linux-kernel - Re: [RFC PATCH 1/2] sched: Rate limit migrations to 1 per 2ms per task

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Date:   Wed, 13 Sep 2023 11:46:47 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Chen Yu <yu.c.chen@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Swapnil Sapkal <Swapnil.Sapkal@....com>,
        Aaron Lu <aaron.lu@...el.com>,
        Julien Desfossez <jdesfossez@...italocean.com>, x86@...nel.org
Subject: Re: [RFC PATCH 1/2] sched: Rate limit migrations to 1 per 2ms per
 task

On 9/10/23 03:03, Chen Yu wrote:
> On 2023-09-06 at 09:57:04 -0400, Mathieu Desnoyers wrote:
[...]
>>
>> I suspect we could try something like this then:
>>
>> When a cpu enters idle state, it could grab a sched_clock() timestamp
>> and store it into this_rq()->enter_idle_time. Then, when it exits
>> idle and reenters idle again, it could save rq->enter_idle_time to
>> rq->prev_enter_idle_time, and sample enter_idle_time again.
>>
>> When considering the CPU as a target for task migration, if it is
>> idle but the delta between sched_clock_cpu(cpu_of(rq)) and that
>> prev_enter_idle_time is below a threshold (e.g. a few ms), this means
>> the CPU got out of idle and went back to idle pretty quickly, which
>> means it's not a good target for pulling tasks for a short while.
>>
> 
> Do you mean inhit the newidle balance? Currently the newidle balance
> checks if the average idle duration of that rq is below the total cost
> to do a load balance:
>     this_rq->avg_idle < sd->max_newidle_lb_cost

Not quite but..

> 
>> I'll try something along these lines and see how it goes.

anyway this approach did not work based on my testing.

>>
> 
> Consider the sleep time looks like a good idea! What you suggests that
> inspires me that, maybe we could consider the task's sleep duration,
> and decide whether to migrate it or not in the next wakeup.
> 
> Say, if a task p sleeps and woken up quickly, can we reserve its previous
> CPU as idle for a short while? So other tasks can not choose p's previous
> CPU during their wakeup. A short while later, when p is woken up, it finds
> that its previous CPU is still idle and chooses that.
> 
> I created a draft patch based on that, and it shows some improvements on
> a 224 CPUs system. I'll post the draft patch and Cc you.

I think your approach is very promising, let's keep digging into that 
direction.

Thanks,

Mathieu

> 
> thanks,
> Chenyu

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