linux-kernel - Re: [PATCH v5 1/3] sched/fair: Introduce the burstable CFS controller

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Message-ID: <YKzWKxD+G4wTd2hf@hirez.programming.kicks-ass.net>
Date:   Tue, 25 May 2021 12:49:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     changhuaixin <changhuaixin@...ux.alibaba.com>
Cc:     Benjamin Segall <bsegall@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        dtcccc@...ux.alibaba.com, Juri Lelli <juri.lelli@...hat.com>,
        khlebnikov@...dex-team.ru,
        open list <linux-kernel@...r.kernel.org>,
        Mel Gorman <mgorman@...e.de>, Ingo Molnar <mingo@...hat.com>,
        Odin Ugedal <odin@...d.al>, Odin Ugedal <odin@...dal.com>,
        pauld@...head.com, Paul Turner <pjt@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Shanpei Chen <shanpeic@...ux.alibaba.com>,
        Tejun Heo <tj@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        xiyou.wangcong@...il.com
Subject: Re: [PATCH v5 1/3] sched/fair: Introduce the burstable CFS controller

On Mon, May 24, 2021 at 08:42:03PM +0800, changhuaixin wrote:

> >> --- a/kernel/sysctl.c
> >> +++ b/kernel/sysctl.c
> >> @@ -1816,6 +1816,15 @@ static struct ctl_table kern_table[] = {
> >> 		.proc_handler	= proc_dointvec_minmax,
> >> 		.extra1		= SYSCTL_ONE,
> >> 	},
> >> +	{
> >> +		.procname	= "sched_cfs_bw_burst_enabled",
> >> +		.data		= &sysctl_sched_cfs_bw_burst_enabled,
> >> +		.maxlen		= sizeof(unsigned int),
> >> +		.mode		= 0644,
> >> +		.proc_handler	= proc_dointvec_minmax,
> >> +		.extra1		= SYSCTL_ZERO,
> >> +		.extra2		= SYSCTL_ONE,
> >> +	},
> >> #endif
> >> #if defined(CONFIG_ENERGY_MODEL) && defined(CONFIG_CPU_FREQ_GOV_SCHEDUTIL)
> >> 	{
> > 
> > What's the purpose of this new sysctl? By default it is disabled because
> > burst==0, only if you set burst to some !0 value does this actually do
> > anything.
> 
> Originally, this is designed to turn burst feature off when the system becomes unstable.
> Guess we can remove this as you have questioned it.

Is stability a concern? This is CFS after all, if we overload, we simply
share time as per usual.

If there is a real use-case for a global knob to limit/disable this I
don't object too much, but then please explicitly mention it.