linux-kernel - Re: [PATCH/for-next v3 2/3] cgroup/cpuset: Defer housekeeping_update() calls from CPU hotplug to workqueue

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Message-ID: <617f19e4-cac0-4dab-801c-f1a31f0233a8@redhat.com>
Date: Tue, 3 Feb 2026 23:51:38 -0500
From: Waiman Long <llong@...hat.com>
To: Chen Ridong <chenridong@...weicloud.com>, Tejun Heo <tj@...nel.org>,
 Johannes Weiner <hannes@...xchg.org>, Michal Koutný
 <mkoutny@...e.com>, Ingo Molnar <mingo@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Steven Rostedt <rostedt@...dmis.org>, Ben Segall <bsegall@...gle.com>,
 Mel Gorman <mgorman@...e.de>, Valentin Schneider <vschneid@...hat.com>,
 Anna-Maria Behnsen <anna-maria@...utronix.de>,
 Frederic Weisbecker <frederic@...nel.org>,
 Thomas Gleixner <tglx@...utronix.de>, Shuah Khan <shuah@...nel.org>
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-kselftest@...r.kernel.org
Subject: Re: [PATCH/for-next v3 2/3] cgroup/cpuset: Defer
 housekeeping_update() calls from CPU hotplug to workqueue

On 2/3/26 10:27 PM, Chen Ridong wrote:
>
> On 2026/2/3 4:11, Waiman Long wrote:
>> The update_isolation_cpumasks() function can be called either directly
>> from regular cpuset control file write with cpuset_full_lock() called
>> or via the CPU hotplug path with cpus_write_lock and cpuset_mutex held.
>>
>> As we are going to enable dynamic update to the nozh_full housekeeping
>> cpumask (HK_TYPE_KERNEL_NOISE) soon with the help of CPU hotplug,
>> allowing the CPU hotplug path to call into housekeeping_update() directly
>> from update_isolation_cpumasks() will likely cause deadlock. So we
>> have to defer any call to housekeeping_update() after the CPU hotplug
>> operation has finished. This is now done via the workqueue where
>> the actual housekeeping_update() call, if needed, will happen after
>> cpus_write_lock is released.
>>
>> We can't use the synchronous task_work API as call from CPU hotplug
>> path happen in the per-cpu kthread of the CPU that is being shut down
>> or brought up. Because of the asynchronous nature of workqueue, the
>> HK_TYPE_DOMAIN housekeeping cpumask will be updated a bit later than the
>> "cpuset.cpus.isolated" control file in this case.
>>
>> Also add a check in test_cpuset_prs.sh and modify some existing
>> test cases to confirm that "cpuset.cpus.isolated" and HK_TYPE_DOMAIN
>> housekeeping cpumask will both be updated.
>>
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>>   kernel/cgroup/cpuset.c                        | 37 +++++++++++++++++--
>>   .../selftests/cgroup/test_cpuset_prs.sh       | 13 +++++--
>>   2 files changed, 44 insertions(+), 6 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index d705c5ba64a7..e98a2e953392 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -1302,6 +1302,17 @@ static bool prstate_housekeeping_conflict(int prstate, struct cpumask *new_cpus)
>>   	return false;
>>   }
>>   
>> +static void isolcpus_workfn(struct work_struct *work)
>> +{
>> +	cpuset_full_lock();
>> +	if (isolated_cpus_updating) {
>> +		isolated_cpus_updating = false;
>> +		WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
>> +		rebuild_sched_domains_locked();
>> +	}
>> +	cpuset_full_unlock();
>> +}
>> +
>>   /*
>>    * update_isolation_cpumasks - Update external isolation related CPU masks
>>    *
>> @@ -1310,14 +1321,34 @@ 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 or via CPU hotplug. In the latter case, it is
>> +	 * the per-cpu kthread that calls cpuset_handle_hotplug() on behalf
>> +	 * of the task that initiates CPU shutdown or bringup.
>> +	 *
>> +	 * To have better flexibility and prevent the possibility of deadlock
>> +	 * when calling from CPU hotplug, we defer the housekeeping_update()
>> +	 * call to after the current cpuset critical section has finished.
>> +	 * This is done via workqueue.
>> +	 */
>> +	if (current->flags & PF_KTHREAD) {
>> +		/*
>> +		 * We rely on WORK_STRUCT_PENDING_BIT to not requeue a work
>> +		 * item that is still pending.
>> +		 */
>> +		queue_work(system_unbound_wq, &isolcpus_work);
>> +		/* Also defer sched domains regeneration to the work function */
>> +		force_sd_rebuild = false;
> Eh, looking at the call path:
>
> cpuset_hotplug_update_tasks
> 	update_parent_effective_cpumask
> 		update_isolation_cpumasks
> 		force_sd_rebuild = false;
> 	cpuset_force_rebuild();
>
> Setting force_sd_rebuild to false here might be redundant, given that
> cpuset_force_rebuild() is called immediately afterward.

Thank for spotting that. I will try to address this.

Thanks,
Longman