linux-kernel - Re: [PATCH cgroup/for-6.20 v4 4/5] cgroup/cpuset: Don't invalidate sibling partitions on cpuset.cpus conflict

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Message-ID: <86b578f3-70f5-4a72-9371-e35478ec1c01@redhat.com>
Date: Mon, 12 Jan 2026 11:02:28 -0500
From: Waiman Long <llong@...hat.com>
To: Waiman Long <llong@...hat.com>, Michal Koutný
 <mkoutny@...e.com>
Cc: Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
 Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>,
 linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
 linux-kselftest@...r.kernel.org, linux-doc@...r.kernel.org,
 Sun Shaojie <sunshaojie@...inos.cn>, Chen Ridong
 <chenridong@...weicloud.com>, Chen Ridong <chenridong@...wei.com>
Subject: Re: [PATCH cgroup/for-6.20 v4 4/5] cgroup/cpuset: Don't invalidate
 sibling partitions on cpuset.cpus conflict

On 1/12/26 10:15 AM, Waiman Long wrote:
> On 1/12/26 10:08 AM, Michal Koutný wrote:
>> On Mon, Jan 12, 2026 at 09:51:28AM -0500, Waiman Long 
>> <llong@...hat.com> wrote:
>>> Sorry, I might have missed this comment of yours. The
>>> "cpuset.cpus.exclusive" file lists all the CPUs that can be granted 
>>> to its
>>> children as exclusive CPUs. The cgroup root is an implicit partition 
>>> root
>>> where all its CPUs can be granted to its children whether they are 
>>> online or
>>> offline. "cpuset.cpus.effective" OTOH ignores the offline CPUs as 
>>> well as
>>> exclusive CPUs that have been passed down to existing descendant 
>>> partition
>>> roots so it may differ from the implicit "cpuset.cpus.exclusive".
>> Howewer, there's no "cpuset.cpus" configurable nor visible on the root
>> cgroup. So possibly drop this hunk altogether for simplicity?
>
> Ah, you are right. I thought there was a read-only copy in cgroup 
> root. Will correct that.
>
Below is the doc diff between v4 and v5:

diff --git a/Documentation/admin-guide/cgroup-v2.rst 
b/Documentation/admin-guide/cgroup-v2.rst
index a3446db96cea..28613c0e1c90 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -2641,9 +2641,10 @@ Cpuset Interface Files

      The root cgroup is always a partition root and its state cannot
      be changed.  All other non-root cgroups start out as "member".
-    Even though the "cpuset.cpus.exclusive*" control files are not
-    present in the root cgroup, they are implicitly the same as
-    "cpuset.cpus".
+    Even though the "cpuset.cpus.exclusive*" and "cpuset.cpus"
+    control files are not present in the root cgroup, they are
+    implicitly the same as the "/sys/devices/system/cpu/possible"
+    sysfs file.

      When set to "root", the current cgroup is the root of a new
      partition or scheduling domain.  The set of exclusive CPUs is

Cheers,
Longman