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

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Date:   Tue, 29 May 2018 08:40:21 -0400
From:   Waiman Long <longman@...hat.com>
To:     Juri Lelli <juri.lelli@...hat.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>
Subject: Re: [PATCH v8 4/6] cpuset: Make generate_sched_domains() recognize
 isolated_cpus

On 05/29/2018 02:27 AM, Juri Lelli wrote:
> On 28/05/18 21:24, Waiman Long wrote:
>> On 05/28/2018 09:12 PM, Waiman Long wrote:
>>> On 05/24/2018 06:28 AM, Juri Lelli wrote:
>>>> On 17/05/18 16:55, Waiman Long wrote:
>>>>
>>>> [...]
>>>>
>>>>> @@ -849,7 +860,12 @@ static void rebuild_sched_domains_locked(void)
>>>>>  	 * passing doms with offlined cpu to partition_sched_domains().
>>>>>  	 * Anyways, hotplug work item will rebuild sched domains.
>>>>>  	 */
>>>>> -	if (!cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
>>>>> +	if (!top_cpuset.isolation_count &&
>>>>> +	    !cpumask_equal(top_cpuset.effective_cpus, cpu_active_mask))
>>>>> +		goto out;
>>>>> +
>>>>> +	if (top_cpuset.isolation_count &&
>>>>> +	   !cpumask_subset(top_cpuset.effective_cpus, cpu_active_mask))
>>>>>  		goto out;
>>>> Do we cover the case in which hotplug removed one of the isolated cpus
>>>> from cpu_active_mask?
>>> Yes, you are right. That is the remnant of my original patch that allows
>>> only one isolated_cpus at root. Thanks for spotting that.
>> I am sorry. I would like to take it back my previous comment. The code
>> above looks for inconsistency in the state of the effective_cpus mask to
>> find out if it is racing with a hotplug event. If it is, we can skip the
>> domain generation as the hotplug event will do that too. The checks are
>> still valid with the current patchset. So I don't think we need to make
>> any change here.
> Yes, these checks are valid, but don't we also need to check for hotplug
> races w.r.t. isolated CPUs (of some other sub domain)?

It is not actually a race. Both the hotplug event and any changes to cpu
lists or flags are serialized by the cpuset_mutex. It is just that we
may be doing the same work twice that we are wasting cpu cycles. So we
are doing a quick check to avoid this. The check isn't exhaustive and we
can certainly miss some cases. Doing a more throughout check may need as
much time as doing the sched domain generation itself and so you are
actually wasting more CPU cycles on average as the chance of a hotplug
event is very low.

Cheers,
Longman