Re: [PATCH/for-next 1/2] cgroup/cpuset: Defer housekeeping_update() call from CPU hotplug to task_work
From: Waiman Long
Date: Thu Jan 29 2026 - 20:38:02 EST
On 1/29/26 2:15 AM, Chen Ridong wrote:
Thanks for the suggestions. I will adopt some of them. BTW, I am switching to use workqueue instead of the task_work as the latter is suitable in this use case.
On 2026/1/29 12:03, Chen Ridong wrote:
It can be much clearer:
On 2026/1/28 12:42, Waiman Long wrote:
The update_isolation_cpumasks() function can be called either directlyThe logic is not straightforward; perhaps we can simplify it as follows,
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@xxxxxxxxxx>
---
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;
}
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);
}
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
* @defer
*
* The following external CPU masks will be updated if necessary:
* - workqueue unbound cpumask
*/
static void __update_isolation_cpumasks(bool defer)
{
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 (defer) {
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);
lockdep_assert_cpus_held();
lockdep_assert_cpuset_lock_held();
/*
* 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);
}
static inline void asyn_update_isolation_cpumasks(void)
{
__update_isolation_cpumasks(true);
}
The hotplug path just calls asyn_update_isolation_cpumasks(), cpuset_full_locked
and isolation_task_work_queued can be removed.
Cheers,
Longman