linux-kernel - Re: [PATCH v1] cpuset: Avoid unnecessary partition invalidation

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Message-ID: <85f438b2-5131-4794-bb2d-09ca611fb246@redhat.com>
Date: Wed, 12 Nov 2025 13:05:15 -0500
From: Waiman Long <llong@...hat.com>
To: Sun Shaojie <sunshaojie@...inos.cn>, chenridong@...weicloud.com
Cc: tj@...nel.org, hannes@...xchg.org, mkoutny@...e.com, shuah@...nel.org,
 cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v1] cpuset: Avoid unnecessary partition invalidation

On 11/12/25 4:46 AM, Sun Shaojie wrote:
> Hi Ridong,
>
> Thank you for your response.
>
>  From your reply "in case 1, A1 can also be converted to a partition," I
> realize there might be a misunderstanding. The scenario I'm addressing
> involves two sibling cgroups where one is an effective partition root and
> the other is not, and both have empty cpuset.cpus.exclusive. Let me
> explain the intention behind case 1 in detail, which will also illustrate
> why this has negative impacts on our product.
>
> In case 1, after #3 completes, A1 is already a valid partition root - this
> is correct.After #4, B1 was generated, and B1 is no-exclusive. After #5,
> A1 changes from "root" to "root invalid". But A1 becoming "root invalid"
> could be unnecessary because having A1 remain as "root" might be more
> acceptable. Here's the analysis:
>
> As documented in cgroup-v2.rst regarding cpuset.cpus: "The actual list of
> CPUs to be granted, however, is subjected to constraints imposed by its
> parent and can differ from the requested CPUs". This means that although
> we're requesting CPUs 0-3 for B1, we can accept that the actual available
> CPUs in B1 might not be 0-3.
>
> Based on this characteristic, in our product's implementation for case 1,
> before writing to B1's cpuset.cpus in #5, we check B1's parent
> cpuset.cpus.effective and know that the CPUs available for B1 don't include
> 0-1 (since 0-1 are exclusively used by A1). However, we still want to set
> B1's cpuset.cpus to 0-3 because we hope that when 0-1 become available in
> the future, B1 can use them without affecting the normal operation of other
> cgroups.
>
> The reality is that because B1's requested cpuset.cpus (0-3) conflicts with
> A1's exclusive CPUs (0-1) at that moment, it destroys the validity of A1's
> partition root. So why must the current rule sacrifice A1's validity to
> accommodate B1's CPU request? In this situation, B1 can clearly use 2-3
> while A1 exclusively uses 0-1 - they don't need to conflict.
>
> This patch narrows the exclusivity conflict check scope to only between
> partitions. Moreover, user-specified CPUs (including cpuset.cpus and
> cpuset.cpus.exclusive) only have true exclusive meaning within effective
> partitions. So why should the current rule perform exclusivity conflict
> checks between an exclusive partition and a non-exclusive member? This is
> clearly unnecessary.

As I have said in the other thread, v2 exclusive cpuset checking follows 
the v1 rule. However, the behavior of setting cpuset.cpus differs 
between v1 and v2. In v1, setting cpuset.cpus can fail if there is some 
conflict. In v2, users are allow to set whatever value they want without 
failure, but the effective CPUs granted will be subjected to constraints 
and differ from cpuset.cpus. So in that sense, I think it makes sense to 
relax the exclusive cpuset check for v2, but we still need to keep the 
current v1 behavior. Please update your patch to do that.

Cheers,
Longman