Re: [PATCH/for-next v2 1/2] cgroup/cpuset: Defer housekeeping_update() call from CPU hotplug to workqueue

[Waiman Long <llong@redhat.com>]

On 2/2/26 8:05 AM, Peter Zijlstra wrote:
> On Fri, Jan 30, 2026 at 10:42:53AM -0500, Waiman Long wrote:
>
>> +/* Both cpuset_mutex and cpus_read_locked acquired */
>> +static bool cpuset_locked;
>> +
>>   /*
>>    * A flag to force sched domain rebuild at the end of an operation.
>>    * It can be set in
>> @@ -285,10 +288,12 @@ void cpuset_full_lock(void)
>>   {
>>   	cpus_read_lock();
>>   	mutex_lock(&cpuset_mutex);
>> +	cpuset_locked = true;
>>   }
>>   
>>   void cpuset_full_unlock(void)
>>   {
>> +	cpuset_locked = false;
>>   	mutex_unlock(&cpuset_mutex);
>>   	cpus_read_unlock();
>>   }
>> @@ -1293,14 +1308,30 @@ static bool prstate_housekeeping_conflict(int prstate, struct cpumask *new_cpus)
>>    */
>>   static void update_isolation_cpumasks(void)
>>   {
>> -	int ret;
>> +	static DECLARE_WORK(isolcpus_work, isolcpus_workfn);
>>   
>>   	if (!isolated_cpus_updating)
>>   		return;
>>   
>> -	ret = housekeeping_update(isolated_cpus);
>> -	WARN_ON_ONCE(ret < 0);
>> +	/*
>> +	 * This function can be reached either directly from regular cpuset
>> +	 * control file write (cpuset_locked) or via hotplug (cpus_write_lock
>> +	 * && cpuset_mutex held). In the later case, we defer the
>> +	 * housekeeping_update() call to the system_unbound_wq to avoid the
>> +	 * possibility of deadlock. This also means that there will be a short
>> +	 * period of time where HK_TYPE_DOMAIN housekeeping cpumask will lag
>> +	 * behind isolated_cpus.
>> +	 */
>> +	if (!cpuset_locked) {
> I agree with Chen that this is bloody terrible.
>
> At the very least this should have:
>
> 	lockdep_assert_held(&cpuset_mutex);
>
> But ideally you'd do patches against this and tip/locking/core that add
> proper __guarded_by() annotations to this.

Yes, I am going to remove cpuset_locked in the next version. As for 
__guarded_by() annotation, I need to set up a clang environment that I 
can use to test it before I will work on that. I usually just use gcc 
for my compilation need.

Cheers,
Longman