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

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Date:   Fri, 21 Oct 2022 12:03:08 +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 2022-10-19 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?
[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'?


thanks,
Chenyu