linux-kernel - Re: [PATCH 2/2] cgroup/cpuset: Change callback_lock to raw_spinlock

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Message-ID: <05900ad4-fdbd-4505-a080-1e71bedb5f4a@redhat.com>
Date: Mon, 20 Oct 2025 15:25:28 -0400
From: Waiman Long <llong@...hat.com>
To: Tejun Heo <tj@...nel.org>
Cc: Johannes Weiner <hannes@...xchg.org>, Michal Koutný
 <mkoutny@...e.com>, cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 Chen Ridong <chenridong@...wei.com>
Subject: Re: [PATCH 2/2] cgroup/cpuset: Change callback_lock to raw_spinlock_t

On 10/20/25 1:00 PM, Tejun Heo wrote:
> On Sun, Oct 19, 2025 at 10:32:07PM -0400, Waiman Long wrote:
>> The callback_lock is acquired either to read a stable set of cpu
>> or node masks or to modify those masks when cpuset_mutex is also
>> acquired. Sometime, it may need to go up the cgroup hierarchy while
>> holding the lock to find the right set of masks to use. Assuming that
>> the depth of the cgroup hierarch is finite and typically small, the
>> lock hold time should be limited.
>>
>> As a result, it can be converted to raw_spinlock_t to reduce overhead
>> in a PREEMPT_RT setting without introducing excessive latency.
>>
>> Signed-off-by: Waiman Long <longman@...hat.com>
> Is this something that RT people would like? Why does the overhead of the
> lock matter? I think this patch requires more justifications.

Yes, I will push a new patch and cc the RT guys to see if they have any 
objection. I had seen that callback_lock was changed to raw_spinlock_t 
in one or more of the releases of the linux-rt-devel tree before the 
mainline merge.

Thanks,
Longman