling first (ordering) or even reach the sibling
(throttling). select_idle_smt() is a last-ditch effort.

-- 
Mel Gorman
SUSE Labs