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From:   Waiman Long <llong@redhat.com>
Subject: Re: [PATCH v2 2/6] cgroup/cpuset: Clarify the use of invalid
 partition root
To:     Tejun Heo <tj@kernel.org>
Cc:     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>
References: <20210621184924.27493-1-longman@redhat.com>
 <20210621184924.27493-3-longman@redhat.com>
 <YNcHOe3o//pIiByh@mtj.duckdns.org>
Message-ID: <6ea1ac38-73e1-3f78-a5d2-a4c23bcd8dd1@redhat.com>
Date:   Mon, 28 Jun 2021 09:06:50 -0400
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On 6/26/21 6:53 AM, Tejun Heo wrote:
> Hello, Waiman.
>
> On Mon, Jun 21, 2021 at 02:49:20PM -0400, Waiman Long wrote:
>>   1) A partition root can't be changed to member if it has child partition
>>      roots.
>>   2) Removing CPUs from cpuset.cpus that causes it to become invalid is
>>      not allowed.
> I'm not a fan of this approach. No matter what we have to be able to handle
> CPU removals which are user-iniated operations anyway, so I don't see why
> we're adding a different way of handling a different set of operations. Just
> handle them the same?

The main reason for doing this is because normal cpuset control file 
actions are under the direct control of the cpuset code. So it is up to 
us to decide whether to grant it or deny it. Hotplug, on the other hand, 
is not under the control of cpuset code. It can't deny a hotplug 
operation. This is the main reason why the partition root error state 
was added in the first place.

Normally, users can set cpuset.cpus to whatever value they want even 
though they are not actually granted. However, turning on partition root 
is under more strict control. You can't turn on partition root if the 
CPUs requested cannot actually be granted. The problem with setting the 
state to just partition error is that users may not be aware that the 
partition creation operation fails.  We can't assume all users will do 
the proper error checking. I would rather let them know the operation 
fails rather than relying on them doing the proper check afterward.

Yes, I agree that it is a different philosophy than the original cpuset 
code, but I thought one reason of doing cgroup v2 is to simplify the 
interface and make it a bit more erorr-proof. Since partition root 
creation is a relatively rare operation, we can afford to make it more 
strict than the other operations.

Cheers,
Longman