linux-kernel - Re: [PATCH] sched/fair: Optimize select_idle

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Date:   Fri, 13 Dec 2019 12:20:03 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     "chengjian (D)" <cj.chengjian@...wei.com>, mingo@...nel.org,
        linux-kernel@...r.kernel.org, chenwandun@...wei.com,
        xiexiuqi@...wei.com, liwei391@...wei.com, huawei.libin@...wei.com,
        bobo.shaobowang@...wei.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org
Subject: Re: [PATCH] sched/fair: Optimize select_idle_cpu

On 13/12/2019 12:09, Peter Zijlstra wrote:
>> Like you said the gains here would probably be small - the highest SMT
>> count I'm aware of is SMT8 (POWER9). Still, if we end up with both
>> select_idle_core() and select_idle_cpu() using that pattern, it would make
>> sense IMO to align select_idle_smt() with those.
> 
> The cpumask_and() operation added would also have cost. I really don't
> see that paying off.
> 
> The other sites have the problem that we combine an iteration limit with
> affinity constraints. This loop doesn't do that and therefore doesn't
> suffer the problem.
> 

select_idle_core() doesn't really have an iteration limit, right? That
being said, yeah, the cpumask_and() for e.g. SMT2 systems would be
mostly wasteful.