Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Subject: Re: [PATCH v8 4/6] cpuset: Make generate_sched_domains() recognize
 isolated_cpus
To:     Juri Lelli <juri.lelli@redhat.com>
Cc:     Tejun Heo <tj@kernel.org>, Li Zefan <lizefan@huawei.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        kernel-team@fb.com, pjt@google.com, luto@amacapital.net,
        Mike Galbraith <efault@gmx.de>, torvalds@linux-foundation.org,
        Roman Gushchin <guro@fb.com>
References: <1526590545-3350-1-git-send-email-longman@redhat.com>
 <1526590545-3350-5-git-send-email-longman@redhat.com>
 <20180524102837.GA3948@localhost.localdomain>
 <45d70c88-e9f5-716a-ee9a-33dc111159cc@redhat.com>
 <8e610b98-970c-a309-5821-fc8e6aca892f@redhat.com>
 <20180529062703.GA8985@localhost.localdomain>
From:   Waiman Long <longman@redhat.com>
Organization: Red Hat
Message-ID: <8164a41b-3218-c618-64a6-52747344c4db@redhat.com>
Date:   Tue, 29 May 2018 08:40:21 -0400
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.2.0
MIME-Version: 1.0
In-Reply-To: <20180529062703.GA8985@localhost.localdomain>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8BIT
Content-Language: en-US
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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