linux-kernel - Re: [PATCH v4 2/6] cgroup/cpuset: Properly handle partition root tree

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Date:   Thu, 12 Aug 2021 18:56:00 -0400
From:   Waiman Long <llong@...hat.com>
To:     Tejun Heo <tj@...nel.org>, Waiman Long <llong@...hat.com>
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>,
        Frederic Weisbecker <frederic@...nel.org>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        Michal Koutný <mkoutny@...e.com>
Subject: Re: [PATCH v4 2/6] cgroup/cpuset: Properly handle partition root tree

On 8/12/21 6:18 PM, Tejun Heo wrote:
> Hello,
>
> On Wed, Aug 11, 2021 at 03:27:20PM -0400, Waiman Long wrote:
>> Disabling partition at the parent level does invalidate all the child
>> partitions under it. So it must be done with care when we disable a
>> partition.
>>
>> How about we give some indication that a child partition exist when reading
>> cpuset.cpus.partition and recommend double-checking it before disabling a
>> partition? For example, we keep track of the number of cpus delegated to
>> child partitions. Perhaps we can list that information on read.
>>
>> With that information available, I have no objection to allow disabling a
>> parent partition with child partitions under it.
> This is a general problem which has always existed regardless of whether the
> errors are synchronous or not. There are many different reasons that a write
> to a cpuset interface file could fail and it has never been easy to tell why
> a given operation was rejected. Making error notifications asynchronous
> doesn't really change anything fundamental although it does make the
> situation a bit more opaque.
>
> I'm all for improving visibility. Now that we can consolidate most error
> states into a unified failure state, this might actually be easier to do.
> IOW, we now just have to explain why a given cgroup is in an invalid state
> rather than additionally having to explain why a given write has been
> rejected, which is pretty awkward to do as those failures are transient and
> local to the writer.
>
> So, if you wanna tackle this, let's do it right and provide something
> comprehensive rather than explaining just one failure.

That sounds reasonable. My current idea is to add invalid partition 
reason string to cpuset. So when users read cpuset.cpus.partition of an 
invalid partition, it will read something like "root invalid  (<reason>)".

What do you think?

Cheers,
Longman