linux-kernel - Re: [PATCH v5] cpuset: Avoid invalidating sibling partitions on cpuset.cpus conflict.

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Message-ID: <unk64xmcj5kt5c5gaauwaeld5qsshaldw7utgzk362w33y3zr7@s765trmj5ccs>
Date: Wed, 19 Nov 2025 14:20:25 +0100
From: Michal Koutný <mkoutny@...e.com>
To: Sun Shaojie <sunshaojie@...inos.cn>
Cc: llong@...hat.com, chenridong@...weicloud.com, cgroups@...r.kernel.org, 
	hannes@...xchg.org, linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	shuah@...nel.org, tj@...nel.org
Subject: Re: [PATCH v5] cpuset: Avoid invalidating sibling partitions on
 cpuset.cpus conflict.

On Wed, Nov 19, 2025 at 06:57:49PM +0800, Sun Shaojie <sunshaojie@...inos.cn> wrote:
> Currently, when setting a cpuset's cpuset.cpus to a value that conflicts
> with its sibling partition, the sibling's partition state becomes invalid.
> However, this invalidation is often unnecessary. If the cpuset being
> modified is exclusive, it should invalidate itself upon conflict.
> 
> This patch applies only to the following two cases：
> 
> Assume the machine has 4 CPUs (0-3).
> 
>    root cgroup
>       /    \
>     A1      B1
> 
> Case 1: A1 is exclusive, B1 is non-exclusive, set B1's cpuset.cpus
> 
>  Table 1.1: Before applying this patch
>  Step                                       | A1's prstate | B1's prstate |
>  #1> echo "0-1" > A1/cpuset.cpus            | member       | member       |
>  #2> echo "root" > A1/cpuset.cpus.partition | root         | member       |
>  #3> echo "0" > B1/cpuset.cpus              | root invalid | member       |
> 
> After step #3, A1 changes from "root" to "root invalid" because its CPUs
> (0-1) overlap with those requested by B1 (0). However, B1 can actually
> use CPUs 2-3(from B1's parent), so it would be more reasonable for A1 to
> remain as "root."
> 
>  Table 1.2: After applying this patch
>  Step                                       | A1's prstate | B1's prstate |
>  #1> echo "0-1" > A1/cpuset.cpus            | member       | member       |
>  #2> echo "root" > A1/cpuset.cpus.partition | root         | member       |
>  #3> echo "0" > B1/cpuset.cpus              | root         | member       |
> 
> Case 2: Both A1 and B1 are exclusive, set B1's cpuset.cpus

(Thanks for working this out, Shaojie.)

> 
>  Table 2.1: Before applying this patch
>  Step                                       | A1's prstate | B1's prstate |
>  #1> echo "0-1" > A1/cpuset.cpus            | member       | member       |
>  #2> echo "root" > A1/cpuset.cpus.partition | root         | member       |
>  #3> echo "2" > B1/cpuset.cpus              | root         | member       |
>  #4> echo "root" > B1/cpuset.cpus.partition | root         | root         |
>  #5> echo "1-2" > B1/cpuset.cpus            | root invalid | root invalid |
> 
> After step #4, B1 can exclusively use CPU 2. Therefore, at step #5,
> regardless of what conflicting value B1 writes to cpuset.cpus, it will
> always have at least CPU 2 available. This makes it unnecessary to mark
> A1 as "root invalid".
> 
>  Table 2.2: After applying this patch
>  Step                                       | A1's prstate | B1's prstate |
>  #1> echo "0-1" > A1/cpuset.cpus            | member       | member       |
>  #2> echo "root" > A1/cpuset.cpus.partition | root         | member       |
>  #3> echo "2" > B1/cpuset.cpus              | root         | member       |
>  #4> echo "root" > B1/cpuset.cpus.partition | root         | root         |
>  #5> echo "1-2" > B1/cpuset.cpus            | root         | root invalid |
> 
> In summary, regardless of how B1 configures its cpuset.cpus, there will
> always be available CPUs in B1's cpuset.cpus.effective. Therefore, there
> is no need to change A1 from "root" to "root invalid".

Admittedly, I don't like this change because it relies on implicit
preference ordering between siblings (here first comes, first served)
and so the effective config cannot be derived just from the applied
values :-/

Do you actually want to achieve this or is it an implementation
side-effect of the Case 1 scenario that you want to achieve?


Thanks,
Michal

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