Message-ID: <ec6e2b89-385a-fcc7-7cfa-7e9119fc34bc@redhat.com>
Date:   Wed, 1 Dec 2021 09:56:21 -0500
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Subject: Re: [PATCH v8 5/6] cgroup/cpuset: Update description of
 cpuset.cpus.partition in cgroup-v2.rst
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To:     =?UTF-8?Q?Michal_Koutn=c3=bd?= <mkoutny@suse.com>
Cc:     Tejun Heo <tj@kernel.org>, Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Shuah Khan <shuah@kernel.org>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kselftest@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>, Phil Auld <pauld@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Marcelo Tosatti <mtosatti@redhat.com>
References: <20211018143619.205065-1-longman@redhat.com>
 <20211018143619.205065-6-longman@redhat.com>
 <20211115193122.GA16798@blackbody.suse.cz>
 <8f68692b-bd8f-33fd-44ae-f6f83bf2dc00@redhat.com>
 <20211116175411.GA50019@blackbody.suse.cz>
 <293d7abf-aff6-fcd8-c999-b1dbda1cffb8@redhat.com>
 <YaZbXArNIMNvwJD/@slm.duckdns.org>
 <2347fe66-dc68-6d58-e63b-7ed2b8077b48@redhat.com>
 <20211201141350.GA54766@blackbody.suse.cz>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <20211201141350.GA54766@blackbody.suse.cz>
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On 12/1/21 09:13, Michal Koutný wrote:
> On Tue, Nov 30, 2021 at 10:56:34PM -0500, Waiman Long <longman@redhat.com> wrote:
>>>>       A valid parent partition may distribute out all its CPUs to
>>>>       its child partitions as long as it is not the root cgroup and
>>>>       there is no task associated with it.
>>> A valid parent partition which isn't root never has tasks in them to begin
>>> with.
>> I believe there is some corner cases where it is possible to put task in an
>> intermediate partition. That is why I put down this statement.
> Just mind the threads -- cpuset controller is threaded and having tasks
> in inner cgroup nodes is a real scenario. I wouldn't consider it a
> corner case.
>
> [ Actually, the paragraph could IMO be simplified:
Right, I shouldn't say corner cases. Having task in an intermediate 
partition is possible depending on event sequence. I am aware that there 
are code in the cpuset code to prevent that, but it didn't block all cases.
>> A valid parent partition may distribute out all its CPUs to
>>   its child partitions as long as there is no task associated with it.
> Assuming there's always at least one kernel thread in the root cgroup
> that can't be migrated anyway.]

I am aware of that. That is why I said root cgroup must have at least 
one cpu in its "cpuset.cpus.effective".

Cheers,
Longman