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

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Date:   Wed, 06 Sep 2023 13:51:10 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        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 Wed, 2023-09-06 at 11:47 +0200, Peter Zijlstra wrote:
> On Tue, Sep 05, 2023 at 03:44:57PM -0700, Tim Chen wrote:
> 
> > Reading up on sched_clock() documentation and seems like it should 
> > indeed be monotonic.
> 
> It tries very hard to be monotonic but cannot guarantee. The moment TSC
> is found unstable it's too late to fix up everything.
> 

Yes, if TSC becomes unstable and could cause sched_clock to reset and go way backward.
Perhaps we can add the following check in Mathieu's original
patch to fix things up:

+static bool should_migrate_task(struct task_struct *p, int prev_cpu)
> +{
	/* sched_clock reset causing next migration time to be too far ahead */
	if (p->se.next_migration_time > sched_clock_cpu(prev_cpu) + SCHED_MIGRATION_RATELIMIT_WINDOW)
		p->se.next_migration_time = sched_clock_cpu(prev_cpu) + SCHED_MIGRATION_RATELIMIT_WINDOW;

> +	/* Rate limit task migration. */
> +	if (sched_clock_cpu(prev_cpu) < p->se.next_migration_time)
> +	       return false;
> +	return true;
> +}
> +

Tim