linux-kernel - Re: [cgroup/for-6.19 PATCH] cgroup/cpuset: Make callback_lock a raw_spinlock

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Message-ID: <318f1024-ba7a-4d88-aac5-af9040c31021@redhat.com>
Date: Wed, 12 Nov 2025 13:21:12 -0500
From: Waiman Long <llong@...hat.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
 Michal Koutný <mkoutny@...e.com>,
 Clark Williams <clrkwllms@...nel.org>, Steven Rostedt <rostedt@...dmis.org>,
 linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
 linux-rt-devel@...ts.linux.dev, Chen Ridong <chenridong@...wei.com>,
 Pingfan Liu <piliu@...hat.com>, Juri Lelli <juri.lelli@...hat.com>
Subject: Re: [cgroup/for-6.19 PATCH] cgroup/cpuset: Make callback_lock a
 raw_spinlock_t

On 11/12/25 3:51 AM, Sebastian Andrzej Siewior wrote:
> On 2025-11-11 22:57:59 [-0500], Waiman Long wrote:
>> The callback_lock is a spinlock_t which 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.
> We can't assume that, can we?
We can theoretically create a cgroup hierarchy with many levels, but no 
sane users will actually do that. If this is a concern to you, I can 
certainly drop this patch.
>> Some externally callable cpuset APIs like cpuset_cpus_allowed() and
> cpuset_cpus_allowed() has three callers in kernel/sched/ and all use
> GFP_KERNEL shortly before invoking the function in question.
The current callers of these APIs are fine. What I am talking is about 
new callers that may want to call them when holding a raw_spinlock_t.
>
>> cpuset_mems_allowed() acquires callback_lock with irq disabled to ensure
> This I did not find. But I would ask to rework it somehow that we don't
> need to use raw_spinlock_t as a hammer that solves it all.

OK.

Cheers,
Longman

>> stable cpuset data. These APIs currently have the restriction that they
>> can't be called when a raw spinlock is being held. This is needed to
>> work correctly in a PREEMPT_RT kernel. This requires additional code
>> changes to work around this limitation. See [1] for a discussion of that.
>>
>> Make these external cpuset APIs more useful by changing callback_lock
>> to a raw_spinlock_t to remove this limitation so that they can be called
>> from within other raw spinlock critical sections if needed.
>>
>> [1] https://lore.kernel.org/lkml/20251110014706.8118-1-piliu@redhat.com/
>>
>> Signed-off-by: Waiman Long <longman@...hat.com>
> Sebastian
>