linux-kernel - Re: [PATCH v9 6/7] cgroup/cpuset: Update description of cpuset.cpus.partition in cgroup-v2.rst

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Message-ID: <Ybe0YWEo7Wp7wib9@slm.duckdns.org>
Date:   Mon, 13 Dec 2021 11:00:17 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Waiman Long <longman@...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 v9 6/7] cgroup/cpuset: Update description of
 cpuset.cpus.partition in cgroup-v2.rst

Hello,

On Sun, Dec 05, 2021 at 01:32:19PM -0500, Waiman Long wrote:
> +	In the case of an invalid partition root, a descriptive string on
> +	why the partition is invalid is included within parentheses.
> +
> +	Almost all possible state transitions among "member", valid
> +	and invalid partition roots are allowed except from "member"
> +	to invalid partition root.

So, this part still bothers me for the following two reasons that I brought
up earlier:

* When a valid partition turns invalid, now we have a reliable way of
  discovering what exactly caused the transition. However, when a user now
  fails to turn a member into partition, all they get is -EINVAL and there's
  no way to discover why it failed and the failure conditions that -EINVAL
  represents aren't simple.

* In an automated configuration scenarios, this operation mode may be
  difficult to make reliable and lead to sporadic failures which can be
  tricky to track down. The core problem is that whether a given operation
  succeeds or not may depend on external states (CPU on/offline) which may
  change asynchronously in a way that the configuring entity doesn't have
  any control over.

It's true that both are existing problems with the current partition
interface and given that this is a pretty spcialized feature, this can be
okay. Michal, what are your thoughts?

Thanks.

-- 
tejun