Message-ID: <ccca8790-2bf9-46db-b355-ce8695ea7093@huaweicloud.com>
Date: Thu, 29 Jan 2026 12:03:43 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Waiman Long <longman@...hat.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 1/2] cgroup/cpuset: Defer housekeeping_update()
 call from CPU hotplug to task_work



On 2026/1/28 12:42, 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 cause deadlock. So we
> have to defer any call to housekeeping_update() after the CPU hotplug
> operation has finished. This can be done via the task_work_add(...,
> TWA_RESUME) API where the actual housekeeping_update() call, if needed,
> will happen right before existing back to userspace.
> 
> Since the HK_TYPE_DOMAIN housekeeping cpumask should now track the
> changes in "cpuset.cpus.isolated", add a check in test_cpuset_prs.sh to
> confirm that the CPU hotplug deferral, if needed, is working as expected.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  kernel/cgroup/cpuset.c                        | 49 ++++++++++++++++++-
>  .../selftests/cgroup/test_cpuset_prs.sh       |  9 ++++
>  2 files changed, 56 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 7b7d12ab1006..98c7cb732206 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -84,6 +84,10 @@ static cpumask_var_t	isolated_cpus;
>   */
>  static bool isolated_cpus_updating;
>  
> +/* Both cpuset_mutex and cpus_read_locked acquired */
> +static bool cpuset_full_locked;
> +static bool isolation_task_work_queued;
> +
>  /*
>   * A flag to force sched domain rebuild at the end of an operation.
>   * It can be set in
> @@ -285,10 +289,12 @@ void cpuset_full_lock(void)
>  {
>  	cpus_read_lock();
>  	mutex_lock(&cpuset_mutex);
> +	cpuset_full_locked = true;
>  }
>  
>  void cpuset_full_unlock(void)
>  {
> +	cpuset_full_locked = false;
>  	mutex_unlock(&cpuset_mutex);
>  	cpus_read_unlock();
>  }
> @@ -1285,25 +1291,64 @@ static bool prstate_housekeeping_conflict(int prstate, struct cpumask *new_cpus)
>  	return false;
>  }
>  
> +static void __update_isolation_cpumasks(bool twork);
> +static void isolation_task_work_fn(struct callback_head *cb)
> +{
> +	cpuset_full_lock();
> +	__update_isolation_cpumasks(true);
> +	cpuset_full_lock();
> +}
> +
>  /*
> - * update_isolation_cpumasks - Update external isolation related CPU masks
> + * __update_isolation_cpumasks - Update external isolation related CPU masks
> + * @twork - set if call from isolation_task_work_fn()
>   *
>   * The following external CPU masks will be updated if necessary:
>   * - workqueue unbound cpumask
>   */
> -static void update_isolation_cpumasks(void)
> +static void __update_isolation_cpumasks(bool twork)
>  {
>  	int ret;
>  
> +	if (twork)
> +		isolation_task_work_queued = false;
> +
>  	if (!isolated_cpus_updating)
>  		return;
>  
> +	/*
> +	 * This function can be reached either directly from regular cpuset
> +	 * control file write (cpuset_full_locked) or via hotplug
> +	 * (cpus_write_lock && cpuset_mutex held). In the later case, we
> +	 * defer the housekeeping_update() call to a task_work to avoid
> +	 * the possibility of deadlock. The task_work will be run right
> +	 * before exiting back to userspace.
> +	 */
> +	if (!cpuset_full_locked) {
> +		static struct callback_head twork_cb;
> +
> +		if (!isolation_task_work_queued) {
> +			init_task_work(&twork_cb, isolation_task_work_fn);
> +			if (!task_work_add(current, &twork_cb, TWA_RESUME))
> +				isolation_task_work_queued = true;
> +			else
> +				/* Current task shouldn't be exiting */
> +				WARN_ON_ONCE(1);
> +		}
> +		return;
> +	}
> +
>  	ret = housekeeping_update(isolated_cpus);
>  	WARN_ON_ONCE(ret < 0);
>  
>  	isolated_cpus_updating = false;
>  }
>  

The logic is not straightforward; perhaps we can simplify it as follows,
maybe I missed something, just correct me.

static void isolation_task_work_fn(struct callback_head *cb)
{
	guard(mutex)(&isolcpus_update_mutex);
	WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
}

/*
 * __update_isolation_cpumasks - Update external isolation related CPU masks
 * @twork - set if call from isolation_task_work_fn()
 *
 * The following external CPU masks will be updated if necessary:
 * - workqueue unbound cpumask
 */
static void __update_isolation_cpumasks(bool twork)
{
	if (!isolated_cpus_updating)
		return;

	/*
	 * This function can be reached either directly from regular cpuset
	 * control file write (cpuset_full_locked) or via hotplug
	 * (cpus_write_lock && cpuset_mutex held). In the later case, we
	 * defer the housekeeping_update() call to a task_work to avoid
	 * the possibility of deadlock. The task_work will be run right
	 * before exiting back to userspace.
	 */
	if (twork) {
		static struct callback_head twork_cb;

		init_task_work(&twork_cb, isolation_task_work_fn);
		if (task_work_add(current, &twork_cb, TWA_RESUME))
			/* Current task shouldn't be exiting */
			WARN_ON_ONCE(1);

		return;
	}

	lockdep_assert_held(&isolcpus_update_mutex);
	/*
	 * Release cpus_read_lock & cpuset_mutex before calling
	 * housekeeping_update() and re-acquiring them afterward if not
	 * calling from task_work.
	 */

	cpuset_full_unlock();
	WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
	cpuset_full_lock();

	isolated_cpus_updating = false;
}

static inline void update_isolation_cpumasks(void)
{
	__update_isolation_cpumasks(false);
}

If twork is true, add it directly to the work queue — note that this requires
holding isolcpus_update_mutex and then calling housekeeping_update. Otherwise,
release the 'full' lock before invoking housekeeping_update.

> +static inline void update_isolation_cpumasks(void)
> +{
> +	__update_isolation_cpumasks(false);
> +}
> +
>  /**
>   * rm_siblings_excl_cpus - Remove exclusive CPUs that are used by sibling cpusets
>   * @parent: Parent cpuset containing all siblings
> diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> index 5dff3ad53867..af4a2532cb3e 100755
> --- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> +++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> @@ -773,6 +773,7 @@ check_isolcpus()
>  	EXPECTED_ISOLCPUS=$1
>  	ISCPUS=${CGROUP2}/cpuset.cpus.isolated
>  	ISOLCPUS=$(cat $ISCPUS)
> +	HKICPUS=$(cat /sys/devices/system/cpu/isolated)
>  	LASTISOLCPU=
>  	SCHED_DOMAINS=/sys/kernel/debug/sched/domains
>  	if [[ $EXPECTED_ISOLCPUS = . ]]
> @@ -810,6 +811,14 @@ check_isolcpus()
>  	ISOLCPUS=
>  	EXPECTED_ISOLCPUS=$EXPECTED_SDOMAIN
>  
> +	#
> +	# The inverse of HK_TYPE_DOMAIN cpumask in $HKICPUS should match $ISOLCPUS
> +	#
> +	[[ "$ISOLCPUS" != "$HKICPUS" ]] && {
> +		echo "Housekeeping isolated CPUs mismatch - $HKICPUS"
> +		return 1
> +	}
> +
>  	#
>  	# Use the sched domain in debugfs to check isolated CPUs, if available
>  	#

-- 
Best regards,
Ridong

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