Re: [PATCH 1/3] cgroup/cpuset: Protect is_in_v2_mode() in cpuset_num_cpus()

[Waiman Long <longman@redhat.com>]

On 9/29/26 3:08 PM, Peter Zijlstra wrote:
> On Tue, Sep 29, 2026 at 02:10:40PM -0400, Waiman Long wrote:
>> On 9/29/26 1:35 PM, Andrea Righi wrote:
>>>> I think it is simpler to just change is_in_v2_mode() to cpuset_v2(). Almost
>>>> all the cpuset functions should either take the callback_lock with interrupt
>>>> disabled (which is a RCU read-side critical section) or with rcu_read_lock()
>>>> and cpuset_mutex() acquired. This cpuset_num_cpus() function is an
>>>> exception. Given what is said in the comment, this function is not supposed
>>>> to be used with v1 mounted. We should change it to cpuset_v2().
>>> The comment says that, outside cgroup v2, cpuset_num_cpus() falls back to
>>> num_online_cpus(). However, on v1 with cpu and cpuset mounted together using
>>> cpuset_v2_mode, it returns the group's effective cpuset count and fair.c uses
>>> that count in the default "concur" group share calculation and in "max" mode.
>>>
>>> So replacing is_in_v2_mode() with cpuset_v2() would change scheduler behavior
>>> for that setup. I guess we could either preserve the current behavior, fix the
>>> comment and protect the root lookup with RCU; or make the code follow the
>>> documented v2-only behavior. Which one would you prefer?
>> I will let Peter decide if he wants to support the cpuset_v2_mode mount
>> option of cgroup v1 since he is the original author of cpuset_num_cpus(). If
>> this is supported, we have to update the function comment as well.
> So I was not aware of this weird mount option at all. That said, ideally
> it would work in the widest possible setting.
>
> The main constraint is going from a cpu-cgroup to a cpuset-cgroup. It
> was my understanding that this transition only works in v2, but if that
> mount option is sufficient to make that cross-cgroup transition
> meaningful, then yay I suppose.

The cpuset_v2_mode mount option is only for making the cpuset.cpus and 
cpuset.mems behave like in v2. The cpu-cgroup and cpuset-cgroup can 
still be in separate hierarchies. If cross-cgroup transition is the main 
point, it won't work with the cpuset_v2_mode mount option. We should 
switch to use cpuset_v2().

Cheers,
Longman