linux-kernel - Re: [-next v2 4/4] cpuset: add helpers for cpus read and cpuset

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Message-ID: <c88c6e2e-5988-405c-a037-651a8800ba83@huaweicloud.com>
Date: Thu, 14 Aug 2025 11:58:49 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Waiman Long <llong@...hat.com>, tj@...nel.org, hannes@...xchg.org,
 mkoutny@...e.com
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 lujialin4@...wei.com, chenridong@...wei.com, christophe.jaillet@...adoo.fr
Subject: Re: [-next v2 4/4] cpuset: add helpers for cpus read and cpuset_mutex
 locks



On 2025/8/14 11:27, Waiman Long wrote:
> On 8/13/25 11:13 PM, Waiman Long wrote:
>> On 8/13/25 8:44 PM, Chen Ridong wrote:
>>>
>>> On 2025/8/14 4:09, Waiman Long wrote:
>>>> On 8/13/25 4:29 AM, Chen Ridong wrote:
>>>>> From: Chen Ridong <chenridong@...wei.com>
>>>>>
>>>>> cpuset: add helpers for cpus_read_lock and cpuset_mutex
>>>>>
>>>>> Replace repetitive locking patterns with new helpers:
>>>>> - cpus_read_cpuset_lock()
>>>>> - cpus_read_cpuset_unlock()
>>>>>
>>>>> This makes the code cleaner and ensures consistent lock ordering.
>>>>>
>>>>> Signed-off-by: Chen Ridong <chenridong@...wei.com>
>>>>> ---
>>>>>    kernel/cgroup/cpuset-internal.h |  2 ++
>>>>>    kernel/cgroup/cpuset-v1.c       | 12 +++------
>>>>>    kernel/cgroup/cpuset.c          | 48 +++++++++++++++------------------
>>>>>    3 files changed, 28 insertions(+), 34 deletions(-)
>>>>>
>>>>> diff --git a/kernel/cgroup/cpuset-internal.h b/kernel/cgroup/cpuset-internal.h
>>>>> index 75b3aef39231..6fb00c96044d 100644
>>>>> --- a/kernel/cgroup/cpuset-internal.h
>>>>> +++ b/kernel/cgroup/cpuset-internal.h
>>>>> @@ -276,6 +276,8 @@ int cpuset_update_flag(cpuset_flagbits_t bit, struct cpuset *cs, int
>>>>> turning_on)
>>>>>    ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
>>>>>                        char *buf, size_t nbytes, loff_t off);
>>>>>    int cpuset_common_seq_show(struct seq_file *sf, void *v);
>>>>> +void cpus_read_cpuset_lock(void);
>>>>> +void cpus_read_cpuset_unlock(void);
>>>> The names are not intuitive. I would prefer just extend the cpuset_lock/unlock to include
>>>> cpus_read_lock/unlock and we use cpuset_lock/unlock consistently in the cpuset code. Also, there is
>>>> now no external user of cpuset_lock/unlock, we may as well remove them from include/linux/cpuset.h.
>>>>
>>>> Cheers,
>>>> Longman
>>> I like the idea and have considered it.
>>> However, I noticed that cpuset_locked is being used in __sched_setscheduler.
>>
>> Right, I overloooked the cpuset_lock() call in kernel/sched/syscall.c. So we can't remove it from
>> include/linux/cpuset.h.
>>
>> This call is invoked to ensure cpusets information is stable. However, it doesn't hurt if the
>> cpus_read_lock() is also acquired as a result. Alternatively, we can use a name like
>> cpuset_full_lock() to include cpus_read_lock().
> 
> I have a correction. According to commit d74b27d63a8b ("cgroup/cpuset: Change cpuset_rwsem and
> hotplug lock order") , sched_scheduler() can be called while holding cpus_hotplug_lock. So we should
> keep cpuset_lock() as it is.
> 
> Cheers,
> Longman

Thank you Longman, this is very helpful.

I had considered whether we can add cpus_read_lock() to the cpuset_lock, but based on your
explanation, I now understand this approach would not work.

For clarity, would it be acceptable to rename:
cpus_read_cpuset_lock() -> cpuset_full_lock()
cpus_read_cpuset_unlock() -> cpuset_full_unlock()

-- 
Best regards,
Ridong