Date:   Fri, 16 Jul 2021 14:59:44 -0400
From:   Waiman Long <llong@...hat.com>
To:     Waiman Long <llong@...hat.com>, Tejun Heo <tj@...nel.org>
Cc:     Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <shuah@...nel.org>, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kselftest@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>, Phil Auld <pauld@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [PATCH v2 2/6] cgroup/cpuset: Clarify the use of invalid
 partition root

On 7/16/21 2:44 PM, Waiman Long wrote:
> On 7/5/21 1:51 PM, Tejun Heo wrote:
>> Hello, Waiman.
>>
>> On Mon, Jun 28, 2021 at 09:06:50AM -0400, Waiman Long wrote:
>>> 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.
>> I have a difficult time convincing myself that this difference 
>> justifies the
>> behavior difference and it keeps bothering me that there is a state 
>> which
>> can be reached through one path but rejected by the other. I'll continue
>> below.
>>
>>> 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.
>> So, IMO, one of the reasons why cgroup1 interface was such a mess was
>> because each piece of interaction was designed ad-hoc without regard 
>> to the
>> overall consistency. One person feels a particular way of interacting 
>> with
>> the interface is "correct" and does it that way and another person does
>> another part in a different way. In the end, we ended up with a messy
>> patchwork.
>>
>> One problematic aspect of cpuset in cgroup1 was the handling of failure
>> modes, which was caused by the same exact approach - we wanted the 
>> interface
>> to reject invalid configurations outright even though we didn't have the
>> ability to prevent those configurations from occurring through other 
>> paths,
>> which makes the failure mode more subtle by further obscuring them.
>>
>> I think a better approach would be having a clear signal and 
>> mechanism to
>> watch the state and explicitly requiring users to verify and monitor the
>> state transitions.
>
> Sorry for the late reply as I was busy with other works.
>
> I agree with you on principle. However, the reason why there are more 
> restrictions on enabling partition is because I want to avoid forcing 
> the users to always read back cpuset.partition.type to see if the 
> operation succeeds instead of just getting an error from the 
> operation. The former approach is more error prone. If you don't want 
> changes in existing behavior, I can relax the checking and allow them 
> to become an invalid partition if an illegal operation happens.
>
> Also there is now another cpuset patch to extend cpu isolation to 
> cgroup v1 [1]. I think it is better suit to the cgroup v2 partition 
> scheme, but cgroup v1 is still quite heavily out there.
>
> Please let me know what you want me to do and I will send out a v3 
> version. 

Note that the current cpuset partition implementation have implemented 
some restrictions on when a partition can be enabled. However, I missed 
some corner cases in the original implementation that allow certain 
cpuset operations to make a partition invalid. I tried to plug those 
holes in this patchset. However, if maintaining backward compatibility 
is more important, I can leave those holes and update the documentation 
to make sure that people check cpuset.partition.type to confirm if their 
operation succeeds.

Cheers,
Longman

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