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

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Message-ID: <20230906084455.GD38741@noisy.programming.kicks-ass.net>
Date:   Wed, 6 Sep 2023 10:44:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Tim Chen <tim.c.chen@...ux.intel.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 Tue, Sep 05, 2023 at 05:16:25PM -0400, Mathieu Desnoyers wrote:
> On 9/5/23 16:28, Tim Chen wrote:
> > On Tue, 2023-09-05 at 13:11 -0400, Mathieu Desnoyers wrote:
> > > Rate limit migrations to 1 migration per 2 milliseconds per task. On a
> > > kernel with EEVDF scheduler (commit b97d64c722598ffed42ece814a2cb791336c6679),
> > > this speeds up hackbench from 62s to 45s on AMD EPYC 192-core (over 2 sockets).
> > > 
> > > 
> > > 
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index 479db611f46e..0d294fce261d 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -4510,6 +4510,7 @@ static void __sched_fork(unsigned long clone_flags, struct task_struct *p)
> > >   	p->se.vruntime			= 0;
> > >   	p->se.vlag			= 0;
> > >   	p->se.slice			= sysctl_sched_base_slice;
> > > +	p->se.next_migration_time	= 0;
> > 
> > It seems like the next_migration_time should be initialized to the current time,
> > in case the system run for a long time and clock wrap around could cause problem.
> 
> next_migration_time is a u64, which should "never" overflow. Other scheduler
> code comparing with sched_clock() don't appear to care about u64 overflow.

Much code actually considers overflow. We also have monotonicity filters
where it really matters.