linux-kernel - Re: [PATCH 2/2] sched/fair: skip the cache hot CPU in select_idle

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Date:   Wed, 27 Sep 2023 12:11:33 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Chen Yu <yu.c.chen@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>
Cc:     Tim Chen <tim.c.chen@...el.com>, Aaron Lu <aaron.lu@...el.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        linux-kernel@...r.kernel.org, Chen Yu <yu.chen.surf@...il.com>
Subject: Re: [PATCH 2/2] sched/fair: skip the cache hot CPU in
 select_idle_cpu()

On 9/26/23 06:11, Chen Yu wrote:
> Problem statement:
> When task p is woken up, the scheduler leverages select_idle_sibling()
> to find an idle CPU for it. p's previous CPU is usually a preference
> because it can improve cache locality. However in many cases, the
> previous CPU has already been taken by other wakees, thus p has to
> find another idle CPU.
> 
> Proposal:
> Inspired by Mathieu's idea[1], introduce the SIS_CACHE. It considers
> the sleep time of the task for better task placement. Based on the
> task's short sleeping history, keep p's previous CPU idle for a short
> while. Later when p is woken up, it can choose its previous CPU in
> select_idle_sibling(). When p's previous CPU is reserved, another wakee
> is not allowed to choose this CPU in select_idle_idle(). The reservation
> period is set to the task's average short sleep time, AKA, se->sis_rsv_avg.
> 
> This does not break the work conservation of the scheduler, because
> wakee will still try its best to find an idle CPU. The difference is that
> different idle CPUs might have different priorities.
> 
> Prateek pointed out that, with SIS_CACHE enabled, if all idle CPUs are
> cache-hot, select_idle_cpu() might have to choose a non-idle target CPU,
> which brings task stacking. Mitigate this by returning the first cache-hot
> idle CPU if no cache-cold CPU is found.

I've tried your patches on my reference hackbench workload:

./hackbench -g 32 -f 20 --threads --pipe -l 480000 -s 100

Unfortunately they don't appear to help for that specific load.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com