linux-kernel - Re: [PATCH v6 3/4] sched/fair: Introduce SIS

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Date:   Fri, 21 Oct 2022 12:34:08 +0800
From:   Chen Yu <yu.c.chen@...el.com>
To:     Abel Wu <wuyun.abel@...edance.com>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, Mel Gorman <mgorman@...e.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Valentin Schneider <valentin.schneider@....com>,
        Josh Don <joshdon@...gle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        Aubrey Li <aubrey.li@...el.com>,
        Qais Yousef <qais.yousef@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Rik van Riel <riel@...riel.com>,
        Yicong Yang <yangyicong@...wei.com>,
        Barry Song <21cnbao@...il.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 3/4] sched/fair: Introduce SIS_CORE

On 2022-10-21 at 12:30:56 +0800, Abel Wu wrote:
> Hi Chen, thanks for your reviewing!
> 
> On 10/21/22 12:03 PM, Chen Yu wrote:
> > On 2022-10-19 at 20:28:58 +0800, Abel Wu wrote:
> > [cut]
> > > A major concern is the accuracy of the idle cpumask. A cpu present
> > > in the mask might not be idle any more, which is called the false
> > > positive cpu. Such cpus will negate lots of benefit this feature
> > > brings. The strategy against the false positives will be introduced
> > > in next patch.
> > > 
> > I was thinking that, if patch[3/4] needs [4/4] to fix the false positives,
> > maybe SIS_CORE could be disabled by default in 3/4 but enabled
> > in 4/4? So this might facilicate git bisect in case of any regression
> > check?
> 
> Agreed. Will fix in next version.
> 
> > [cut]
> > > + * To honor the rule of CORE granule update, set this cpu to the LLC idle
> > > + * cpumask only if there is no cpu of this core showed up in the cpumask.
> > > + */
> > > +static void update_idle_cpu(int cpu)
> > > +{
> > > +	struct sched_domain_shared *sds;
> > > +
> > > +	if (!sched_feat(SIS_CORE))
> > > +		return;
> > > +
> > > +	sds = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> > > +	if (sds) {
> > > +		struct cpumask *icpus = to_cpumask(sds->icpus);
> > > +
> > > +		/*
> > > +		 * This is racy against clearing in select_idle_cpu(),
> > > +		 * and can lead to idle cpus miss the chance to be set to
> > > +		 * the idle cpumask, thus the idle cpus are temporarily
> > > +		 * out of reach in SIS domain scan. But it should be rare
> > > +		 * and we still have ILB to kick them working.
> > > +		 */
> > > +		if (!cpumask_intersects(cpu_smt_mask(cpu), icpus))
> > > +			cpumask_set_cpu(cpu, icpus);
> > Maybe I miss something, here we only set one CPU in the icpus, but
> > when we reach update_idle_cpu(), all SMT siblings of 'cpu' are idle,
> > is this intended for 'CORE granule update'?
> 
> The __update_idle_core() is called by all the cpus that need to go idle
> to update has_idle_core if necessary, and update_idle_cpu() is called
> before that check.
>
I see.

Since __update_idle_core() has checked all SMT siblings of 'cpu' if
they are idle, can that information also be updated to icpus?

thanks,
Chenyu 
> Thanks,
> Abel
>