linux-kernel - Re: [PATCH v8 4/6] cpuset: Make generate_sched_domains() recognize isolated

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Message-ID: <bf4cb72b-9ff3-eb8b-ca2c-f6c4fee5c123@redhat.com>
Date:   Wed, 23 May 2018 16:18:53 -0400
From:   Waiman Long <longman@...hat.com>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        kernel-team@...com, pjt@...gle.com, luto@...capital.net,
        Mike Galbraith <efault@....de>, torvalds@...ux-foundation.org,
        Roman Gushchin <guro@...com>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH v8 4/6] cpuset: Make generate_sched_domains() recognize
 isolated_cpus

On 05/23/2018 01:34 PM, Patrick Bellasi wrote:
> Hi Waiman,
>
> On 17-May 16:55, Waiman Long wrote:
>
> [...]
>
>> @@ -672,13 +672,14 @@ static int generate_sched_domains(cpumask_var_t **domains,
>>  	int ndoms = 0;		/* number of sched domains in result */
>>  	int nslot;		/* next empty doms[] struct cpumask slot */
>>  	struct cgroup_subsys_state *pos_css;
>> +	bool root_load_balance = is_sched_load_balance(&top_cpuset);
>>  
>>  	doms = NULL;
>>  	dattr = NULL;
>>  	csa = NULL;
>>  
>>  	/* Special case for the 99% of systems with one, full, sched domain */
>> -	if (is_sched_load_balance(&top_cpuset)) {
>> +	if (root_load_balance && !top_cpuset.isolation_count) {
> Perhaps I'm missing something but, it seems to me that, when the two
> conditions above are true, then we are going to destroy and rebuild
> the exact same scheduling domains.
>
> IOW, on 99% of systems where:
>
>    is_sched_load_balance(&top_cpuset)
>    top_cpuset.isolation_count = 0
>
> since boot time and forever, then every time we update a value for
> cpuset.cpus we keep rebuilding the same SDs.
>
> It's not strictly related to this patch, the same already happens in
> mainline based just on the first condition, but since you are extending
> that optimization, perhaps you can tell me where I'm possibly wrong or
> which cases I'm not considering.
>
> I'm interested mainly because on Android systems those conditions
> are always true and we see SDs rebuilds every time we write
> something in cpuset.cpus, which ultimately accounts for almost all the
> 6-7[ms] time required for the write to return, depending on the CPU
> frequency.
>
> Cheers Patrick
>
Yes, that is true. I will look into how to further optimize this. Thanks
for the suggestion.

-Longman