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

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Message-ID: <03be9c79-bc59-cd4a-869b-ed4c85c61224@bytedance.com>
Date:   Fri, 21 Oct 2022 17:35:06 +0800
From:   Abel Wu <wuyun.abel@...edance.com>
To:     Chen Yu <yu.c.chen@...el.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 10/21/22 12:34 PM, Chen Yu wrote:
> 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?

I think this will simply fallback to the original per-cpu proposal and
lose the opportunity to spread tasks to different cores.