[PATCH/for-next v3 0/3] cgroup/cpuset: Fix partition related locking issues

[PATCH/for-next v3 0/3] cgroup/cpuset: Fix partition related locking issues

[Waiman Long]

 v3:
  - Add a new patch to clarify the locking rules for internal variables
  - Defer all housekeeping_update() calls with associated
    rebuild_sched_domains*() calls to either workqueue or task_work.

 v2:
  - Change patch 1 to use workqueue instead of task run as it is a
    per-cpu kthread that performs the cpuset shutdown and bringup work.
  - Simplify and streamline some of the code.

After booting the latest cgroup for-next debug kernel with the latest
cgroup changes as well as Federic's "cpuset/isolation: Honour kthreads
preferred affinity" patch series [1] merged on top and running the
test-cpuset-prs.sh test, a circular locking dependency lockdep splat
was reported. See patch 2 for details.

To fix this issue, a new top level cpuset_top_mutex is added and the
call to housekeeping_update() is deferred to either a task_work or to
a workqueue.

With these changes in place, the cpuset test ran to completion with no
failure and no lockdep splat.

[1] https://lore.kernel.org/lkml/20260125224541.50226-1-frederic@kernel.org/

Waiman Long (3):
  cgroup/cpuset: Clarify exclusion rules for cpuset internal variables
  cgroup/cpuset: Defer housekeeping_update() calls from CPU hotplug to
    workqueue
  cgroup/cpuset: Call housekeeping_update() without holding
    cpus_read_lock

 kernel/cgroup/cpuset.c                        | 211 ++++++++++++++----
 kernel/sched/isolation.c                      |   4 +-
 kernel/time/timer_migration.c                 |   3 +-
 .../selftests/cgroup/test_cpuset_prs.sh       |  13 +-
 4 files changed, 174 insertions(+), 57 deletions(-)

-- 
2.52.0

[PATCH/for-next v3 1/3] cgroup/cpuset: Clarify exclusion rules for cpuset internal variables

[Waiman Long]

Clarify the locking rules associated with file level internal variables
inside the cpuset code. There is no functional change.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset.c | 105 ++++++++++++++++++++++++-----------------
 1 file changed, 61 insertions(+), 44 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index c43efef7df71..d705c5ba64a7 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -61,6 +61,58 @@ static const char * const perr_strings[] = {
 	[PERR_REMOTE]    = "Have remote partition underneath",
 };
 
+/*
+ * CPUSET Locking Convention
+ * -------------------------
+ *
+ * Below are the three global locks guarding cpuset structures in lock
+ * acquisition order:
+ *  - cpu_hotplug_lock (cpus_read_lock/cpus_write_lock)
+ *  - cpuset_mutex
+ *  - callback_lock (raw spinlock)
+ *
+ * A task must hold all the three locks to modify externally visible or
+ * used fields of cpusets, though some of the internally used cpuset fields
+ * and internal variables can be modified without holding callback_lock. If only
+ * reliable read access of the externally used fields are needed, a task can
+ * hold either cpuset_mutex or callback_lock which are exposed to other
+ * external subsystems.
+ *
+ * If a task holds cpu_hotplug_lock and cpuset_mutex, it blocks others,
+ * ensuring that it is the only task able to also acquire callback_lock and
+ * be able to modify cpusets.  It can perform various checks on the cpuset
+ * structure first, knowing nothing will change. It can also allocate memory
+ * without holding callback_lock. While it is performing these checks, various
+ * callback routines can briefly acquire callback_lock to query cpusets.  Once
+ * it is ready to make the changes, it takes callback_lock, blocking everyone
+ * else.
+ *
+ * Calls to the kernel memory allocator cannot be made while holding
+ * callback_lock which is a spinlock, as the memory allocator may sleep or
+ * call back into cpuset code and acquire callback_lock.
+ *
+ * Now, the task_struct fields mems_allowed and mempolicy may be changed
+ * by other task, we use alloc_lock in the task_struct fields to protect
+ * them.
+ *
+ * The cpuset_common_seq_show() handlers only hold callback_lock across
+ * small pieces of code, such as when reading out possibly multi-word
+ * cpumasks and nodemasks.
+ */
+
+static DEFINE_MUTEX(cpuset_mutex);
+
+/*
+ * File level internal variables below follow one of the following exclusion
+ * rules.
+ *
+ * RWCS: Read/write-able by holding both cpus_read_lock/cpus_write_lock and
+ *	 cpuset_mutex.
+ *
+ * CSCB: Readable by holding either cpuset_mutex or callback_lock. Writable
+ *	 by holding both cpuset_mutex and callback_lock.
+ */
+
 /*
  * For local partitions, update to subpartitions_cpus & isolated_cpus is done
  * in update_parent_effective_cpumask(). For remote partitions, it is done in
@@ -70,19 +122,18 @@ static const char * const perr_strings[] = {
  * Exclusive CPUs distributed out to local or remote sub-partitions of
  * top_cpuset
  */
-static cpumask_var_t	subpartitions_cpus;
+static cpumask_var_t	subpartitions_cpus;	/* RWCS */
 
 /*
- * Exclusive CPUs in isolated partitions
+ * Exclusive CPUs in isolated partitions (shown in cpuset.cpus.isolated)
  */
-static cpumask_var_t	isolated_cpus;
+static cpumask_var_t	isolated_cpus;		/* CSCB */
 
 /*
- * isolated_cpus updating flag (protected by cpuset_mutex)
- * Set if isolated_cpus is going to be updated in the current
- * cpuset_mutex crtical section.
+ * Set if isolated_cpus is being updated in the current cpuset_mutex
+ * critical section.
  */
-static bool isolated_cpus_updating;
+static bool		isolated_cpus_updating;	/* RWCS */
 
 /*
  * A flag to force sched domain rebuild at the end of an operation.
@@ -98,7 +149,7 @@ static bool isolated_cpus_updating;
  * Note that update_relax_domain_level() in cpuset-v1.c can still call
  * rebuild_sched_domains_locked() directly without using this flag.
  */
-static bool force_sd_rebuild;
+static bool force_sd_rebuild;			/* RWCS */
 
 /*
  * Partition root states:
@@ -218,42 +269,6 @@ struct cpuset top_cpuset = {
 	.partition_root_state = PRS_ROOT,
 };
 
-/*
- * There are two global locks guarding cpuset structures - cpuset_mutex and
- * callback_lock. The cpuset code uses only cpuset_mutex. Other kernel
- * subsystems can use cpuset_lock()/cpuset_unlock() to prevent change to cpuset
- * structures. Note that cpuset_mutex needs to be a mutex as it is used in
- * paths that rely on priority inheritance (e.g. scheduler - on RT) for
- * correctness.
- *
- * A task must hold both locks to modify cpusets.  If a task holds
- * cpuset_mutex, it blocks others, ensuring that it is the only task able to
- * also acquire callback_lock and be able to modify cpusets.  It can perform
- * various checks on the cpuset structure first, knowing nothing will change.
- * It can also allocate memory while just holding cpuset_mutex.  While it is
- * performing these checks, various callback routines can briefly acquire
- * callback_lock to query cpusets.  Once it is ready to make the changes, it
- * takes callback_lock, blocking everyone else.
- *
- * Calls to the kernel memory allocator can not be made while holding
- * callback_lock, as that would risk double tripping on callback_lock
- * from one of the callbacks into the cpuset code from within
- * __alloc_pages().
- *
- * If a task is only holding callback_lock, then it has read-only
- * access to cpusets.
- *
- * Now, the task_struct fields mems_allowed and mempolicy may be changed
- * by other task, we use alloc_lock in the task_struct fields to protect
- * them.
- *
- * The cpuset_common_seq_show() handlers only hold callback_lock across
- * small pieces of code, such as when reading out possibly multi-word
- * cpumasks and nodemasks.
- */
-
-static DEFINE_MUTEX(cpuset_mutex);
-
 /**
  * cpuset_lock - Acquire the global cpuset mutex
  *
@@ -1163,6 +1178,8 @@ static void reset_partition_data(struct cpuset *cs)
 static void isolated_cpus_update(int old_prs, int new_prs, struct cpumask *xcpus)
 {
 	WARN_ON_ONCE(old_prs == new_prs);
+	lockdep_assert_held(&callback_lock);
+	lockdep_assert_held(&cpuset_mutex);
 	if (new_prs == PRS_ISOLATED)
 		cpumask_or(isolated_cpus, isolated_cpus, xcpus);
 	else
-- 
2.52.0

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

[Waiman Long]

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@redhat.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;
+		return;
+	}
 
+	WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
 	isolated_cpus_updating = false;
 }
 
diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
index 5dff3ad53867..0502b156582b 100755
--- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
+++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
@@ -245,8 +245,9 @@ TEST_MATRIX=(
 	"C2-3:P1:S+  C3:P2  .      .     O2=0   O2=1    .      .     0 A1:2|A2:3 A1:P1|A2:P2"
 	"C2-3:P1:S+  C3:P1  .      .     O2=0    .      .      .     0 A1:|A2:3 A1:P1|A2:P1"
 	"C2-3:P1:S+  C3:P1  .      .     O3=0    .      .      .     0 A1:2|A2: A1:P1|A2:P1"
-	"C2-3:P1:S+  C3:P1  .      .    T:O2=0   .      .      .     0 A1:3|A2:3 A1:P1|A2:P-1"
-	"C2-3:P1:S+  C3:P1  .      .      .    T:O3=0   .      .     0 A1:2|A2:2 A1:P1|A2:P-1"
+	"C2-3:P1:S+  C3:P2  .      .    T:O2=0   .      .      .     0 A1:3|A2:3 A1:P1|A2:P-2"
+	"C1-3:P1:S+  C3:P2  .      .      .    T:O3=0   .      .     0 A1:1-2|A2:1-2 A1:P1|A2:P-2 3|"
+	"C1-3:P1:S+  C3:P2  .      .      .    T:O3=0  O3=1    .     0 A1:1-2|A2:3 A1:P1|A2:P2  3"
 	"$SETUP_A123_PARTITIONS    .     O1=0    .      .      .     0 A1:|A2:2|A3:3 A1:P1|A2:P1|A3:P1"
 	"$SETUP_A123_PARTITIONS    .     O2=0    .      .      .     0 A1:1|A2:|A3:3 A1:P1|A2:P1|A3:P1"
 	"$SETUP_A123_PARTITIONS    .     O3=0    .      .      .     0 A1:1|A2:2|A3: A1:P1|A2:P1|A3:P1"
@@ -764,7 +765,7 @@ check_cgroup_states()
 # only CPUs in isolated partitions as well as those that are isolated at
 # boot time.
 #
-# $1 - expected isolated cpu list(s) <isolcpus1>{,<isolcpus2>}
+# $1 - expected isolated cpu list(s) <isolcpus1>{|<isolcpus2>}
 # <isolcpus1> - expected sched/domains value
 # <isolcpus2> - cpuset.cpus.isolated value = <isolcpus1> if not defined
 #
@@ -773,6 +774,7 @@ 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 [[ $EXPECTED_ISOLCPUS = . ]]
@@ -810,6 +812,11 @@ check_isolcpus()
 	ISOLCPUS=
 	EXPECTED_ISOLCPUS=$EXPECTED_SDOMAIN
 
+	#
+	# The inverse of HK_TYPE_DOMAIN cpumask in $HKICPUS should match $ISOLCPUS
+	#
+	[[ "$ISOLCPUS" != "$HKICPUS" ]] && return 1
+
 	#
 	# Use the sched domain in debugfs to check isolated CPUs, if available
 	#
-- 
2.52.0

[PATCH/for-next v3 3/3] cgroup/cpuset: Call housekeeping_update() without holding cpus_read_lock

[Waiman Long]

The current cpuset partition code is able to dynamically update
the sched domains of a running system and the corresponding
HK_TYPE_DOMAIN housekeeping cpumask to perform what is essentally the
"isolcpus=domain,..." boot command line feature at run time.

The housekeeping cpumask update requires flushing a number of different
workqueues which may not be safe with cpus_read_lock() held as the
workqueue flushing code may acquire cpus_read_lock() or acquiring locks
which have locking dependency with cpus_read_lock() down the chain. Below
is an example of such circular locking problem.

  ======================================================
  WARNING: possible circular locking dependency detected
  6.18.0-test+ #2 Tainted: G S
  ------------------------------------------------------
  test_cpuset_prs/10971 is trying to acquire lock:
  ffff888112ba4958 ((wq_completion)sync_wq){+.+.}-{0:0}, at: touch_wq_lockdep_map+0x7a/0x180

  but task is already holding lock:
  ffffffffae47f450 (cpuset_mutex){+.+.}-{4:4}, at: cpuset_partition_write+0x85/0x130

  which lock already depends on the new lock.

  the existing dependency chain (in reverse order) is:
  -> #4 (cpuset_mutex){+.+.}-{4:4}:
  -> #3 (cpu_hotplug_lock){++++}-{0:0}:
  -> #2 (rtnl_mutex){+.+.}-{4:4}:
  -> #1 ((work_completion)(&arg.work)){+.+.}-{0:0}:
  -> #0 ((wq_completion)sync_wq){+.+.}-{0:0}:

  Chain exists of:
    (wq_completion)sync_wq --> cpu_hotplug_lock --> cpuset_mutex

  5 locks held by test_cpuset_prs/10971:
   #0: ffff88816810e440 (sb_writers#7){.+.+}-{0:0}, at: ksys_write+0xf9/0x1d0
   #1: ffff8891ab620890 (&of->mutex#2){+.+.}-{4:4}, at: kernfs_fop_write_iter+0x260/0x5f0
   #2: ffff8890a78b83e8 (kn->active#187){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x2b6/0x5f0
   #3: ffffffffadf32900 (cpu_hotplug_lock){++++}-{0:0}, at: cpuset_partition_write+0x77/0x130
   #4: ffffffffae47f450 (cpuset_mutex){+.+.}-{4:4}, at: cpuset_partition_write+0x85/0x130

  Call Trace:
   <TASK>
     :
   touch_wq_lockdep_map+0x93/0x180
   __flush_workqueue+0x111/0x10b0
   housekeeping_update+0x12d/0x2d0
   update_parent_effective_cpumask+0x595/0x2440
   update_prstate+0x89d/0xce0
   cpuset_partition_write+0xc5/0x130
   cgroup_file_write+0x1a5/0x680
   kernfs_fop_write_iter+0x3df/0x5f0
   vfs_write+0x525/0xfd0
   ksys_write+0xf9/0x1d0
   do_syscall_64+0x95/0x520
   entry_SYSCALL_64_after_hwframe+0x76/0x7e

To avoid such a circular locking dependency problem, we have to
call housekeeping_update() without holding the cpus_read_lock() and
cpuset_mutex. The current set of wq's flushed by housekeeping_update()
may not have work functions that call cpus_read_lock() directly,
but we are likely to extend the list of wq's that are flushed in the
future. Moreover, the current set of work functions may hold locks that
may have cpu_hotplug_lock down the dependency chain.

One way to do that is to defer the housekeeping_update() call after
the current cpuset critical section has finished without holding
cpus_read_lock. For cpuset control file write, this can be done by
deferring it using task_work right before returning to userspace.

To enable mutual exclusion between the housekeeping_update() call and
other cpuset control file write actions, a new top level cpuset_top_mutex
is introduced. This new mutex will be acquired first to allow sharing
variables used by both code paths. However, cpuset update from CPU
hotplug can still happen in parallel with the housekeeping_update()
call, though that should be rare in production environment.

As cpus_read_lock() is now no longer held when
tmigr_isolated_exclude_cpumask() is called, it needs to acquire it
directly.

The lockdep_is_cpuset_held() is also updated to check the new
cpuset_top_mutex.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset.c        | 103 +++++++++++++++++++++++++++-------
 kernel/sched/isolation.c      |   4 +-
 kernel/time/timer_migration.c |   3 +-
 3 files changed, 86 insertions(+), 24 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index e98a2e953392..d2f51f40f87e 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -65,14 +65,28 @@ static const char * const perr_strings[] = {
  * CPUSET Locking Convention
  * -------------------------
  *
- * Below are the three global locks guarding cpuset structures in lock
+ * Below are the four global/local locks guarding cpuset structures in lock
  * acquisition order:
+ *  - cpuset_top_mutex
  *  - cpu_hotplug_lock (cpus_read_lock/cpus_write_lock)
  *  - cpuset_mutex
  *  - callback_lock (raw spinlock)
  *
- * A task must hold all the three locks to modify externally visible or
- * used fields of cpusets, though some of the internally used cpuset fields
+ * As cpuset will now indirectly flush a number of different workqueues in
+ * housekeeping_update() to update housekeeping cpumasks when the set of
+ * isolated CPUs is going to be changed, it may be vulnerable to deadlock
+ * if we hold cpus_read_lock while calling into housekeeping_update().
+ *
+ * The first cpuset_top_mutex will be held except when calling into
+ * cpuset_handle_hotplug() from the CPU hotplug code where cpus_write_lock
+ * and cpuset_mutex will be held instead. The main purpose of this mutex
+ * is to prevent regular cpuset control file write actions from interfering
+ * with the call to housekeeping_update(), though CPU hotplug operation can
+ * still happen in parallel. This mutex also provides protection for some
+ * internal variables.
+ *
+ * A task must hold all the remaining three locks to modify externally visible
+ * or used fields of cpusets, though some of the internally used cpuset fields
  * and internal variables can be modified without holding callback_lock. If only
  * reliable read access of the externally used fields are needed, a task can
  * hold either cpuset_mutex or callback_lock which are exposed to other
@@ -100,6 +114,7 @@ static const char * const perr_strings[] = {
  * cpumasks and nodemasks.
  */
 
+static DEFINE_MUTEX(cpuset_top_mutex);
 static DEFINE_MUTEX(cpuset_mutex);
 
 /*
@@ -111,6 +126,8 @@ static DEFINE_MUTEX(cpuset_mutex);
  *
  * CSCB: Readable by holding either cpuset_mutex or callback_lock. Writable
  *	 by holding both cpuset_mutex and callback_lock.
+ *
+ * T:	 Read/write-able by holding the cpuset_top_mutex.
  */
 
 /*
@@ -135,6 +152,13 @@ static cpumask_var_t	isolated_cpus;		/* CSCB */
  */
 static bool		isolated_cpus_updating;	/* RWCS */
 
+/*
+ * Copy of isolated_cpus to be processed by housekeeping_update()
+ */
+static cpumask_var_t	isolated_hk_cpus;	/* T */
+static bool		isolcpus_twork_queued;	/* T */
+
+
 /*
  * A flag to force sched domain rebuild at the end of an operation.
  * It can be set in
@@ -298,6 +322,7 @@ void lockdep_assert_cpuset_lock_held(void)
  */
 void cpuset_full_lock(void)
 {
+	mutex_lock(&cpuset_top_mutex);
 	cpus_read_lock();
 	mutex_lock(&cpuset_mutex);
 }
@@ -306,12 +331,13 @@ void cpuset_full_unlock(void)
 {
 	mutex_unlock(&cpuset_mutex);
 	cpus_read_unlock();
+	mutex_unlock(&cpuset_top_mutex);
 }
 
 #ifdef CONFIG_LOCKDEP
 bool lockdep_is_cpuset_held(void)
 {
-	return lockdep_is_held(&cpuset_mutex);
+	return lockdep_is_held(&cpuset_top_mutex);
 }
 #endif
 
@@ -1302,15 +1328,38 @@ static bool prstate_housekeeping_conflict(int prstate, struct cpumask *new_cpus)
 	return false;
 }
 
-static void isolcpus_workfn(struct work_struct *work)
+/*
+ * housekeeping_update() will only be called if isolated_cpus differs
+ * from isolated_hk_cpus. To be safe, rebuild_sched_domains() will always
+ * be called just in case there are still pending sched domains changes.
+ */
+static void do_housekeeping_update(bool *flag)
 {
-	cpuset_full_lock();
-	if (isolated_cpus_updating) {
-		isolated_cpus_updating = false;
-		WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
-		rebuild_sched_domains_locked();
+	bool update_hk = true;
+
+	guard(mutex)(&cpuset_top_mutex);
+	if (flag)
+		*flag = false;
+	scoped_guard(spinlock_irq, &callback_lock) {
+		if (cpumask_equal(isolated_hk_cpus, isolated_cpus))
+			update_hk = false;
+		else
+			cpumask_copy(isolated_hk_cpus, isolated_cpus);
 	}
-	cpuset_full_unlock();
+	if (update_hk)
+		WARN_ON_ONCE(housekeeping_update(isolated_hk_cpus) < 0);
+	rebuild_sched_domains();
+}
+
+static void isolcpus_workfn(struct work_struct *work)
+{
+	do_housekeeping_update(NULL);
+}
+
+static void isolcpus_tworkfn(struct callback_head *cb)
+{
+	/* Clear isolcpus_twork_queued */
+	do_housekeeping_update(&isolcpus_twork_queued);
 }
 
 /*
@@ -1322,9 +1371,15 @@ static void isolcpus_workfn(struct work_struct *work)
 static void update_isolation_cpumasks(void)
 {
 	static DECLARE_WORK(isolcpus_work, isolcpus_workfn);
+	static struct callback_head twork_cb;
 
 	if (!isolated_cpus_updating)
 		return;
+	else
+		isolated_cpus_updating = false;
+
+	/* Also defer sched domains regeneration to the wq or task_work */
+	force_sd_rebuild = false;
 
 	/*
 	 * This function can be reached either directly from regular cpuset
@@ -1332,10 +1387,10 @@ static void update_isolation_cpumasks(void)
 	 * 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.
+	 * To have better flexibility and prevent the possibility of deadlock,
+	 * we defer the housekeeping_update() call to after the current
+	 * cpuset critical section has finished. This is done via task_work
+	 * for cpuset control file write and workqueue for CPU hotplug.
 	 */
 	if (current->flags & PF_KTHREAD) {
 		/*
@@ -1343,13 +1398,22 @@ static void update_isolation_cpumasks(void)
 		 * 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;
 		return;
 	}
 
-	WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
-	isolated_cpus_updating = false;
+	/*
+	 * update_isolation_cpumasks() may be called more than once in the
+	 * same cpuset_mutex critical section.
+	 */
+	lockdep_assert_held(&cpuset_top_mutex);
+	if (isolcpus_twork_queued)
+		return;
+
+	init_task_work(&twork_cb, isolcpus_tworkfn);
+	if (!task_work_add(current, &twork_cb, TWA_RESUME))
+		isolcpus_twork_queued = true;
+	else
+		WARN_ON_ONCE(1);	/* Current task shouldn't be exiting */
 }
 
 /**
@@ -3657,6 +3721,7 @@ int __init cpuset_init(void)
 	BUG_ON(!alloc_cpumask_var(&top_cpuset.exclusive_cpus, GFP_KERNEL));
 	BUG_ON(!zalloc_cpumask_var(&subpartitions_cpus, GFP_KERNEL));
 	BUG_ON(!zalloc_cpumask_var(&isolated_cpus, GFP_KERNEL));
+	BUG_ON(!zalloc_cpumask_var(&isolated_hk_cpus, GFP_KERNEL));
 
 	cpumask_setall(top_cpuset.cpus_allowed);
 	nodes_setall(top_cpuset.mems_allowed);
diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index 3b725d39c06e..ef152d401fe2 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -123,8 +123,6 @@ int housekeeping_update(struct cpumask *isol_mask)
 	struct cpumask *trial, *old = NULL;
 	int err;
 
-	lockdep_assert_cpus_held();
-
 	trial = kmalloc(cpumask_size(), GFP_KERNEL);
 	if (!trial)
 		return -ENOMEM;
@@ -136,7 +134,7 @@ int housekeeping_update(struct cpumask *isol_mask)
 	}
 
 	if (!housekeeping.flags)
-		static_branch_enable_cpuslocked(&housekeeping_overridden);
+		static_branch_enable(&housekeeping_overridden);
 
 	if (housekeeping.flags & HK_FLAG_DOMAIN)
 		old = housekeeping_cpumask_dereference(HK_TYPE_DOMAIN);
diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
index 6da9cd562b20..244a8d025e78 100644
--- a/kernel/time/timer_migration.c
+++ b/kernel/time/timer_migration.c
@@ -1559,8 +1559,6 @@ int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
 	cpumask_var_t cpumask __free(free_cpumask_var) = CPUMASK_VAR_NULL;
 	int cpu;
 
-	lockdep_assert_cpus_held();
-
 	if (!works)
 		return -ENOMEM;
 	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
@@ -1570,6 +1568,7 @@ int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
 	 * First set previously isolated CPUs as available (unisolate).
 	 * This cpumask contains only CPUs that switched to available now.
 	 */
+	guard(cpus_read_lock)();
 	cpumask_andnot(cpumask, cpu_online_mask, exclude_cpumask);
 	cpumask_andnot(cpumask, cpumask, tmigr_available_cpumask);
 
-- 
2.52.0

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

[Peter Zijlstra]

On Mon, Feb 02, 2026 at 03:11:43PM -0500, Waiman Long wrote:

> @@ -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) {

		/* Serializes the static isolcpus_workfn. */
		lockdep_assert_held(&cpuset_mutex);

> +		/*
> +		 * 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;
> +		return;
> +	}
>  
> +	WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
>  	isolated_cpus_updating = false;
>  }

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

[Waiman Long]

On 2/2/26 3:18 PM, Peter Zijlstra wrote:
> On Mon, Feb 02, 2026 at 03:11:43PM -0500, Waiman Long wrote:
>
>> @@ -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) {
> 		/* Serializes the static isolcpus_workfn. */
> 		lockdep_assert_held(&cpuset_mutex);

Do we require synchronization between the the queue_work() call and the 
execution of the work function? I thought it is not needed, but I may be 
wrong.

Thanks,
Longman

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

[Peter Zijlstra]

On Mon, Feb 02, 2026 at 03:32:03PM -0500, Waiman Long wrote:
> On 2/2/26 3:18 PM, Peter Zijlstra wrote:
> > On Mon, Feb 02, 2026 at 03:11:43PM -0500, Waiman Long wrote:
> > 
> > > @@ -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) {
> > 		/* Serializes the static isolcpus_workfn. */
> > 		lockdep_assert_held(&cpuset_mutex);
> 
> Do we require synchronization between the the queue_work() call and the
> execution of the work function? I thought it is not needed, but I may be
> wrong.

Well, something needs to ensure there aren't two threads trying to use
this one work thing at the same time, no?

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

[Waiman Long]

On 2/2/26 3:48 PM, Peter Zijlstra wrote:
> On Mon, Feb 02, 2026 at 03:32:03PM -0500, Waiman Long wrote:
>> On 2/2/26 3:18 PM, Peter Zijlstra wrote:
>>> On Mon, Feb 02, 2026 at 03:11:43PM -0500, Waiman Long wrote:
>>>
>>>> @@ -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) {
>>> 		/* Serializes the static isolcpus_workfn. */
>>> 		lockdep_assert_held(&cpuset_mutex);
>> Do we require synchronization between the the queue_work() call and the
>> execution of the work function? I thought it is not needed, but I may be
>> wrong.
> Well, something needs to ensure there aren't two threads trying to use
> this one work thing at the same time, no?

isolcpus_workfn() does touches the work struct and there can't be more 
than one thread calling queue_work() with the same work. However it is 
possible that if isolcpus_workfn() and this code path are completely 
async, there is a chance that we may miss a call to 
housekeeping_update(). So I need to take a further look into that.

Cheers,
Longman

Re: [PATCH/for-next v3 3/3] cgroup/cpuset: Call housekeeping_update() without holding cpus_read_lock

[Chen Ridong]

On 2026/2/3 4:11, Waiman Long wrote:
> The current cpuset partition code is able to dynamically update
> the sched domains of a running system and the corresponding
> HK_TYPE_DOMAIN housekeeping cpumask to perform what is essentally the
> "isolcpus=domain,..." boot command line feature at run time.
> 
> The housekeeping cpumask update requires flushing a number of different
> workqueues which may not be safe with cpus_read_lock() held as the
> workqueue flushing code may acquire cpus_read_lock() or acquiring locks
> which have locking dependency with cpus_read_lock() down the chain. Below
> is an example of such circular locking problem.
> 
>   ======================================================
>   WARNING: possible circular locking dependency detected
>   6.18.0-test+ #2 Tainted: G S
>   ------------------------------------------------------
>   test_cpuset_prs/10971 is trying to acquire lock:
>   ffff888112ba4958 ((wq_completion)sync_wq){+.+.}-{0:0}, at: touch_wq_lockdep_map+0x7a/0x180
> 
>   but task is already holding lock:
>   ffffffffae47f450 (cpuset_mutex){+.+.}-{4:4}, at: cpuset_partition_write+0x85/0x130
> 
>   which lock already depends on the new lock.
> 
>   the existing dependency chain (in reverse order) is:
>   -> #4 (cpuset_mutex){+.+.}-{4:4}:
>   -> #3 (cpu_hotplug_lock){++++}-{0:0}:
>   -> #2 (rtnl_mutex){+.+.}-{4:4}:
>   -> #1 ((work_completion)(&arg.work)){+.+.}-{0:0}:
>   -> #0 ((wq_completion)sync_wq){+.+.}-{0:0}:
> 
>   Chain exists of:
>     (wq_completion)sync_wq --> cpu_hotplug_lock --> cpuset_mutex
> 
>   5 locks held by test_cpuset_prs/10971:
>    #0: ffff88816810e440 (sb_writers#7){.+.+}-{0:0}, at: ksys_write+0xf9/0x1d0
>    #1: ffff8891ab620890 (&of->mutex#2){+.+.}-{4:4}, at: kernfs_fop_write_iter+0x260/0x5f0
>    #2: ffff8890a78b83e8 (kn->active#187){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x2b6/0x5f0
>    #3: ffffffffadf32900 (cpu_hotplug_lock){++++}-{0:0}, at: cpuset_partition_write+0x77/0x130
>    #4: ffffffffae47f450 (cpuset_mutex){+.+.}-{4:4}, at: cpuset_partition_write+0x85/0x130
> 
>   Call Trace:
>    <TASK>
>      :
>    touch_wq_lockdep_map+0x93/0x180
>    __flush_workqueue+0x111/0x10b0
>    housekeeping_update+0x12d/0x2d0
>    update_parent_effective_cpumask+0x595/0x2440
>    update_prstate+0x89d/0xce0
>    cpuset_partition_write+0xc5/0x130
>    cgroup_file_write+0x1a5/0x680
>    kernfs_fop_write_iter+0x3df/0x5f0
>    vfs_write+0x525/0xfd0
>    ksys_write+0xf9/0x1d0
>    do_syscall_64+0x95/0x520
>    entry_SYSCALL_64_after_hwframe+0x76/0x7e
> 
> To avoid such a circular locking dependency problem, we have to
> call housekeeping_update() without holding the cpus_read_lock() and
> cpuset_mutex. The current set of wq's flushed by housekeeping_update()
> may not have work functions that call cpus_read_lock() directly,
> but we are likely to extend the list of wq's that are flushed in the
> future. Moreover, the current set of work functions may hold locks that
> may have cpu_hotplug_lock down the dependency chain.
> 
> One way to do that is to defer the housekeeping_update() call after
> the current cpuset critical section has finished without holding
> cpus_read_lock. For cpuset control file write, this can be done by
> deferring it using task_work right before returning to userspace.
> 
> To enable mutual exclusion between the housekeeping_update() call and
> other cpuset control file write actions, a new top level cpuset_top_mutex
> is introduced. This new mutex will be acquired first to allow sharing
> variables used by both code paths. However, cpuset update from CPU
> hotplug can still happen in parallel with the housekeeping_update()
> call, though that should be rare in production environment.
> 
> As cpus_read_lock() is now no longer held when
> tmigr_isolated_exclude_cpumask() is called, it needs to acquire it
> directly.
> 
> The lockdep_is_cpuset_held() is also updated to check the new
> cpuset_top_mutex.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/cgroup/cpuset.c        | 103 +++++++++++++++++++++++++++-------
>  kernel/sched/isolation.c      |   4 +-
>  kernel/time/timer_migration.c |   3 +-
>  3 files changed, 86 insertions(+), 24 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index e98a2e953392..d2f51f40f87e 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -65,14 +65,28 @@ static const char * const perr_strings[] = {
>   * CPUSET Locking Convention
>   * -------------------------
>   *
> - * Below are the three global locks guarding cpuset structures in lock
> + * Below are the four global/local locks guarding cpuset structures in lock
>   * acquisition order:
> + *  - cpuset_top_mutex
>   *  - cpu_hotplug_lock (cpus_read_lock/cpus_write_lock)
>   *  - cpuset_mutex
>   *  - callback_lock (raw spinlock)
>   *
> - * A task must hold all the three locks to modify externally visible or
> - * used fields of cpusets, though some of the internally used cpuset fields
> + * As cpuset will now indirectly flush a number of different workqueues in
> + * housekeeping_update() to update housekeeping cpumasks when the set of
> + * isolated CPUs is going to be changed, it may be vulnerable to deadlock
> + * if we hold cpus_read_lock while calling into housekeeping_update().
> + *
> + * The first cpuset_top_mutex will be held except when calling into
> + * cpuset_handle_hotplug() from the CPU hotplug code where cpus_write_lock
> + * and cpuset_mutex will be held instead. The main purpose of this mutex
> + * is to prevent regular cpuset control file write actions from interfering
> + * with the call to housekeeping_update(), though CPU hotplug operation can
> + * still happen in parallel. This mutex also provides protection for some
> + * internal variables.
> + *
> + * A task must hold all the remaining three locks to modify externally visible
> + * or used fields of cpusets, though some of the internally used cpuset fields
>   * and internal variables can be modified without holding callback_lock. If only
>   * reliable read access of the externally used fields are needed, a task can
>   * hold either cpuset_mutex or callback_lock which are exposed to other
> @@ -100,6 +114,7 @@ static const char * const perr_strings[] = {
>   * cpumasks and nodemasks.
>   */
>  
> +static DEFINE_MUTEX(cpuset_top_mutex);
>  static DEFINE_MUTEX(cpuset_mutex);
>  
>  /*
> @@ -111,6 +126,8 @@ static DEFINE_MUTEX(cpuset_mutex);
>   *
>   * CSCB: Readable by holding either cpuset_mutex or callback_lock. Writable
>   *	 by holding both cpuset_mutex and callback_lock.
> + *
> + * T:	 Read/write-able by holding the cpuset_top_mutex.
>   */
>  
>  /*
> @@ -135,6 +152,13 @@ static cpumask_var_t	isolated_cpus;		/* CSCB */
>   */
>  static bool		isolated_cpus_updating;	/* RWCS */
>  
> +/*
> + * Copy of isolated_cpus to be processed by housekeeping_update()
> + */
> +static cpumask_var_t	isolated_hk_cpus;	/* T */
> +static bool		isolcpus_twork_queued;	/* T */
> +
> +
>  /*
>   * A flag to force sched domain rebuild at the end of an operation.
>   * It can be set in
> @@ -298,6 +322,7 @@ void lockdep_assert_cpuset_lock_held(void)
>   */
>  void cpuset_full_lock(void)
>  {
> +	mutex_lock(&cpuset_top_mutex);
>  	cpus_read_lock();
>  	mutex_lock(&cpuset_mutex);
>  }
> @@ -306,12 +331,13 @@ void cpuset_full_unlock(void)
>  {
>  	mutex_unlock(&cpuset_mutex);
>  	cpus_read_unlock();
> +	mutex_unlock(&cpuset_top_mutex);
>  }
>  
>  #ifdef CONFIG_LOCKDEP
>  bool lockdep_is_cpuset_held(void)
>  {
> -	return lockdep_is_held(&cpuset_mutex);
> +	return lockdep_is_held(&cpuset_top_mutex);
>  }
>  #endif
>  

void cpuset_lock(void)
{
	mutex_lock(&cpuset_mutex);
}

void cpuset_unlock(void)
{
	mutex_unlock(&cpuset_mutex);
}

void lockdep_assert_cpuset_lock_held(void)
{
	lockdep_assert_held(&cpuset_mutex);
}

A potential issue is that lockdep_is_cpuset_held() only checks cpuset_top_mutex.
In the call chain below, only cpuset_mutex is acquired:

rebuild_sched_domains_cpuslocked ---only cpuset_mutex is acquired
rebuild_sched_domains_locked
partition_sched_domains
dl_rebuild_rd_accounting
dl_rebuild_rd_accounting
dl_update_tasks_root_domain
dl_add_task_root_domain
dl_get_task_effective_cpus
housekeeping_cpumask
housekeeping_dereference_check
if (IS_ENABLED(CONFIG_CPUSETS) && lockdep_is_cpuset_held())

Since lockdep_is_cpuset_held() validates cpuset_top_mutex rather than
cpuset_mutex, could this lead to false lockdep warnings?

-- 
Best regards,
Ridong

Re: [PATCH/for-next v3 3/3] cgroup/cpuset: Call housekeeping_update() without holding cpus_read_lock

[Chen Ridong]

On 2026/2/3 4:11, Waiman Long wrote:
> --- a/kernel/time/timer_migration.c
> +++ b/kernel/time/timer_migration.c
> @@ -1559,8 +1559,6 @@ int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
>  	cpumask_var_t cpumask __free(free_cpumask_var) = CPUMASK_VAR_NULL;
>  	int cpu;
>  
> -	lockdep_assert_cpus_held();
> -
>  	if (!works)
>  		return -ENOMEM;
>  	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
> @@ -1570,6 +1568,7 @@ int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
>  	 * First set previously isolated CPUs as available (unisolate).
>  	 * This cpumask contains only CPUs that switched to available now.
>  	 */
> +	guard(cpus_read_lock)();
>  	cpumask_andnot(cpumask, cpu_online_mask, exclude_cpumask);
>  	cpumask_andnot(cpumask, cpumask, tmigr_available_cpumask);
>  

It may lead to lockdep issue.

tmigr_init_isolation
	guard(cpus_read_lock)()
	tmigr_isolated_exclude_cpumask(cpumask)
		guard(cpus_read_lock)()

-- 
Best regards,
Ridong

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

[Chen Ridong]

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@redhat.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.

> +		return;
> +	}
>  
> +	WARN_ON_ONCE(housekeeping_update(isolated_cpus) < 0);
>  	isolated_cpus_updating = false;
>  }
>  
> diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> index 5dff3ad53867..0502b156582b 100755
> --- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> +++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> @@ -245,8 +245,9 @@ TEST_MATRIX=(
>  	"C2-3:P1:S+  C3:P2  .      .     O2=0   O2=1    .      .     0 A1:2|A2:3 A1:P1|A2:P2"
>  	"C2-3:P1:S+  C3:P1  .      .     O2=0    .      .      .     0 A1:|A2:3 A1:P1|A2:P1"
>  	"C2-3:P1:S+  C3:P1  .      .     O3=0    .      .      .     0 A1:2|A2: A1:P1|A2:P1"
> -	"C2-3:P1:S+  C3:P1  .      .    T:O2=0   .      .      .     0 A1:3|A2:3 A1:P1|A2:P-1"
> -	"C2-3:P1:S+  C3:P1  .      .      .    T:O3=0   .      .     0 A1:2|A2:2 A1:P1|A2:P-1"
> +	"C2-3:P1:S+  C3:P2  .      .    T:O2=0   .      .      .     0 A1:3|A2:3 A1:P1|A2:P-2"
> +	"C1-3:P1:S+  C3:P2  .      .      .    T:O3=0   .      .     0 A1:1-2|A2:1-2 A1:P1|A2:P-2 3|"
> +	"C1-3:P1:S+  C3:P2  .      .      .    T:O3=0  O3=1    .     0 A1:1-2|A2:3 A1:P1|A2:P2  3"
>  	"$SETUP_A123_PARTITIONS    .     O1=0    .      .      .     0 A1:|A2:2|A3:3 A1:P1|A2:P1|A3:P1"
>  	"$SETUP_A123_PARTITIONS    .     O2=0    .      .      .     0 A1:1|A2:|A3:3 A1:P1|A2:P1|A3:P1"
>  	"$SETUP_A123_PARTITIONS    .     O3=0    .      .      .     0 A1:1|A2:2|A3: A1:P1|A2:P1|A3:P1"
> @@ -764,7 +765,7 @@ check_cgroup_states()
>  # only CPUs in isolated partitions as well as those that are isolated at
>  # boot time.
>  #
> -# $1 - expected isolated cpu list(s) <isolcpus1>{,<isolcpus2>}
> +# $1 - expected isolated cpu list(s) <isolcpus1>{|<isolcpus2>}
>  # <isolcpus1> - expected sched/domains value
>  # <isolcpus2> - cpuset.cpus.isolated value = <isolcpus1> if not defined
>  #
> @@ -773,6 +774,7 @@ 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 [[ $EXPECTED_ISOLCPUS = . ]]
> @@ -810,6 +812,11 @@ check_isolcpus()
>  	ISOLCPUS=
>  	EXPECTED_ISOLCPUS=$EXPECTED_SDOMAIN
>  
> +	#
> +	# The inverse of HK_TYPE_DOMAIN cpumask in $HKICPUS should match $ISOLCPUS
> +	#
> +	[[ "$ISOLCPUS" != "$HKICPUS" ]] && return 1
> +
>  	#
>  	# Use the sched domain in debugfs to check isolated CPUs, if available
>  	#

-- 
Best regards,
Ridong

Re: [PATCH/for-next v3 3/3] cgroup/cpuset: Call housekeeping_update() without holding cpus_read_lock

[Waiman Long]

On 2/3/26 9:51 PM, Chen Ridong wrote:
>
> On 2026/2/3 4:11, Waiman Long wrote:
>> --- a/kernel/time/timer_migration.c
>> +++ b/kernel/time/timer_migration.c
>> @@ -1559,8 +1559,6 @@ int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
>>   	cpumask_var_t cpumask __free(free_cpumask_var) = CPUMASK_VAR_NULL;
>>   	int cpu;
>>   
>> -	lockdep_assert_cpus_held();
>> -
>>   	if (!works)
>>   		return -ENOMEM;
>>   	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
>> @@ -1570,6 +1568,7 @@ int tmigr_isolated_exclude_cpumask(struct cpumask *exclude_cpumask)
>>   	 * First set previously isolated CPUs as available (unisolate).
>>   	 * This cpumask contains only CPUs that switched to available now.
>>   	 */
>> +	guard(cpus_read_lock)();
>>   	cpumask_andnot(cpumask, cpu_online_mask, exclude_cpumask);
>>   	cpumask_andnot(cpumask, cpumask, tmigr_available_cpumask);
>>   
> It may lead to lockdep issue.
>
> tmigr_init_isolation
> 	guard(cpus_read_lock)()
> 	tmigr_isolated_exclude_cpumask(cpumask)
> 		guard(cpus_read_lock)()
>
Good catch. I haven't set up "isolcpus" in my test environment. That is 
why this lockdep splat didn't get triggered. I will fix that in the next 
version.

Cheers,
Longman

Re: [PATCH/for-next v3 3/3] cgroup/cpuset: Call housekeeping_update() without holding cpus_read_lock

[Waiman Long]

On 2/3/26 9:44 PM, Chen Ridong wrote:
>
> On 2026/2/3 4:11, Waiman Long wrote:
>> The current cpuset partition code is able to dynamically update
>> the sched domains of a running system and the corresponding
>> HK_TYPE_DOMAIN housekeeping cpumask to perform what is essentally the
>> "isolcpus=domain,..." boot command line feature at run time.
>>
>> The housekeeping cpumask update requires flushing a number of different
>> workqueues which may not be safe with cpus_read_lock() held as the
>> workqueue flushing code may acquire cpus_read_lock() or acquiring locks
>> which have locking dependency with cpus_read_lock() down the chain. Below
>> is an example of such circular locking problem.
>>
>>    ======================================================
>>    WARNING: possible circular locking dependency detected
>>    6.18.0-test+ #2 Tainted: G S
>>    ------------------------------------------------------
>>    test_cpuset_prs/10971 is trying to acquire lock:
>>    ffff888112ba4958 ((wq_completion)sync_wq){+.+.}-{0:0}, at: touch_wq_lockdep_map+0x7a/0x180
>>
>>    but task is already holding lock:
>>    ffffffffae47f450 (cpuset_mutex){+.+.}-{4:4}, at: cpuset_partition_write+0x85/0x130
>>
>>    which lock already depends on the new lock.
>>
>>    the existing dependency chain (in reverse order) is:
>>    -> #4 (cpuset_mutex){+.+.}-{4:4}:
>>    -> #3 (cpu_hotplug_lock){++++}-{0:0}:
>>    -> #2 (rtnl_mutex){+.+.}-{4:4}:
>>    -> #1 ((work_completion)(&arg.work)){+.+.}-{0:0}:
>>    -> #0 ((wq_completion)sync_wq){+.+.}-{0:0}:
>>
>>    Chain exists of:
>>      (wq_completion)sync_wq --> cpu_hotplug_lock --> cpuset_mutex
>>
>>    5 locks held by test_cpuset_prs/10971:
>>     #0: ffff88816810e440 (sb_writers#7){.+.+}-{0:0}, at: ksys_write+0xf9/0x1d0
>>     #1: ffff8891ab620890 (&of->mutex#2){+.+.}-{4:4}, at: kernfs_fop_write_iter+0x260/0x5f0
>>     #2: ffff8890a78b83e8 (kn->active#187){.+.+}-{0:0}, at: kernfs_fop_write_iter+0x2b6/0x5f0
>>     #3: ffffffffadf32900 (cpu_hotplug_lock){++++}-{0:0}, at: cpuset_partition_write+0x77/0x130
>>     #4: ffffffffae47f450 (cpuset_mutex){+.+.}-{4:4}, at: cpuset_partition_write+0x85/0x130
>>
>>    Call Trace:
>>     <TASK>
>>       :
>>     touch_wq_lockdep_map+0x93/0x180
>>     __flush_workqueue+0x111/0x10b0
>>     housekeeping_update+0x12d/0x2d0
>>     update_parent_effective_cpumask+0x595/0x2440
>>     update_prstate+0x89d/0xce0
>>     cpuset_partition_write+0xc5/0x130
>>     cgroup_file_write+0x1a5/0x680
>>     kernfs_fop_write_iter+0x3df/0x5f0
>>     vfs_write+0x525/0xfd0
>>     ksys_write+0xf9/0x1d0
>>     do_syscall_64+0x95/0x520
>>     entry_SYSCALL_64_after_hwframe+0x76/0x7e
>>
>> To avoid such a circular locking dependency problem, we have to
>> call housekeeping_update() without holding the cpus_read_lock() and
>> cpuset_mutex. The current set of wq's flushed by housekeeping_update()
>> may not have work functions that call cpus_read_lock() directly,
>> but we are likely to extend the list of wq's that are flushed in the
>> future. Moreover, the current set of work functions may hold locks that
>> may have cpu_hotplug_lock down the dependency chain.
>>
>> One way to do that is to defer the housekeeping_update() call after
>> the current cpuset critical section has finished without holding
>> cpus_read_lock. For cpuset control file write, this can be done by
>> deferring it using task_work right before returning to userspace.
>>
>> To enable mutual exclusion between the housekeeping_update() call and
>> other cpuset control file write actions, a new top level cpuset_top_mutex
>> is introduced. This new mutex will be acquired first to allow sharing
>> variables used by both code paths. However, cpuset update from CPU
>> hotplug can still happen in parallel with the housekeeping_update()
>> call, though that should be rare in production environment.
>>
>> As cpus_read_lock() is now no longer held when
>> tmigr_isolated_exclude_cpumask() is called, it needs to acquire it
>> directly.
>>
>> The lockdep_is_cpuset_held() is also updated to check the new
>> cpuset_top_mutex.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   kernel/cgroup/cpuset.c        | 103 +++++++++++++++++++++++++++-------
>>   kernel/sched/isolation.c      |   4 +-
>>   kernel/time/timer_migration.c |   3 +-
>>   3 files changed, 86 insertions(+), 24 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index e98a2e953392..d2f51f40f87e 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -65,14 +65,28 @@ static const char * const perr_strings[] = {
>>    * CPUSET Locking Convention
>>    * -------------------------
>>    *
>> - * Below are the three global locks guarding cpuset structures in lock
>> + * Below are the four global/local locks guarding cpuset structures in lock
>>    * acquisition order:
>> + *  - cpuset_top_mutex
>>    *  - cpu_hotplug_lock (cpus_read_lock/cpus_write_lock)
>>    *  - cpuset_mutex
>>    *  - callback_lock (raw spinlock)
>>    *
>> - * A task must hold all the three locks to modify externally visible or
>> - * used fields of cpusets, though some of the internally used cpuset fields
>> + * As cpuset will now indirectly flush a number of different workqueues in
>> + * housekeeping_update() to update housekeeping cpumasks when the set of
>> + * isolated CPUs is going to be changed, it may be vulnerable to deadlock
>> + * if we hold cpus_read_lock while calling into housekeeping_update().
>> + *
>> + * The first cpuset_top_mutex will be held except when calling into
>> + * cpuset_handle_hotplug() from the CPU hotplug code where cpus_write_lock
>> + * and cpuset_mutex will be held instead. The main purpose of this mutex
>> + * is to prevent regular cpuset control file write actions from interfering
>> + * with the call to housekeeping_update(), though CPU hotplug operation can
>> + * still happen in parallel. This mutex also provides protection for some
>> + * internal variables.
>> + *
>> + * A task must hold all the remaining three locks to modify externally visible
>> + * or used fields of cpusets, though some of the internally used cpuset fields
>>    * and internal variables can be modified without holding callback_lock. If only
>>    * reliable read access of the externally used fields are needed, a task can
>>    * hold either cpuset_mutex or callback_lock which are exposed to other
>> @@ -100,6 +114,7 @@ static const char * const perr_strings[] = {
>>    * cpumasks and nodemasks.
>>    */
>>   
>> +static DEFINE_MUTEX(cpuset_top_mutex);
>>   static DEFINE_MUTEX(cpuset_mutex);
>>   
>>   /*
>> @@ -111,6 +126,8 @@ static DEFINE_MUTEX(cpuset_mutex);
>>    *
>>    * CSCB: Readable by holding either cpuset_mutex or callback_lock. Writable
>>    *	 by holding both cpuset_mutex and callback_lock.
>> + *
>> + * T:	 Read/write-able by holding the cpuset_top_mutex.
>>    */
>>   
>>   /*
>> @@ -135,6 +152,13 @@ static cpumask_var_t	isolated_cpus;		/* CSCB */
>>    */
>>   static bool		isolated_cpus_updating;	/* RWCS */
>>   
>> +/*
>> + * Copy of isolated_cpus to be processed by housekeeping_update()
>> + */
>> +static cpumask_var_t	isolated_hk_cpus;	/* T */
>> +static bool		isolcpus_twork_queued;	/* T */
>> +
>> +
>>   /*
>>    * A flag to force sched domain rebuild at the end of an operation.
>>    * It can be set in
>> @@ -298,6 +322,7 @@ void lockdep_assert_cpuset_lock_held(void)
>>    */
>>   void cpuset_full_lock(void)
>>   {
>> +	mutex_lock(&cpuset_top_mutex);
>>   	cpus_read_lock();
>>   	mutex_lock(&cpuset_mutex);
>>   }
>> @@ -306,12 +331,13 @@ void cpuset_full_unlock(void)
>>   {
>>   	mutex_unlock(&cpuset_mutex);
>>   	cpus_read_unlock();
>> +	mutex_unlock(&cpuset_top_mutex);
>>   }
>>   
>>   #ifdef CONFIG_LOCKDEP
>>   bool lockdep_is_cpuset_held(void)
>>   {
>> -	return lockdep_is_held(&cpuset_mutex);
>> +	return lockdep_is_held(&cpuset_top_mutex);
>>   }
>>   #endif
>>   
> void cpuset_lock(void)
> {
> 	mutex_lock(&cpuset_mutex);
> }
>
> void cpuset_unlock(void)
> {
> 	mutex_unlock(&cpuset_mutex);
> }
>
> void lockdep_assert_cpuset_lock_held(void)
> {
> 	lockdep_assert_held(&cpuset_mutex);
> }
>
> A potential issue is that lockdep_is_cpuset_held() only checks cpuset_top_mutex.
> In the call chain below, only cpuset_mutex is acquired:
>
> rebuild_sched_domains_cpuslocked ---only cpuset_mutex is acquired
> rebuild_sched_domains_locked
> partition_sched_domains
> dl_rebuild_rd_accounting
> dl_rebuild_rd_accounting
> dl_update_tasks_root_domain
> dl_add_task_root_domain
> dl_get_task_effective_cpus
> housekeeping_cpumask
> housekeeping_dereference_check
> if (IS_ENABLED(CONFIG_CPUSETS) && lockdep_is_cpuset_held())
>
> Since lockdep_is_cpuset_held() validates cpuset_top_mutex rather than
> cpuset_mutex, could this lead to false lockdep warnings?

Right, it should check for either cpuset_mutex or cpuset_top_mutex.

Thanks,
Longman

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

[Waiman Long]

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@redhat.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