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

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Date:   Tue, 12 Sep 2023 11:18:33 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Chen Yu <yu.c.chen@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        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
Subject: Re: [RFC PATCH 2/2] sched/fair: skip the cache hot CPU in
 select_idle_cpu()

On 9/12/23 10:14, Chen Yu wrote:
> On 2023-09-12 at 10:06:27 -0400, Mathieu Desnoyers wrote:
[...]
>>
>> One more tweak: given that more than one task can update the cache_hot_timeout forward
>> one after another, and given that some tasks have larger burst_sleep_avg values than
>> others, I suspect we want to keep the forward movement monotonic with something like:
>>
>> if (sched_feat(SIS_CACHE) && task_sleep && !rq->nr_running && p->se.burst_sleep_avg &&
>>      rq->cache_hot_timeout < now + p->se.burst_sleep_avg)
>> 	rq->cache_hot_timeout = now + p->se.burst_sleep_avg;
>>
> 
> Yeah, Aaron has mentioned this too:
> https://lore.kernel.org/lkml/ZP7SYu+gxlc%2FYjHu@chenyu5-mobl2/
> May I know the benefit of keeping forward movement monotonic?
> I thought that, should we only honor the latest dequeued task's burst_sleep_avg?
> Because we don't know whether the old deuqued task's cache has been scribbled by the latest
> dequeued task or not, does it still make sense to wake up the old dequeued task on its
> previous CPU?

Here is my reasoning:

If a second task is scheduled after the first dequeued task (a
task with large burst_sleep_avg) is dequeued, that second task (with
small burst_sleep_avg) would need to entirely scribble the other task's
cache lines within the time given by sysctl_sched_migration_cost, which
I suspect is typically not very large. So I doubt that the second task
can entirely kick out the first task cache lines within that time frame,
and therefore that second task should not move the cache_hot_timeout
value backwards.

But perhaps I'm missing something ?

Thanks,

Mathieu


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