[PATCH v8 00/11] cgroup/cpuset: Support multiple source/destination cpusets for cpuset_*attach()

[PATCH v8 00/11] cgroup/cpuset: Support multiple source/destination cpusets for cpuset_*attach()

[Waiman Long]

 v8:
  - Update patch 3 to make attach_in_progress a global flag which block
    all cpuset write operations until the attach is done.
  - Add a new patch 4 to collect most of the attach related variables
    into a attach_ctx structure as suggested by Ridong.
  - Add a test_cpuset selftest patch contributed by Michal Koutný.
  - Keep the existing Reviewed-by tags as there is no functional change
    to the other patches other than rename of some variables. Also add
    some new tags.

 v7: https://lore.kernel.org/lkml/20260621032816.1806773-1-longman@redhat.com/
  - Include the fix patch from Farhad Alemi to fix a div/0 crash that
    was part of the old patch 1.
  - Integrated v6 patch 7 into earlier patches.
  - Add a new "cgroup/cpuset: Prevent race between task attach and
    cpuset state change" patch to ensure that there will be no cpuset
    state change between cpuset_can_attach() and cpuset_attach() or
    cpuset_cancel_attach().
  - Break v6 patch 6 into 2 separate patches for supporting multiple
    source cpusets and multiple destination cpusets respectively and
    further simplify and streamline the code.

 v6:
  - Make guarantee_online_mems() to only return cs->effective_mems with v2
    in patch 1.
  - Remove obsolete commit description text from patch 3.
  - Add Reviewed-by tags.
  - In patch 6, add WARN_ON_ONCE() test in cpuset_can_attach() to
    confirm that cs != oldcs.

Sashiko AI review of another cpuset patch had found that cpuset_attach()
and cpuset_can_attach() can be passed a cgroup_taskset with tasks
migrating from one source cpuset to multiple destination cpusets and
vice versa.  Further testing of the cpuset code indicates that this is
indeed the case when the v2 cpuset controller is enabled or disabled.

Unfortunately, cpuset_attach() and cpuset_can_attach() still assume that
there will be one source and one destinaton cpuset which may result in
inocrrect behavior.

This patch series is created to fix this issue.

Patch 1 is a fix that fix a cgroup v2 div/0 crash due to bug in
cpuset_update_tasks_nodemask().

Patch 2 is to fix an inconsistency in the way node mask update is being
handled in cpuset_update_tasks_nodemask() and cpuset_attach() so that
they match each other.

Patch 3 makes any cpuset state change to wait for the completion of the
pending cpuset attach operation, if any.

Patch 4 collects task attach related data into a common attach_ctx
structure.

Patches 5 and 6 are just preparatory patches to make the remaining
patches easier to review.

Patch 7 makes attach_ctx.old_cs track group leader for use by
cpuset_migrate_mm().

Patch 8 moves mpol_rebind_mm() and cpuset_migrate_mm() inside
cpuset_attach_task() to make CLONE_INTO_CGROUP flag of clone(2) works
more like moving task from one cpuset to another one, while also make
supporting multiple source and destination cpusets easier.

Patch 9 makes the necessary changes to enable the support of multiple
source cpusets by keeping all the source cpusets found during task
iterations in a singly linked lists.

Patch 10 enables the support of multiple destination cpusets during the
the enabling of cpuset v2 controller.

Patch 11 adds new test case to test_cpuset to test disabling of cpuset
controller.

Farhad Alemi (1):
  cgroup/cpuset: rebind mm mempolicy to effective_mems, not mems_allowed

Michal Koutný (1):
  selftests/cgroup: Add test for cpuset affinity on controller disable

Waiman Long (9):
  cgroup/cpuset: Fix node inconsistencies between
    cpuset_update_tasks_nodemask() and cpuset_attach()
  cgroup/cpuset: Prevent race between task attach and cpuset state
    change
  cgroup/cpuset: Put all task attach related variables into attach_ctx
  cgroup/cpuset: Add a cpuset_reserve_dl_bw() helper
  cgroup/cpuset: Expand the scope of cpuset_can_attach_check()
  cgroup/cpuset: Make attach_ctx.old_cs track task group leader
  cgroup/cpuset: Move mpol_rebind_mm/cpuset_migrate_mm() calls inside
    cpuset_attach_task()
  cgroup/cpuset: Support multiple source cpusets for cpuset_*attach()
  cgroup/cpuset: Support multiple destination cpusets for
    cpuset_*attach()

 kernel/cgroup/cpuset-internal.h              |  11 +-
 kernel/cgroup/cpuset.c                       | 442 ++++++++++++-------
 tools/testing/selftests/cgroup/test_cpuset.c | 243 ++++++++++
 3 files changed, 541 insertions(+), 155 deletions(-)

-- 
2.54.0

[PATCH v8 01/11] cgroup/cpuset: rebind mm mempolicy to effective_mems, not mems_allowed

[Waiman Long]

From: Farhad Alemi <farhad.alemi@berkeley.edu>

Creating a child cpuset where cpuset.mems is never set leads to a div/0
when a VMA mempolicy with MPOL_F_RELATIVE_NODES rebinds in response to a
CPU hotplug event.

Reproduction steps:
 1) Create a cgroup w/ cpuset controls (do not set cpuset.mems)
 2) Move the task into the child cpuset
 3) Create a VMA mempolicy for that task with MPOL_F_RELATIVE_NODES
 4) unplug and hotplug a cpu
      echo 0 > /sys/devices/system/cpu/cpu1/online
      echo 1 > /sys/devices/system/cpu/cpu1/online
 5) mempolicy rebind does a div/0 in mpol_relative_nodemask on the
    call to __nodes_fold()

The cpuset code passes (cs->mems_allowed) which is not guaranteed to have
nodes to the rebind routine.  Use cs->effective_mems instead, which is
guaranteed to have a non-empty nodemask.

Closes: https://lore.kernel.org/linux-mm/CA+0ovCgxbZkXa+OU8w3s84R3KNPNxxRfmsNR-udh+afQBbGNmw@mail.gmail.com/
Link: https://lore.kernel.org/all/CA+0ovCiEz6SP_sn3kN4Tb+_oC=eHMXy_Ffj=usV3wREdQrUtww@mail.gmail.com/
Fixes: ae1c802382f7 ("cpuset: apply cs->effective_{cpus,mems}")
Suggested-by: Gregory Price <gourry@gourry.net>
Suggested-by: Waiman Long <longman@redhat.com>
Signed-off-by: Farhad Alemi <farhad.alemi@berkeley.edu>
Acked-by: Waiman Long <longman@redhat.com>
Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
Reviewed-by: Gregory Price <gourry@gourry.net>
Cc: stable@vger.kernel.org
---
 kernel/cgroup/cpuset.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index e53f35e2726f..49d8564d1a48 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2671,7 +2671,7 @@ void cpuset_update_tasks_nodemask(struct cpuset *cs)
 
 		migrate = is_memory_migrate(cs);
 
-		mpol_rebind_mm(mm, &cs->mems_allowed);
+		mpol_rebind_mm(mm, &cs->effective_mems);
 		if (migrate)
 			cpuset_migrate_mm(mm, &cs->old_mems_allowed, &newmems);
 		else
-- 
2.54.0

[PATCH v8 02/11] cgroup/cpuset: Fix node inconsistencies between cpuset_update_tasks_nodemask() and cpuset_attach()

[Waiman Long]

Whenever memory node mask is changed, there are 4 places where the node
mask has to be updated or used.
 1) task's node mask via cpuset_change_task_nodemask()
 2) memory policy binding via mpol_rebind_mm()
 3) if memory migration is enabled, migrate from old_mems_allowed to
    the new node mask via cpuset_migrate_mm().
 4) setting old_mems_allowed

These memory actions are done in cpuset_update_tasks_nodemask() and
cpuset_attach(). However there are inconsistencies in what node masks
are being used in these 2 functions.

In cpuset_update_tasks_nodemask(),
 - cpuset_change_task_nodemask(): guarantee_online_mems()
 - mpol_rebind_mm(): mems_allowed
 - cpuset_migrate_mm(): guarantee_online_mems()
 - old_mems_allowed: guarantee_online_mems()

In cpuset_attach(),
 - cpuset_change_task_nodemask(): guarantee_online_mems()
 - mpol_rebind_mm(): effective_mems
 - cpuset_migrate_mm(): effective_mems
 - old_mems_allowed: effective_mems

These inconsistencies dates back to quite a long time ago and it is
hard to say what should be the correct values.

The guarantee_online_mems() function returns a node mask from current or
an ancestor cpuset that is a subset of node_states[N_MEMORY]. Nodes in
node_states[N_MEMORY] are all online, i.e. in node_states[N_ONLINE].
However, node in node_states[N_ONLINE] may not have memory. So
node_states[N_MEMORY] should be a subset of node_states[N_ONLINE].

The guarantee_online_mems() function should mostly be useful for v1
where mems_allowed is the same as effective_mems. With v2, the memory
nodes in effective_mems should be a subset of node_states[N_MEMORY]
except when a memory hot-unplug operation is in progress and a memory
node is removed from node_states[N_MEMORY] but not yet reflected in
the effective_mems's as cpuset_handle_hotplug() has not been called
from cpuset_track_online_nodes().

Let use the following setup for both of them and make them consistent.
 - cpuset_change_task_nodemask(): guarantee_online_mems()
 - mpol_rebind_mm(): effective_mems
 - cpuset_migrate_mm(): guarantee_online_mems()
 - old_mems_allowed: guarantee_online_mems()

So for v2, it is effectively all effective_mems most of the time. For
v1, mpol_rebind_mm() uses mems_allowed which may differ from what
guarantee_online_mems() returns, but it conforms to what the cpuset v1
documentation says with respect to setting memory policy.

Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
Reviewed-by: Gregory Price <gourry@gourry.net>
---
 kernel/cgroup/cpuset.c | 30 ++++++++++++++++++------------
 1 file changed, 18 insertions(+), 12 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 49d8564d1a48..d108c2083e86 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -489,7 +489,10 @@ static void guarantee_active_cpus(struct task_struct *tsk,
  * Return in *pmask the portion of a cpusets's mems_allowed that
  * are online, with memory.  If none are online with memory, walk
  * up the cpuset hierarchy until we find one that does have some
- * online mems.  The top cpuset always has some mems online.
+ * online mems.  The top cpuset always has some mems online. With v2,
+ * effective_mems should always contain online memory nodes except
+ * during the transition period where a memory node hotunplug operation
+ * is in progress.
  *
  * One way or another, we guarantee to return some non-empty subset
  * of node_states[N_MEMORY].
@@ -2633,6 +2636,14 @@ static void *cpuset_being_rebound;
  * Iterate through each task of @cs updating its mems_allowed to the
  * effective cpuset's.  As this function is called with cpuset_mutex held,
  * cpuset membership stays stable.
+ *
+ * - cpuset_change_task_nodemask(): guarantee_online_mems()
+ * - mpol_rebind_mm(): effective_mems
+ * - cpuset_migrate_mm(): guarantee_online_mems()
+ * - old_mems_allowed: guarantee_online_mems()
+ *
+ * For v2, guarantee_online_mems() should return a node mask that is the same
+ * as the effective_mems of current cpuset.
  */
 void cpuset_update_tasks_nodemask(struct cpuset *cs)
 {
@@ -2641,7 +2652,6 @@ void cpuset_update_tasks_nodemask(struct cpuset *cs)
 	struct task_struct *task;
 
 	cpuset_being_rebound = cs;		/* causes mpol_dup() rebind */
-
 	guarantee_online_mems(cs, &newmems);
 
 	/*
@@ -3159,19 +3169,16 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 	cpus_updated = !cpumask_equal(cs->effective_cpus,
 				      oldcs->effective_cpus);
 	mems_updated = !nodes_equal(cs->effective_mems, oldcs->effective_mems);
+	guarantee_online_mems(cs, &cpuset_attach_nodemask_to);
 
 	/*
 	 * In the default hierarchy, enabling cpuset in the child cgroups
-	 * will trigger a number of cpuset_attach() calls with no change
-	 * in effective cpus and mems. In that case, we can optimize out
-	 * by skipping the task iteration and update.
+	 * will trigger a cpuset_attach() call with no change in effective cpus
+	 * and mems. In that case, we can optimize out by skipping the task
+	 * iteration and update.
 	 */
-	if (cpuset_v2() && !cpus_updated && !mems_updated) {
-		cpuset_attach_nodemask_to = cs->effective_mems;
+	if (cpuset_v2() && !cpus_updated && !mems_updated)
 		goto out;
-	}
-
-	guarantee_online_mems(cs, &cpuset_attach_nodemask_to);
 
 	cgroup_taskset_for_each(task, css, tset)
 		cpuset_attach_task(cs, task);
@@ -3182,7 +3189,6 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 	 * if there is no change in effective_mems and CS_MEMORY_MIGRATE is
 	 * not set.
 	 */
-	cpuset_attach_nodemask_to = cs->effective_mems;
 	if (!is_memory_migrate(cs) && !mems_updated)
 		goto out;
 
@@ -3190,7 +3196,7 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 		struct mm_struct *mm = get_task_mm(leader);
 
 		if (mm) {
-			mpol_rebind_mm(mm, &cpuset_attach_nodemask_to);
+			mpol_rebind_mm(mm, &cs->effective_mems);
 
 			/*
 			 * old_mems_allowed is the same with mems_allowed
-- 
2.54.0

[PATCH v8 03/11] cgroup/cpuset: Prevent race between task attach and cpuset state change

[Waiman Long]

Commit e44193d39e8d ("cpuset: let hotplug propagation work wait for
task attaching") was introduced to let hotplug operation to wait
until the completion of task attach operation. However, it is still
possible that the states of the source or destination cpuset can
be changed between the cpuset_can_attach() call and the subsequent
cpuset_attach()/cpuset_cacnel_attach() call.

As a result, data gathered during cpuset_can_attach() cannot be reliably
used in the subsequent cpuset_attach()/cpuset_cacnel_attach()
call at all. Make the task attach operation more robust
and allow the sharing of data between cpuset_can_attach() and
cpuset_attach()/cpuset_cacnel_attach() by making cpuset_write_resmask()
and cpuset_partition_write() wait for the completion of task attach
as well.

Ideally, an ongoing task attach operation should block any cpuset write
operation that can change its internal state until the operation is
completed. However, the attach_in_progress flag is currently per cpuset
and only the destination cpuset will have this flag set. The flag is not
set in the source cpuset where the tasks will be moved from. Even if we
extend the scope to include the source cpuset, it will not block cpuset
operation that changes the state of one of its ancestor cpuset which may
indirectly impact the state of the source or destination cpuset. It may
be too costly to set the flag for the whole subtree, it is far easier
to just make the flag global and block all the cpuset write operation
whenever a task attach operation is in progress. Make that change by
creating a new cpuset attach context (attach_ctx) structure to hold the
global in_progress flag and use it for blocking cpuset write operation
if a cpuset attach operation is in progress.

The comments about validate_change() are no longer valid as it won't
be called at all if an attach operation is in progress. So the comments
can be removed.

The per-cpuset attach_in_progress flag is also currently used in
partition_is_populated() and cpuset_is_populated() to determine if
an empty cpuset will have incoming task. This check will no longer be
needed as this function will not be called when there is a task attach
in progress. So the flag check is now removed.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset-internal.h | 11 +-----
 kernel/cgroup/cpuset.c          | 68 +++++++++++++++++++++------------
 2 files changed, 44 insertions(+), 35 deletions(-)

diff --git a/kernel/cgroup/cpuset-internal.h b/kernel/cgroup/cpuset-internal.h
index f7aaf01f7cd5..817b86ba7019 100644
--- a/kernel/cgroup/cpuset-internal.h
+++ b/kernel/cgroup/cpuset-internal.h
@@ -145,12 +145,6 @@ struct cpuset {
 	 */
 	nodemask_t old_mems_allowed;
 
-	/*
-	 * Tasks are being attached to this cpuset.  Used to prevent
-	 * zeroing cpus/mems_allowed between ->can_attach() and ->attach().
-	 */
-	int attach_in_progress;
-
 	/* partition root state */
 	int partition_root_state;
 
@@ -269,10 +263,7 @@ static inline int nr_cpusets(void)
 static inline bool cpuset_is_populated(struct cpuset *cs)
 {
 	lockdep_assert_cpuset_lock_held();
-
-	/* Cpusets in the process of attaching should be considered as populated */
-	return cgroup_is_populated(cs->css.cgroup) ||
-		cs->attach_in_progress;
+	return cgroup_is_populated(cs->css.cgroup);
 }
 
 /**
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index d108c2083e86..dec9785d0271 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -356,6 +356,14 @@ static struct workqueue_struct *cpuset_migrate_mm_wq;
 
 static DECLARE_WAIT_QUEUE_HEAD(cpuset_attach_wq);
 
+/*
+ * Cpuset task attach context
+ * Protected by cpuset_mutex
+ */
+static struct {
+	int in_progress;
+} attach_ctx;
+
 static inline void check_insane_mems_config(nodemask_t *nodes)
 {
 	if (!cpusets_insane_config() &&
@@ -368,22 +376,22 @@ static inline void check_insane_mems_config(nodemask_t *nodes)
 }
 
 /*
- * decrease cs->attach_in_progress.
- * wake_up cpuset_attach_wq if cs->attach_in_progress==0.
+ * decrease attach_ctx.in_progress.
+ * wake_up cpuset_attach_wq if attach_ctx.in_progress==0.
  */
-static inline void dec_attach_in_progress_locked(struct cpuset *cs)
+static inline void dec_attach_in_progress_locked(void)
 {
 	lockdep_assert_cpuset_lock_held();
 
-	cs->attach_in_progress--;
-	if (!cs->attach_in_progress)
+	attach_ctx.in_progress--;
+	if (!attach_ctx.in_progress)
 		wake_up(&cpuset_attach_wq);
 }
 
-static inline void dec_attach_in_progress(struct cpuset *cs)
+static inline void dec_attach_in_progress(void)
 {
 	mutex_lock(&cpuset_mutex);
-	dec_attach_in_progress_locked(cs);
+	dec_attach_in_progress_locked();
 	mutex_unlock(&cpuset_mutex);
 }
 
@@ -432,8 +440,7 @@ static inline bool partition_is_populated(struct cpuset *cs,
 	 * nr_populated_domain_children may include populated
 	 * csets from descendants that are partitions.
 	 */
-	if (cgroup_has_tasks(cs->css.cgroup) ||
-	    cs->attach_in_progress)
+	if (cgroup_has_tasks(cs->css.cgroup))
 		return true;
 
 	rcu_read_lock();
@@ -3091,11 +3098,7 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 	cs->dl_bw_cpu = cpu;
 
 out_success:
-	/*
-	 * Mark attach is in progress.  This makes validate_change() fail
-	 * changes which zero cpus/mems_allowed.
-	 */
-	cs->attach_in_progress++;
+	attach_ctx.in_progress++;
 
 out_unlock:
 	if (ret)
@@ -3113,7 +3116,7 @@ static void cpuset_cancel_attach(struct cgroup_taskset *tset)
 	cs = css_cs(css);
 
 	mutex_lock(&cpuset_mutex);
-	dec_attach_in_progress_locked(cs);
+	dec_attach_in_progress_locked();
 
 	if (cs->dl_bw_cpu >= 0)
 		dl_bw_free(cs->dl_bw_cpu, cs->sum_migrate_dl_bw);
@@ -3226,7 +3229,7 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 		reset_migrate_dl_data(cs);
 	}
 
-	dec_attach_in_progress_locked(cs);
+	dec_attach_in_progress_locked();
 
 	mutex_unlock(&cpuset_mutex);
 }
@@ -3246,10 +3249,19 @@ ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
 		return -EACCES;
 
 	buf = strstrip(buf);
+retry:
+	wait_event(cpuset_attach_wq, attach_ctx.in_progress == 0);
+
 	cpuset_full_lock();
 	if (!is_cpuset_online(cs))
 		goto out_unlock;
 
+	/* Don't race with task attach */
+	if (attach_ctx.in_progress) {
+		cpuset_full_unlock();
+		goto retry;
+	}
+
 	trialcs = dup_or_alloc_cpuset(cs);
 	if (!trialcs) {
 		retval = -ENOMEM;
@@ -3377,7 +3389,17 @@ static ssize_t cpuset_partition_write(struct kernfs_open_file *of, char *buf,
 	else
 		return -EINVAL;
 
+retry:
+	wait_event(cpuset_attach_wq, attach_ctx.in_progress == 0);
+
 	cpuset_full_lock();
+
+	/* Don't race with task attach */
+	if (attach_ctx.in_progress) {
+		cpuset_full_unlock();
+		goto retry;
+	}
+
 	if (is_cpuset_online(cs))
 		retval = update_prstate(cs, val);
 	cpuset_update_sd_hk_unlock();
@@ -3616,11 +3638,7 @@ static int cpuset_can_fork(struct task_struct *task, struct css_set *cset)
 	if (ret)
 		goto out_unlock;
 
-	/*
-	 * Mark attach is in progress.  This makes validate_change() fail
-	 * changes which zero cpus/mems_allowed.
-	 */
-	cs->attach_in_progress++;
+	attach_ctx.in_progress++;
 out_unlock:
 	mutex_unlock(&cpuset_mutex);
 	return ret;
@@ -3638,7 +3656,7 @@ static void cpuset_cancel_fork(struct task_struct *task, struct css_set *cset)
 	if (same_cs)
 		return;
 
-	dec_attach_in_progress(cs);
+	dec_attach_in_progress();
 }
 
 /*
@@ -3670,7 +3688,7 @@ static void cpuset_fork(struct task_struct *task)
 	guarantee_online_mems(cs, &cpuset_attach_nodemask_to);
 	cpuset_attach_task(cs, task);
 
-	dec_attach_in_progress_locked(cs);
+	dec_attach_in_progress_locked();
 	mutex_unlock(&cpuset_mutex);
 }
 
@@ -3775,7 +3793,7 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
 	int partcmd = -1;
 	struct cpuset *parent;
 retry:
-	wait_event(cpuset_attach_wq, cs->attach_in_progress == 0);
+	wait_event(cpuset_attach_wq, attach_ctx.in_progress == 0);
 
 	mutex_lock(&cpuset_mutex);
 
@@ -3783,7 +3801,7 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
 	 * We have raced with task attaching. We wait until attaching
 	 * is finished, so we won't attach a task to an empty cpuset.
 	 */
-	if (cs->attach_in_progress) {
+	if (attach_ctx.in_progress) {
 		mutex_unlock(&cpuset_mutex);
 		goto retry;
 	}
-- 
2.54.0

Re: [PATCH v8 03/11] cgroup/cpuset: Prevent race between task attach and cpuset state change

[Ridong Chen]

On 6/27/2026 2:19 AM, Waiman Long wrote:
> Commit e44193d39e8d ("cpuset: let hotplug propagation work wait for
> task attaching") was introduced to let hotplug operation to wait
> until the completion of task attach operation. However, it is still
> possible that the states of the source or destination cpuset can
> be changed between the cpuset_can_attach() call and the subsequent
> cpuset_attach()/cpuset_cacnel_attach() call.
> 
> As a result, data gathered during cpuset_can_attach() cannot be reliably
> used in the subsequent cpuset_attach()/cpuset_cacnel_attach()
> call at all. Make the task attach operation more robust
> and allow the sharing of data between cpuset_can_attach() and
> cpuset_attach()/cpuset_cacnel_attach() by making cpuset_write_resmask()
> and cpuset_partition_write() wait for the completion of task attach
> as well.
> 
> Ideally, an ongoing task attach operation should block any cpuset write
> operation that can change its internal state until the operation is
> completed. However, the attach_in_progress flag is currently per cpuset
> and only the destination cpuset will have this flag set. The flag is not
> set in the source cpuset where the tasks will be moved from. Even if we
> extend the scope to include the source cpuset, it will not block cpuset
> operation that changes the state of one of its ancestor cpuset which may
> indirectly impact the state of the source or destination cpuset. It may
> be too costly to set the flag for the whole subtree, it is far easier
> to just make the flag global and block all the cpuset write operation
> whenever a task attach operation is in progress. Make that change by
> creating a new cpuset attach context (attach_ctx) structure to hold the
> global in_progress flag and use it for blocking cpuset write operation
> if a cpuset attach operation is in progress.
> 
> The comments about validate_change() are no longer valid as it won't
> be called at all if an attach operation is in progress. So the comments
> can be removed.
> 
> The per-cpuset attach_in_progress flag is also currently used in
> partition_is_populated() and cpuset_is_populated() to determine if
> an empty cpuset will have incoming task. This check will no longer be
> needed as this function will not be called when there is a task attach
> in progress. So the flag check is now removed.
> 
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>   kernel/cgroup/cpuset-internal.h | 11 +-----
>   kernel/cgroup/cpuset.c          | 68 +++++++++++++++++++++------------
>   2 files changed, 44 insertions(+), 35 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset-internal.h b/kernel/cgroup/cpuset-internal.h
> index f7aaf01f7cd5..817b86ba7019 100644
> --- a/kernel/cgroup/cpuset-internal.h
> +++ b/kernel/cgroup/cpuset-internal.h
> @@ -145,12 +145,6 @@ struct cpuset {
>   	 */
>   	nodemask_t old_mems_allowed;
>   
> -	/*
> -	 * Tasks are being attached to this cpuset.  Used to prevent
> -	 * zeroing cpus/mems_allowed between ->can_attach() and ->attach().
> -	 */
> -	int attach_in_progress;
> -
>   	/* partition root state */
>   	int partition_root_state;
>   
> @@ -269,10 +263,7 @@ static inline int nr_cpusets(void)
>   static inline bool cpuset_is_populated(struct cpuset *cs)
>   {
>   	lockdep_assert_cpuset_lock_held();
> -
> -	/* Cpusets in the process of attaching should be considered as populated */
> -	return cgroup_is_populated(cs->css.cgroup) ||
> -		cs->attach_in_progress;
> +	return cgroup_is_populated(cs->css.cgroup);
>   }
>   
>   /**
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index d108c2083e86..dec9785d0271 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -356,6 +356,14 @@ static struct workqueue_struct *cpuset_migrate_mm_wq;
>   
>   static DECLARE_WAIT_QUEUE_HEAD(cpuset_attach_wq);
>   
> +/*
> + * Cpuset task attach context
> + * Protected by cpuset_mutex
> + */
> +static struct {
> +	int in_progress;
> +} attach_ctx;
> +
>   static inline void check_insane_mems_config(nodemask_t *nodes)
>   {
>   	if (!cpusets_insane_config() &&
> @@ -368,22 +376,22 @@ static inline void check_insane_mems_config(nodemask_t *nodes)
>   }
>   
>   /*
> - * decrease cs->attach_in_progress.
> - * wake_up cpuset_attach_wq if cs->attach_in_progress==0.
> + * decrease attach_ctx.in_progress.
> + * wake_up cpuset_attach_wq if attach_ctx.in_progress==0.
>    */
> -static inline void dec_attach_in_progress_locked(struct cpuset *cs)
> +static inline void dec_attach_in_progress_locked(void)
>   {
>   	lockdep_assert_cpuset_lock_held();
>   
> -	cs->attach_in_progress--;
> -	if (!cs->attach_in_progress)
> +	attach_ctx.in_progress--;
> +	if (!attach_ctx.in_progress)
>   		wake_up(&cpuset_attach_wq);
>   }
>   
> -static inline void dec_attach_in_progress(struct cpuset *cs)
> +static inline void dec_attach_in_progress(void)
>   {
>   	mutex_lock(&cpuset_mutex);
> -	dec_attach_in_progress_locked(cs);
> +	dec_attach_in_progress_locked();
>   	mutex_unlock(&cpuset_mutex);
>   }
>   
> @@ -432,8 +440,7 @@ static inline bool partition_is_populated(struct cpuset *cs,
>   	 * nr_populated_domain_children may include populated
>   	 * csets from descendants that are partitions.
>   	 */
> -	if (cgroup_has_tasks(cs->css.cgroup) ||
> -	    cs->attach_in_progress)
> +	if (cgroup_has_tasks(cs->css.cgroup))
>   		return true;
>   
>   	rcu_read_lock();
> @@ -3091,11 +3098,7 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
>   	cs->dl_bw_cpu = cpu;
>   
>   out_success:
> -	/*
> -	 * Mark attach is in progress.  This makes validate_change() fail
> -	 * changes which zero cpus/mems_allowed.
> -	 */
> -	cs->attach_in_progress++;
> +	attach_ctx.in_progress++;
>   
>   out_unlock:
>   	if (ret)
> @@ -3113,7 +3116,7 @@ static void cpuset_cancel_attach(struct cgroup_taskset *tset)
>   	cs = css_cs(css);
>   
>   	mutex_lock(&cpuset_mutex);
> -	dec_attach_in_progress_locked(cs);
> +	dec_attach_in_progress_locked();
>   
>   	if (cs->dl_bw_cpu >= 0)
>   		dl_bw_free(cs->dl_bw_cpu, cs->sum_migrate_dl_bw);
> @@ -3226,7 +3229,7 @@ static void cpuset_attach(struct cgroup_taskset *tset)
>   		reset_migrate_dl_data(cs);
>   	}
>   
> -	dec_attach_in_progress_locked(cs);
> +	dec_attach_in_progress_locked();
>   
>   	mutex_unlock(&cpuset_mutex);
>   }
> @@ -3246,10 +3249,19 @@ ssize_t cpuset_write_resmask(struct kernfs_open_file *of,
>   		return -EACCES;
>   
>   	buf = strstrip(buf);
> +retry:
> +	wait_event(cpuset_attach_wq, attach_ctx.in_progress == 0);
> +
>   	cpuset_full_lock();
>   	if (!is_cpuset_online(cs))
>   		goto out_unlock;
>   
> +	/* Don't race with task attach */
> +	if (attach_ctx.in_progress) {
> +		cpuset_full_unlock();
> +		goto retry;
> +	}
> +
>   	trialcs = dup_or_alloc_cpuset(cs);
>   	if (!trialcs) {
>   		retval = -ENOMEM;
> @@ -3377,7 +3389,17 @@ static ssize_t cpuset_partition_write(struct kernfs_open_file *of, char *buf,
>   	else
>   		return -EINVAL;
>   
> +retry:
> +	wait_event(cpuset_attach_wq, attach_ctx.in_progress == 0);
> +
>   	cpuset_full_lock();
> +
> +	/* Don't race with task attach */
> +	if (attach_ctx.in_progress) {
> +		cpuset_full_unlock();
> +		goto retry;
> +	}
> +

Would it make sense to add a helper like wait_attach_done_locked()?

>   	if (is_cpuset_online(cs))
>   		retval = update_prstate(cs, val);
>   	cpuset_update_sd_hk_unlock();
> @@ -3616,11 +3638,7 @@ static int cpuset_can_fork(struct task_struct *task, struct css_set *cset)
>   	if (ret)
>   		goto out_unlock;
>   
> -	/*
> -	 * Mark attach is in progress.  This makes validate_change() fail
> -	 * changes which zero cpus/mems_allowed.
> -	 */
> -	cs->attach_in_progress++;
> +	attach_ctx.in_progress++;
>   out_unlock:
>   	mutex_unlock(&cpuset_mutex);
>   	return ret;
> @@ -3638,7 +3656,7 @@ static void cpuset_cancel_fork(struct task_struct *task, struct css_set *cset)
>   	if (same_cs)
>   		return;
>   
> -	dec_attach_in_progress(cs);
> +	dec_attach_in_progress();
>   }
>   
>   /*
> @@ -3670,7 +3688,7 @@ static void cpuset_fork(struct task_struct *task)
>   	guarantee_online_mems(cs, &cpuset_attach_nodemask_to);
>   	cpuset_attach_task(cs, task);
>   
> -	dec_attach_in_progress_locked(cs);
> +	dec_attach_in_progress_locked();
>   	mutex_unlock(&cpuset_mutex);
>   }
>   
> @@ -3775,7 +3793,7 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
>   	int partcmd = -1;
>   	struct cpuset *parent;
>   retry:
> -	wait_event(cpuset_attach_wq, cs->attach_in_progress == 0);
> +	wait_event(cpuset_attach_wq, attach_ctx.in_progress == 0);
>   
>   	mutex_lock(&cpuset_mutex);
>   
> @@ -3783,7 +3801,7 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
>   	 * We have raced with task attaching. We wait until attaching
>   	 * is finished, so we won't attach a task to an empty cpuset.
>   	 */
> -	if (cs->attach_in_progress) {
> +	if (attach_ctx.in_progress) {
>   		mutex_unlock(&cpuset_mutex);
>   		goto retry;
>   	}

-- 
Best regards
Ridong

Re: [PATCH v8 03/11] cgroup/cpuset: Prevent race between task attach and cpuset state change

[Waiman Long]

On 6/29/26 3:14 AM, Ridong Chen wrote:
>
>
> On 6/27/2026 2:19 AM, Waiman Long wrote:
>> Commit e44193d39e8d ("cpuset: let hotplug propagation work wait for
>> task attaching") was introduced to let hotplug operation to wait
>> until the completion of task attach operation. However, it is still
>> possible that the states of the source or destination cpuset can
>> be changed between the cpuset_can_attach() call and the subsequent
>> cpuset_attach()/cpuset_cacnel_attach() call.
>>
>> As a result, data gathered during cpuset_can_attach() cannot be reliably
>> used in the subsequent cpuset_attach()/cpuset_cacnel_attach()
>> call at all. Make the task attach operation more robust
>> and allow the sharing of data between cpuset_can_attach() and
>> cpuset_attach()/cpuset_cacnel_attach() by making cpuset_write_resmask()
>> and cpuset_partition_write() wait for the completion of task attach
>> as well.
>>
>> Ideally, an ongoing task attach operation should block any cpuset write
>> operation that can change its internal state until the operation is
>> completed. However, the attach_in_progress flag is currently per cpuset
>> and only the destination cpuset will have this flag set. The flag is not
>> set in the source cpuset where the tasks will be moved from. Even if we
>> extend the scope to include the source cpuset, it will not block cpuset
>> operation that changes the state of one of its ancestor cpuset which may
>> indirectly impact the state of the source or destination cpuset. It may
>> be too costly to set the flag for the whole subtree, it is far easier
>> to just make the flag global and block all the cpuset write operation
>> whenever a task attach operation is in progress. Make that change by
>> creating a new cpuset attach context (attach_ctx) structure to hold the
>> global in_progress flag and use it for blocking cpuset write operation
>> if a cpuset attach operation is in progress.
>>
>> The comments about validate_change() are no longer valid as it won't
>> be called at all if an attach operation is in progress. So the comments
>> can be removed.
>>
>> The per-cpuset attach_in_progress flag is also currently used in
>> partition_is_populated() and cpuset_is_populated() to determine if
>> an empty cpuset will have incoming task. This check will no longer be
>> needed as this function will not be called when there is a task attach
>> in progress. So the flag check is now removed.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
>>   kernel/cgroup/cpuset-internal.h | 11 +-----
>>   kernel/cgroup/cpuset.c          | 68 +++++++++++++++++++++------------
>>   2 files changed, 44 insertions(+), 35 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset-internal.h 
>> b/kernel/cgroup/cpuset-internal.h
>> index f7aaf01f7cd5..817b86ba7019 100644
>> --- a/kernel/cgroup/cpuset-internal.h
>> +++ b/kernel/cgroup/cpuset-internal.h
>> @@ -145,12 +145,6 @@ struct cpuset {
>>        */
>>       nodemask_t old_mems_allowed;
>>   -    /*
>> -     * Tasks are being attached to this cpuset.  Used to prevent
>> -     * zeroing cpus/mems_allowed between ->can_attach() and ->attach().
>> -     */
>> -    int attach_in_progress;
>> -
>>       /* partition root state */
>>       int partition_root_state;
>>   @@ -269,10 +263,7 @@ static inline int nr_cpusets(void)
>>   static inline bool cpuset_is_populated(struct cpuset *cs)
>>   {
>>       lockdep_assert_cpuset_lock_held();
>> -
>> -    /* Cpusets in the process of attaching should be considered as 
>> populated */
>> -    return cgroup_is_populated(cs->css.cgroup) ||
>> -        cs->attach_in_progress;
>> +    return cgroup_is_populated(cs->css.cgroup);
>>   }
>>     /**
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index d108c2083e86..dec9785d0271 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -356,6 +356,14 @@ static struct workqueue_struct 
>> *cpuset_migrate_mm_wq;
>>     static DECLARE_WAIT_QUEUE_HEAD(cpuset_attach_wq);
>>   +/*
>> + * Cpuset task attach context
>> + * Protected by cpuset_mutex
>> + */
>> +static struct {
>> +    int in_progress;
>> +} attach_ctx;
>> +
>>   static inline void check_insane_mems_config(nodemask_t *nodes)
>>   {
>>       if (!cpusets_insane_config() &&
>> @@ -368,22 +376,22 @@ static inline void 
>> check_insane_mems_config(nodemask_t *nodes)
>>   }
>>     /*
>> - * decrease cs->attach_in_progress.
>> - * wake_up cpuset_attach_wq if cs->attach_in_progress==0.
>> + * decrease attach_ctx.in_progress.
>> + * wake_up cpuset_attach_wq if attach_ctx.in_progress==0.
>>    */
>> -static inline void dec_attach_in_progress_locked(struct cpuset *cs)
>> +static inline void dec_attach_in_progress_locked(void)
>>   {
>>       lockdep_assert_cpuset_lock_held();
>>   -    cs->attach_in_progress--;
>> -    if (!cs->attach_in_progress)
>> +    attach_ctx.in_progress--;
>> +    if (!attach_ctx.in_progress)
>>           wake_up(&cpuset_attach_wq);
>>   }
>>   -static inline void dec_attach_in_progress(struct cpuset *cs)
>> +static inline void dec_attach_in_progress(void)
>>   {
>>       mutex_lock(&cpuset_mutex);
>> -    dec_attach_in_progress_locked(cs);
>> +    dec_attach_in_progress_locked();
>>       mutex_unlock(&cpuset_mutex);
>>   }
>>   @@ -432,8 +440,7 @@ static inline bool 
>> partition_is_populated(struct cpuset *cs,
>>        * nr_populated_domain_children may include populated
>>        * csets from descendants that are partitions.
>>        */
>> -    if (cgroup_has_tasks(cs->css.cgroup) ||
>> -        cs->attach_in_progress)
>> +    if (cgroup_has_tasks(cs->css.cgroup))
>>           return true;
>>         rcu_read_lock();
>> @@ -3091,11 +3098,7 @@ static int cpuset_can_attach(struct 
>> cgroup_taskset *tset)
>>       cs->dl_bw_cpu = cpu;
>>     out_success:
>> -    /*
>> -     * Mark attach is in progress.  This makes validate_change() fail
>> -     * changes which zero cpus/mems_allowed.
>> -     */
>> -    cs->attach_in_progress++;
>> +    attach_ctx.in_progress++;
>>     out_unlock:
>>       if (ret)
>> @@ -3113,7 +3116,7 @@ static void cpuset_cancel_attach(struct 
>> cgroup_taskset *tset)
>>       cs = css_cs(css);
>>         mutex_lock(&cpuset_mutex);
>> -    dec_attach_in_progress_locked(cs);
>> +    dec_attach_in_progress_locked();
>>         if (cs->dl_bw_cpu >= 0)
>>           dl_bw_free(cs->dl_bw_cpu, cs->sum_migrate_dl_bw);
>> @@ -3226,7 +3229,7 @@ static void cpuset_attach(struct cgroup_taskset 
>> *tset)
>>           reset_migrate_dl_data(cs);
>>       }
>>   -    dec_attach_in_progress_locked(cs);
>> +    dec_attach_in_progress_locked();
>>         mutex_unlock(&cpuset_mutex);
>>   }
>> @@ -3246,10 +3249,19 @@ ssize_t cpuset_write_resmask(struct 
>> kernfs_open_file *of,
>>           return -EACCES;
>>         buf = strstrip(buf);
>> +retry:
>> +    wait_event(cpuset_attach_wq, attach_ctx.in_progress == 0);
>> +
>>       cpuset_full_lock();
>>       if (!is_cpuset_online(cs))
>>           goto out_unlock;
>>   +    /* Don't race with task attach */
>> +    if (attach_ctx.in_progress) {
>> +        cpuset_full_unlock();
>> +        goto retry;
>> +    }
>> +
>>       trialcs = dup_or_alloc_cpuset(cs);
>>       if (!trialcs) {
>>           retval = -ENOMEM;
>> @@ -3377,7 +3389,17 @@ static ssize_t cpuset_partition_write(struct 
>> kernfs_open_file *of, char *buf,
>>       else
>>           return -EINVAL;
>>   +retry:
>> +    wait_event(cpuset_attach_wq, attach_ctx.in_progress == 0);
>> +
>>       cpuset_full_lock();
>> +
>> +    /* Don't race with task attach */
>> +    if (attach_ctx.in_progress) {
>> +        cpuset_full_unlock();
>> +        goto retry;
>> +    }
>> +
>
> Would it make sense to add a helper like wait_attach_done_locked()?

I guess we can add a helper to do that.

Thanks for the suggestions.

Cheers,
Longman

[PATCH v8 04/11] cgroup/cpuset: Put all task attach related variables into attach_ctx

[Waiman Long]

Put the task attach related cpuset_attach_old_cs and
cpuset_attach_nodemask_to static variables into the new attach_ctx
structure to improve readability and ease maintanence.

No functional change is expected.

Suggested-by: Ridong Chen <ridong.chen@linux.dev>
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset.c | 21 ++++++++++-----------
 1 file changed, 10 insertions(+), 11 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index dec9785d0271..1f31f340e0ec 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -362,6 +362,8 @@ static DECLARE_WAIT_QUEUE_HEAD(cpuset_attach_wq);
  */
 static struct {
 	int in_progress;
+	struct cpuset *old_cs;	/* Source cpuset */
+	nodemask_t nodemask_to;
 } attach_ctx;
 
 static inline void check_insane_mems_config(nodemask_t *nodes)
@@ -2996,8 +2998,6 @@ static int update_prstate(struct cpuset *cs, int new_prs)
 	return 0;
 }
 
-static struct cpuset *cpuset_attach_old_cs;
-
 /*
  * Check to see if a cpuset can accept a new task
  * For v1, cpus_allowed and mems_allowed can't be empty.
@@ -3029,8 +3029,8 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 	int cpu, ret;
 
 	/* used later by cpuset_attach() */
-	cpuset_attach_old_cs = task_cs(cgroup_taskset_first(tset, &css));
-	oldcs = cpuset_attach_old_cs;
+	attach_ctx.old_cs = task_cs(cgroup_taskset_first(tset, &css));
+	oldcs = attach_ctx.old_cs;
 	cs = css_cs(css);
 
 	mutex_lock(&cpuset_mutex);
@@ -3133,7 +3133,6 @@ static void cpuset_cancel_attach(struct cgroup_taskset *tset)
  * allocate from cpuset_init().
  */
 static cpumask_var_t cpus_attach;
-static nodemask_t cpuset_attach_nodemask_to;
 
 static void cpuset_attach_task(struct cpuset *cs, struct task_struct *task)
 {
@@ -3150,7 +3149,7 @@ static void cpuset_attach_task(struct cpuset *cs, struct task_struct *task)
 	 */
 	WARN_ON_ONCE(set_cpus_allowed_ptr(task, cpus_attach));
 
-	cpuset_change_task_nodemask(task, &cpuset_attach_nodemask_to);
+	cpuset_change_task_nodemask(task, &attach_ctx.nodemask_to);
 	cpuset1_update_task_spread_flags(cs, task);
 }
 
@@ -3160,7 +3159,7 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 	struct task_struct *leader;
 	struct cgroup_subsys_state *css;
 	struct cpuset *cs;
-	struct cpuset *oldcs = cpuset_attach_old_cs;
+	struct cpuset *oldcs = attach_ctx.old_cs;
 	bool cpus_updated, mems_updated;
 	bool queue_task_work = false;
 
@@ -3172,7 +3171,7 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 	cpus_updated = !cpumask_equal(cs->effective_cpus,
 				      oldcs->effective_cpus);
 	mems_updated = !nodes_equal(cs->effective_mems, oldcs->effective_mems);
-	guarantee_online_mems(cs, &cpuset_attach_nodemask_to);
+	guarantee_online_mems(cs, &attach_ctx.nodemask_to);
 
 	/*
 	 * In the default hierarchy, enabling cpuset in the child cgroups
@@ -3211,7 +3210,7 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 			 */
 			if (is_memory_migrate(cs)) {
 				cpuset_migrate_mm(mm, &oldcs->old_mems_allowed,
-						  &cpuset_attach_nodemask_to);
+						  &attach_ctx.nodemask_to);
 				queue_task_work = true;
 			} else
 				mmput(mm);
@@ -3221,7 +3220,7 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 out:
 	if (queue_task_work)
 		schedule_flush_migrate_mm();
-	cs->old_mems_allowed = cpuset_attach_nodemask_to;
+	cs->old_mems_allowed = attach_ctx.nodemask_to;
 
 	if (cs->nr_migrate_dl_tasks) {
 		cs->nr_deadline_tasks += cs->nr_migrate_dl_tasks;
@@ -3685,7 +3684,7 @@ static void cpuset_fork(struct task_struct *task)
 
 	/* CLONE_INTO_CGROUP */
 	mutex_lock(&cpuset_mutex);
-	guarantee_online_mems(cs, &cpuset_attach_nodemask_to);
+	guarantee_online_mems(cs, &attach_ctx.nodemask_to);
 	cpuset_attach_task(cs, task);
 
 	dec_attach_in_progress_locked();
-- 
2.54.0

Re: [PATCH v8 04/11] cgroup/cpuset: Put all task attach related variables into attach_ctx

[Ridong Chen]

On 6/27/2026 2:19 AM, Waiman Long wrote:
> Put the task attach related cpuset_attach_old_cs and
> cpuset_attach_nodemask_to static variables into the new attach_ctx
> structure to improve readability and ease maintanence.
> 
> No functional change is expected.
> 
> Suggested-by: Ridong Chen <ridong.chen@linux.dev>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>   kernel/cgroup/cpuset.c | 21 ++++++++++-----------
>   1 file changed, 10 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index dec9785d0271..1f31f340e0ec 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -362,6 +362,8 @@ static DECLARE_WAIT_QUEUE_HEAD(cpuset_attach_wq);
>    */
>   static struct {
>   	int in_progress;
> +	struct cpuset *old_cs;	/* Source cpuset */
> +	nodemask_t nodemask_to;
>   } attach_ctx;
>   
>   static inline void check_insane_mems_config(nodemask_t *nodes)
> @@ -2996,8 +2998,6 @@ static int update_prstate(struct cpuset *cs, int new_prs)
>   	return 0;
>   }
>   
> -static struct cpuset *cpuset_attach_old_cs;
> -
>   /*
>    * Check to see if a cpuset can accept a new task
>    * For v1, cpus_allowed and mems_allowed can't be empty.
> @@ -3029,8 +3029,8 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
>   	int cpu, ret;
>   
>   	/* used later by cpuset_attach() */
> -	cpuset_attach_old_cs = task_cs(cgroup_taskset_first(tset, &css));
> -	oldcs = cpuset_attach_old_cs;
> +	attach_ctx.old_cs = task_cs(cgroup_taskset_first(tset, &css));
> +	oldcs = attach_ctx.old_cs;
>   	cs = css_cs(css);
>   
>   	mutex_lock(&cpuset_mutex);
> @@ -3133,7 +3133,6 @@ static void cpuset_cancel_attach(struct cgroup_taskset *tset)
>    * allocate from cpuset_init().
>    */
>   static cpumask_var_t cpus_attach;
> -static nodemask_t cpuset_attach_nodemask_to;
>   
>   static void cpuset_attach_task(struct cpuset *cs, struct task_struct *task)
>   {
> @@ -3150,7 +3149,7 @@ static void cpuset_attach_task(struct cpuset *cs, struct task_struct *task)
>   	 */
>   	WARN_ON_ONCE(set_cpus_allowed_ptr(task, cpus_attach));
>   
> -	cpuset_change_task_nodemask(task, &cpuset_attach_nodemask_to);
> +	cpuset_change_task_nodemask(task, &attach_ctx.nodemask_to);
>   	cpuset1_update_task_spread_flags(cs, task);
>   }
>   
> @@ -3160,7 +3159,7 @@ static void cpuset_attach(struct cgroup_taskset *tset)
>   	struct task_struct *leader;
>   	struct cgroup_subsys_state *css;
>   	struct cpuset *cs;
> -	struct cpuset *oldcs = cpuset_attach_old_cs;
> +	struct cpuset *oldcs = attach_ctx.old_cs;
>   	bool cpus_updated, mems_updated;
>   	bool queue_task_work = false;
>   
> @@ -3172,7 +3171,7 @@ static void cpuset_attach(struct cgroup_taskset *tset)
>   	cpus_updated = !cpumask_equal(cs->effective_cpus,
>   				      oldcs->effective_cpus);
>   	mems_updated = !nodes_equal(cs->effective_mems, oldcs->effective_mems);
> -	guarantee_online_mems(cs, &cpuset_attach_nodemask_to);
> +	guarantee_online_mems(cs, &attach_ctx.nodemask_to);
>   
>   	/*
>   	 * In the default hierarchy, enabling cpuset in the child cgroups
> @@ -3211,7 +3210,7 @@ static void cpuset_attach(struct cgroup_taskset *tset)
>   			 */
>   			if (is_memory_migrate(cs)) {
>   				cpuset_migrate_mm(mm, &oldcs->old_mems_allowed,
> -						  &cpuset_attach_nodemask_to);
> +						  &attach_ctx.nodemask_to);
>   				queue_task_work = true;
>   			} else
>   				mmput(mm);
> @@ -3221,7 +3220,7 @@ static void cpuset_attach(struct cgroup_taskset *tset)
>   out:
>   	if (queue_task_work)
>   		schedule_flush_migrate_mm();
> -	cs->old_mems_allowed = cpuset_attach_nodemask_to;
> +	cs->old_mems_allowed = attach_ctx.nodemask_to;
>   
>   	if (cs->nr_migrate_dl_tasks) {
>   		cs->nr_deadline_tasks += cs->nr_migrate_dl_tasks;
> @@ -3685,7 +3684,7 @@ static void cpuset_fork(struct task_struct *task)
>   
>   	/* CLONE_INTO_CGROUP */
>   	mutex_lock(&cpuset_mutex);
> -	guarantee_online_mems(cs, &cpuset_attach_nodemask_to);
> +	guarantee_online_mems(cs, &attach_ctx.nodemask_to);
>   	cpuset_attach_task(cs, task);
>   
>   	dec_attach_in_progress_locked();

LGTM.

Reviewed-by: Ridong Chen <ridong.chen@linux.dev>

-- 
Best regards
Ridong

[PATCH v8 05/11] cgroup/cpuset: Add a cpuset_reserve_dl_bw() helper

[Waiman Long]

Extract the DL bandwidth allocation code in cpuset_attach() to a new
cpuset_reserve_dl_bw() helper to simplify code.

No functional change is expected.

Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
Reviewed-by: Gregory Price <gourry@gourry.net>
---
 kernel/cgroup/cpuset.c | 42 ++++++++++++++++++++++++------------------
 1 file changed, 24 insertions(+), 18 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 1f31f340e0ec..1bcc3af77c94 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -3012,6 +3012,25 @@ static int cpuset_can_attach_check(struct cpuset *cs)
 	return 0;
 }
 
+static int cpuset_reserve_dl_bw(struct cpuset *cs)
+{
+	int cpu, ret;
+
+	if (!cs->sum_migrate_dl_bw)
+		return 0;
+
+	cpu = cpumask_any_and(cpu_active_mask, cs->effective_cpus);
+	if (unlikely(cpu >= nr_cpu_ids))
+		return -EINVAL;
+
+	ret = dl_bw_alloc(cpu, cs->sum_migrate_dl_bw);
+	if (ret)
+		return ret;
+
+	cs->dl_bw_cpu = cpu;
+	return 0;
+}
+
 static void reset_migrate_dl_data(struct cpuset *cs)
 {
 	cs->nr_migrate_dl_tasks = 0;
@@ -3026,7 +3045,7 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 	struct cpuset *cs, *oldcs;
 	struct task_struct *task;
 	bool setsched_check;
-	int cpu, ret;
+	int ret;
 
 	/* used later by cpuset_attach() */
 	attach_ctx.old_cs = task_cs(cgroup_taskset_first(tset, &css));
@@ -3082,27 +3101,14 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 		}
 	}
 
-	if (!cs->sum_migrate_dl_bw)
-		goto out_success;
-
-	cpu = cpumask_any_and(cpu_active_mask, cs->effective_cpus);
-	if (unlikely(cpu >= nr_cpu_ids)) {
-		ret = -EINVAL;
-		goto out_unlock;
-	}
-
-	ret = dl_bw_alloc(cpu, cs->sum_migrate_dl_bw);
-	if (ret)
-		goto out_unlock;
-
-	cs->dl_bw_cpu = cpu;
-
-out_success:
-	attach_ctx.in_progress++;
+	ret = cpuset_reserve_dl_bw(cs);
 
 out_unlock:
 	if (ret)
 		reset_migrate_dl_data(cs);
+	else
+		attach_ctx.in_progress++;
+
 	mutex_unlock(&cpuset_mutex);
 	return ret;
 }
-- 
2.54.0

[PATCH v8 06/11] cgroup/cpuset: Expand the scope of cpuset_can_attach_check()

[Waiman Long]

Expand the scope of cpuset_can_attach_check() by including the setting
of setsched flag inside cpuset_can_attach_check() with the new @oldcs
and @psetsched argument. As cpuset_can_attach_check() is also called
from cpuset_can_fork(), set the new arguments to NULL from that caller.

Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
---
 kernel/cgroup/cpuset.c | 52 ++++++++++++++++++++++++------------------
 1 file changed, 30 insertions(+), 22 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 1bcc3af77c94..05a81839966b 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -3003,12 +3003,39 @@ static int update_prstate(struct cpuset *cs, int new_prs)
  * For v1, cpus_allowed and mems_allowed can't be empty.
  * For v2, effective_cpus can't be empty.
  * Note that in v1, effective_cpus = cpus_allowed.
+ *
+ * Also set the boolean flag passed in by @psetsched depending on if
+ * security_task_setscheduler() call is needed and @oldcs is not NULL.
  */
-static int cpuset_can_attach_check(struct cpuset *cs)
+static int cpuset_can_attach_check(struct cpuset *cs, struct cpuset *oldcs,
+				   bool *psetsched)
 {
 	if (cpumask_empty(cs->effective_cpus) ||
 	   (!is_in_v2_mode() && nodes_empty(cs->mems_allowed)))
 		return -ENOSPC;
+
+	if (!oldcs)
+		return 0;
+
+	/*
+	 * Skip rights over task setsched check in v2 when nothing changes,
+	 * migration permission derives from hierarchy ownership in
+	 * cgroup_procs_write_permission()).
+	 */
+	*psetsched = !cpuset_v2() ||
+		!cpumask_equal(cs->effective_cpus, oldcs->effective_cpus) ||
+		!nodes_equal(cs->effective_mems, oldcs->effective_mems);
+
+	/*
+	 * A v1 cpuset with tasks will have no CPU left only when CPU hotplug
+	 * brings the last online CPU offline as users are not allowed to empty
+	 * cpuset.cpus when there are active tasks inside. When that happens,
+	 * we should allow tasks to migrate out without security check to make
+	 * sure they will be able to run after migration.
+	 */
+	if (!is_in_v2_mode() && cpumask_empty(oldcs->effective_cpus))
+		*psetsched = false;
+
 	return 0;
 }
 
@@ -3055,29 +3082,10 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 	mutex_lock(&cpuset_mutex);
 
 	/* Check to see if task is allowed in the cpuset */
-	ret = cpuset_can_attach_check(cs);
+	ret = cpuset_can_attach_check(cs, oldcs, &setsched_check);
 	if (ret)
 		goto out_unlock;
 
-	/*
-	 * Skip rights over task setsched check in v2 when nothing changes,
-	 * migration permission derives from hierarchy ownership in
-	 * cgroup_procs_write_permission()).
-	 */
-	setsched_check = !cpuset_v2() ||
-		!cpumask_equal(cs->effective_cpus, oldcs->effective_cpus) ||
-		!nodes_equal(cs->effective_mems, oldcs->effective_mems);
-
-	/*
-	 * A v1 cpuset with tasks will have no CPU left only when CPU hotplug
-	 * brings the last online CPU offline as users are not allowed to empty
-	 * cpuset.cpus when there are active tasks inside. When that happens,
-	 * we should allow tasks to migrate out without security check to make
-	 * sure they will be able to run after migration.
-	 */
-	if (!is_in_v2_mode() && cpumask_empty(oldcs->effective_cpus))
-		setsched_check = false;
-
 	cgroup_taskset_for_each(task, css, tset) {
 		ret = task_can_attach(task);
 		if (ret)
@@ -3631,7 +3639,7 @@ static int cpuset_can_fork(struct task_struct *task, struct css_set *cset)
 	mutex_lock(&cpuset_mutex);
 
 	/* Check to see if task is allowed in the cpuset */
-	ret = cpuset_can_attach_check(cs);
+	ret = cpuset_can_attach_check(cs, NULL, NULL);
 	if (ret)
 		goto out_unlock;
 
-- 
2.54.0

[PATCH v8 07/11] cgroup/cpuset: Make attach_ctx.old_cs track task group leader

[Waiman Long]

There are two possible ways that migration of tasks from multiple source
cpusets to a target cpuset can happen. Either a multithread application
with threads in different cpusets is wholely migrated to a new cpuset
or disabling of v2 cpuset controller will move all the tasks in child
cpusets to the parent cpuset.

In the former case, it is the mm setting of the group leader that
really matters. So attach_ctx.old_cs should track the oldcs of the
thread leader. In the latter case, effective_mems of child cpusets
must always be a subset of the parent. So no real page migration
will not be necessary no matter which child cpuset is selected as
attach_ctx.old_cs.

IOW, attach_ctx.old_cs should be updated to match the latest task
group leader in cpuset_can_attach(), but fall back to that of the first
task if there is no group leader in the taskset.

Suggested-by: Ridong Chen <ridong.chen@linux.dev>
Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
---
 kernel/cgroup/cpuset.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 05a81839966b..e830ba13be9b 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -3086,11 +3086,32 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 	if (ret)
 		goto out_unlock;
 
+	/*
+	 * The attach_ctx.old_cs is used mainly by cpuset_migrate_mm() to get
+	 * the old_mems_allowed value. There are two ways that many-to-one
+	 * cpuset migration can happen:
+	 * 1) A multithread application with threads in different cpusets is
+	 *    wholely migrated to a new cpuset.
+	 * 2) Disabling v2 cpuset controller will move all the tasks in child
+	 *    cpusets to the parent cpuset.
+	 *
+	 * In the former case, it is the mm setting of the group leader that
+	 * really matters. So attach_ctx.old_cs should track the oldcs of the
+	 * group leader. It falls back to the oldcs of the first task if there
+	 * is no group leader in the taskset. In the latter case, effective_mems
+	 * of child cpusets must always be a subset of the parent. So no real
+	 * page migration will be necessary no matter which child cpuset is
+	 * selected as attach_ctx.old_cs.
+	 */
 	cgroup_taskset_for_each(task, css, tset) {
 		ret = task_can_attach(task);
 		if (ret)
 			goto out_unlock;
 
+		/* Update attach_ctx.old_cs to the latest group leader */
+		if (task == task->group_leader)
+			attach_ctx.old_cs = task_cs(task);
+
 		if (setsched_check) {
 			ret = security_task_setscheduler(task);
 			if (ret)
-- 
2.54.0

[PATCH v8 08/11] cgroup/cpuset: Move mpol_rebind_mm/cpuset_migrate_mm() calls inside cpuset_attach_task()

[Waiman Long]

The cpuset_attach_task() was introduced in commit 42a11bf5c543
("cgroup/cpuset: Make cpuset_fork() handle CLONE_INTO_CGROUP properly")
to enable the CLONE_INTO_CGROUP flag of clone(2) to behave more like
moving a task from one cpuset into another one. That commits didn't
move the mpol_rebind_mm() and cpuset_migrate_mm() calls for group leader
into cpuset_attach_task().

When the CLONE_INTO_CGROUP flag is used without CLONE_THREAD, the new
task is its own group leader. So it is still not equivalent to moving
task between cpusets in this case. Make CLONE_INTO_CGROUP behaves
more close to cpuset_attach() by moving the mpol_rebind_mm() and
cpuset_migrate_mm() calls inside cpuset_attach_task().

Also move the stack local cpus_updated, mems_updated and queue_task_work
flags into attach_ctx so that these flags can be accessed inside and
outside of cpuset_attach_task(). The cpuset_fork() function is updated
to set up these flags and do memory migration if necessary.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset.c | 104 ++++++++++++++++++++++++-----------------
 1 file changed, 60 insertions(+), 44 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index e830ba13be9b..ef14ee821b4b 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -362,6 +362,9 @@ static DECLARE_WAIT_QUEUE_HEAD(cpuset_attach_wq);
  */
 static struct {
 	int in_progress;
+	bool cpus_updated;
+	bool mems_updated;
+	bool task_work_queued;
 	struct cpuset *old_cs;	/* Source cpuset */
 	nodemask_t nodemask_to;
 } attach_ctx;
@@ -3171,6 +3174,8 @@ static cpumask_var_t cpus_attach;
 
 static void cpuset_attach_task(struct cpuset *cs, struct task_struct *task)
 {
+	struct mm_struct *mm;
+
 	lockdep_assert_cpuset_lock_held();
 
 	if (cs != &top_cpuset)
@@ -3184,28 +3189,60 @@ static void cpuset_attach_task(struct cpuset *cs, struct task_struct *task)
 	 */
 	WARN_ON_ONCE(set_cpus_allowed_ptr(task, cpus_attach));
 
+	if (cpuset_v2() && !attach_ctx.mems_updated)
+		return;
+
 	cpuset_change_task_nodemask(task, &attach_ctx.nodemask_to);
 	cpuset1_update_task_spread_flags(cs, task);
+
+	if ((task != task->group_leader) ||
+	    (!is_memory_migrate(cs) && !attach_ctx.mems_updated))
+		return;
+
+	/*
+	 * Change mm for threadgroup leader. This is expensive and may
+	 * sleep and should be moved outside migration path proper.
+	 */
+	mm = get_task_mm(task);
+	if (mm) {
+		struct cpuset *oldcs = attach_ctx.old_cs;
+
+		mpol_rebind_mm(mm, &cs->effective_mems);
+
+		/*
+		 * old_mems_allowed is the same with mems_allowed
+		 * here, except if this task is being moved
+		 * automatically due to hotplug.  In that case
+		 * @mems_allowed has been updated and is empty, so
+		 * @old_mems_allowed is the right nodesets that we
+		 * migrate mm from.
+		 */
+		if (is_memory_migrate(cs)) {
+			cpuset_migrate_mm(mm, &oldcs->old_mems_allowed,
+					  &attach_ctx.nodemask_to);
+			attach_ctx.task_work_queued = true;
+		} else {
+			mmput(mm);
+		}
+	}
 }
 
 static void cpuset_attach(struct cgroup_taskset *tset)
 {
 	struct task_struct *task;
-	struct task_struct *leader;
 	struct cgroup_subsys_state *css;
 	struct cpuset *cs;
 	struct cpuset *oldcs = attach_ctx.old_cs;
-	bool cpus_updated, mems_updated;
-	bool queue_task_work = false;
 
 	cgroup_taskset_first(tset, &css);
 	cs = css_cs(css);
 
 	lockdep_assert_cpus_held();	/* see cgroup_attach_lock() */
 	mutex_lock(&cpuset_mutex);
-	cpus_updated = !cpumask_equal(cs->effective_cpus,
-				      oldcs->effective_cpus);
-	mems_updated = !nodes_equal(cs->effective_mems, oldcs->effective_mems);
+	attach_ctx.task_work_queued = false;
+
+	attach_ctx.cpus_updated = !cpumask_equal(cs->effective_cpus, oldcs->effective_cpus);
+	attach_ctx.mems_updated = !nodes_equal(cs->effective_mems, oldcs->effective_mems);
 	guarantee_online_mems(cs, &attach_ctx.nodemask_to);
 
 	/*
@@ -3214,46 +3251,14 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 	 * and mems. In that case, we can optimize out by skipping the task
 	 * iteration and update.
 	 */
-	if (cpuset_v2() && !cpus_updated && !mems_updated)
+	if (cpuset_v2() && !attach_ctx.cpus_updated && !attach_ctx.mems_updated)
 		goto out;
 
 	cgroup_taskset_for_each(task, css, tset)
 		cpuset_attach_task(cs, task);
 
-	/*
-	 * Change mm for all threadgroup leaders. This is expensive and may
-	 * sleep and should be moved outside migration path proper. Skip it
-	 * if there is no change in effective_mems and CS_MEMORY_MIGRATE is
-	 * not set.
-	 */
-	if (!is_memory_migrate(cs) && !mems_updated)
-		goto out;
-
-	cgroup_taskset_for_each_leader(leader, css, tset) {
-		struct mm_struct *mm = get_task_mm(leader);
-
-		if (mm) {
-			mpol_rebind_mm(mm, &cs->effective_mems);
-
-			/*
-			 * old_mems_allowed is the same with mems_allowed
-			 * here, except if this task is being moved
-			 * automatically due to hotplug.  In that case
-			 * @mems_allowed has been updated and is empty, so
-			 * @old_mems_allowed is the right nodesets that we
-			 * migrate mm from.
-			 */
-			if (is_memory_migrate(cs)) {
-				cpuset_migrate_mm(mm, &oldcs->old_mems_allowed,
-						  &attach_ctx.nodemask_to);
-				queue_task_work = true;
-			} else
-				mmput(mm);
-		}
-	}
-
 out:
-	if (queue_task_work)
+	if (attach_ctx.task_work_queued)
 		schedule_flush_migrate_mm();
 	cs->old_mems_allowed = attach_ctx.nodemask_to;
 
@@ -3700,15 +3705,14 @@ static void cpuset_cancel_fork(struct task_struct *task, struct css_set *cset)
  */
 static void cpuset_fork(struct task_struct *task)
 {
-	struct cpuset *cs;
-	bool same_cs;
+	struct cpuset *cs, *oldcs;
 
 	rcu_read_lock();
 	cs = task_cs(task);
-	same_cs = (cs == task_cs(current));
+	oldcs = task_cs(current);
 	rcu_read_unlock();
 
-	if (same_cs) {
+	if (cs == oldcs) {
 		if (cs == &top_cpuset)
 			return;
 
@@ -3720,7 +3724,19 @@ static void cpuset_fork(struct task_struct *task)
 	/* CLONE_INTO_CGROUP */
 	mutex_lock(&cpuset_mutex);
 	guarantee_online_mems(cs, &attach_ctx.nodemask_to);
+	cs->old_mems_allowed = attach_ctx.nodemask_to;
+
+	/*
+	 * Assume CPUs and memory nodes are updated
+	 * A CLONE_INTO_CGROUP operation should have taken the cgroup mutex
+	 * and so there shouldn't be a competing cpuset_attach() operation.
+	 */
+	attach_ctx.cpus_updated = attach_ctx.mems_updated = true;
+	attach_ctx.task_work_queued = false;
+	attach_ctx.old_cs = oldcs;
 	cpuset_attach_task(cs, task);
+	if (attach_ctx.task_work_queued)
+		schedule_flush_migrate_mm();
 
 	dec_attach_in_progress_locked();
 	mutex_unlock(&cpuset_mutex);
-- 
2.54.0

[PATCH v8 09/11] cgroup/cpuset: Support multiple source cpusets for cpuset_*attach()

[Waiman Long]

There are 2 possible scenarios where the cgroup_taskset structure
passed into the cgroup can_attach() and attach() methods can contain
task migration data with multiple source cpusets.

 - A multithread application with threads in different cpusets is
   fully migrated into a new cpuset.
 - Disabling v2 cpuset controller will move all the tasks in child
   cpusets to the parent cpuset.

The current cpuset_can_attach() and cpuset_attach() functions still
expect task migration is from one source cpuset to one destination
cpuset.

Fix that by tracking the set of source (old) cpusets in singly linked
lists. The list will be iterated when necessary to properly update
internal data.

To ensure proper DL tasks accounting, the nr_migrate_dl_tasks in both
the source and destination cpusets are decremented/incremented with
their values added to nr_deadline_tasks when the migration is successful.

The setting of the global attach_ctx.cpus_updated and
attach_ctx.mems_updated flags are also moved from cpuset_attach()
to cpuset_can_attach() as the correct source cpuset can no longer be
determined in cpuset_attach() and cpuset states will not be changed
between cpuset_attach() and cpuset_can_attach() with an earlier patch.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset-internal.h |  5 +++
 kernel/cgroup/cpuset.c          | 65 ++++++++++++++++++++++++++++-----
 2 files changed, 60 insertions(+), 10 deletions(-)

diff --git a/kernel/cgroup/cpuset-internal.h b/kernel/cgroup/cpuset-internal.h
index 817b86ba7019..6636cf5ce326 100644
--- a/kernel/cgroup/cpuset-internal.h
+++ b/kernel/cgroup/cpuset-internal.h
@@ -145,6 +145,11 @@ struct cpuset {
 	 */
 	nodemask_t old_mems_allowed;
 
+	/*
+	 * For linking impacted cpusets during an attach operation.
+	 */
+	struct llist_node attach_node;
+
 	/* partition root state */
 	int partition_root_state;
 
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index ef14ee821b4b..e9e97c6765f0 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -37,6 +37,7 @@
 #include <linux/wait.h>
 #include <linux/workqueue.h>
 #include <linux/task_work.h>
+#include <linux/llist.h>
 
 DEFINE_STATIC_KEY_FALSE(cpusets_pre_enable_key);
 DEFINE_STATIC_KEY_FALSE(cpusets_enabled_key);
@@ -368,6 +369,7 @@ static struct {
 	struct cpuset *old_cs;	/* Source cpuset */
 	nodemask_t nodemask_to;
 } attach_ctx;
+static LLIST_HEAD(src_cs_head);
 
 static inline void check_insane_mems_config(nodemask_t *nodes)
 {
@@ -596,6 +598,7 @@ static struct cpuset *dup_or_alloc_cpuset(struct cpuset *cs)
 		return NULL;
 
 	trial->dl_bw_cpu = -1;
+	init_llist_node(&trial->attach_node);
 
 	/* Setup cpumask pointer array */
 	cpumask_var_t *pmask[4] = {
@@ -3013,6 +3016,8 @@ static int update_prstate(struct cpuset *cs, int new_prs)
 static int cpuset_can_attach_check(struct cpuset *cs, struct cpuset *oldcs,
 				   bool *psetsched)
 {
+	bool cpus_updated, mems_updated;
+
 	if (cpumask_empty(cs->effective_cpus) ||
 	   (!is_in_v2_mode() && nodes_empty(cs->mems_allowed)))
 		return -ENOSPC;
@@ -3020,14 +3025,23 @@ static int cpuset_can_attach_check(struct cpuset *cs, struct cpuset *oldcs,
 	if (!oldcs)
 		return 0;
 
+	if (!llist_on_list(&oldcs->attach_node))
+		llist_add(&oldcs->attach_node, &src_cs_head);
+
+	cpus_updated = !cpumask_equal(cs->effective_cpus, oldcs->effective_cpus);
+	mems_updated = !nodes_equal(cs->effective_mems, oldcs->effective_mems);
+
+	if (cpus_updated)
+		attach_ctx.cpus_updated = true;
+	if (mems_updated)
+		attach_ctx.mems_updated = true;
+
 	/*
 	 * Skip rights over task setsched check in v2 when nothing changes,
 	 * migration permission derives from hierarchy ownership in
 	 * cgroup_procs_write_permission()).
 	 */
-	*psetsched = !cpuset_v2() ||
-		!cpumask_equal(cs->effective_cpus, oldcs->effective_cpus) ||
-		!nodes_equal(cs->effective_mems, oldcs->effective_mems);
+	*psetsched = !cpuset_v2() || cpus_updated || mems_updated;
 
 	/*
 	 * A v1 cpuset with tasks will have no CPU left only when CPU hotplug
@@ -3068,6 +3082,25 @@ static void reset_migrate_dl_data(struct cpuset *cs)
 	cs->dl_bw_cpu = -1;
 }
 
+/*
+ * Clear and optionally apply (@cancel is false) the attach related data in the
+ * source cpusets.
+ */
+static void clear_attach_data(struct llist_head *head, bool cancel)
+{
+	struct cpuset *cs, *next;
+	struct llist_node *lnode = __llist_del_all(head);
+
+	llist_for_each_entry_safe(cs, next, lnode, attach_node) {
+		init_llist_node(&cs->attach_node);
+		if (cs->nr_migrate_dl_tasks) {
+			if (!cancel)
+				cs->nr_deadline_tasks += cs->nr_migrate_dl_tasks;
+			cs->nr_migrate_dl_tasks = 0;
+		}
+	}
+}
+
 /* Called by cgroups to determine if a cpuset is usable; cpuset_mutex held */
 static int cpuset_can_attach(struct cgroup_taskset *tset)
 {
@@ -3083,6 +3116,8 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 	cs = css_cs(css);
 
 	mutex_lock(&cpuset_mutex);
+	attach_ctx.cpus_updated = false;
+	attach_ctx.mems_updated = false;
 
 	/* Check to see if task is allowed in the cpuset */
 	ret = cpuset_can_attach_check(cs, oldcs, &setsched_check);
@@ -3107,6 +3142,15 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 	 * selected as attach_ctx.old_cs.
 	 */
 	cgroup_taskset_for_each(task, css, tset) {
+		struct cpuset *new_oldcs = task_cs(task);
+
+		if (new_oldcs != oldcs) {
+			oldcs = new_oldcs;
+			ret = cpuset_can_attach_check(cs, oldcs, &setsched_check);
+			if (ret)
+				goto out_unlock;
+		}
+
 		ret = task_can_attach(task);
 		if (ret)
 			goto out_unlock;
@@ -3128,6 +3172,7 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 			 * contribute to sum_migrate_dl_bw.
 			 */
 			cs->nr_migrate_dl_tasks++;
+			oldcs->nr_migrate_dl_tasks--;
 			if (dl_task_needs_bw_move(task, cs->effective_cpus))
 				cs->sum_migrate_dl_bw += task->dl.dl_bw;
 		}
@@ -3136,10 +3181,12 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 	ret = cpuset_reserve_dl_bw(cs);
 
 out_unlock:
-	if (ret)
+	if (ret) {
 		reset_migrate_dl_data(cs);
-	else
+		clear_attach_data(&src_cs_head, true);
+	} else {
 		attach_ctx.in_progress++;
+	}
 
 	mutex_unlock(&cpuset_mutex);
 	return ret;
@@ -3155,6 +3202,7 @@ static void cpuset_cancel_attach(struct cgroup_taskset *tset)
 
 	mutex_lock(&cpuset_mutex);
 	dec_attach_in_progress_locked();
+	clear_attach_data(&src_cs_head, true);
 
 	if (cs->dl_bw_cpu >= 0)
 		dl_bw_free(cs->dl_bw_cpu, cs->sum_migrate_dl_bw);
@@ -3232,7 +3280,6 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 	struct task_struct *task;
 	struct cgroup_subsys_state *css;
 	struct cpuset *cs;
-	struct cpuset *oldcs = attach_ctx.old_cs;
 
 	cgroup_taskset_first(tset, &css);
 	cs = css_cs(css);
@@ -3240,9 +3287,6 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 	lockdep_assert_cpus_held();	/* see cgroup_attach_lock() */
 	mutex_lock(&cpuset_mutex);
 	attach_ctx.task_work_queued = false;
-
-	attach_ctx.cpus_updated = !cpumask_equal(cs->effective_cpus, oldcs->effective_cpus);
-	attach_ctx.mems_updated = !nodes_equal(cs->effective_mems, oldcs->effective_mems);
 	guarantee_online_mems(cs, &attach_ctx.nodemask_to);
 
 	/*
@@ -3264,10 +3308,10 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 
 	if (cs->nr_migrate_dl_tasks) {
 		cs->nr_deadline_tasks += cs->nr_migrate_dl_tasks;
-		oldcs->nr_deadline_tasks -= cs->nr_migrate_dl_tasks;
 		reset_migrate_dl_data(cs);
 	}
 
+	clear_attach_data(&src_cs_head, false);
 	dec_attach_in_progress_locked();
 
 	mutex_unlock(&cpuset_mutex);
@@ -3785,6 +3829,7 @@ int __init cpuset_init(void)
 	cpumask_setall(top_cpuset.effective_xcpus);
 	cpumask_setall(top_cpuset.exclusive_cpus);
 	nodes_setall(top_cpuset.effective_mems);
+	init_llist_node(&top_cpuset.attach_node);
 
 	cpuset1_init(&top_cpuset);
 
-- 
2.54.0

[PATCH v8 10/11] cgroup/cpuset: Support multiple destination cpusets for cpuset_*attach()

[Waiman Long]

The only case where the cgroup_taskset structure requires task migration
to multiple cpusets is when enabling a cpuset controller in cgroup v2
where the newly created child cpusets inherits the same effective CPUs
and memory nodes from the parent. In that case, task migration can happen
directly with no update to tasks' CPU and memory nodes assignment and no
further work needed from the cpuset side except updating nr_deadline_tasks
when DL tasks are involved and setting old_mems_allowed in the child
cpusets.

Do that by tracking all the destination cpusets with a new dst_cs_head
singly linked list. The reset_migrate_dl_data() function is integrated
into clear_attach_data() so that it can be used for both source and
destination cpusets.

It is assumed that a given cpuset cannot be both a source and a
destination cpuset. If such condition happens or when there are multiple
destination cpusets with CPU or memory nodes changes, the current code
will not handle it correctly. So it will print a warning and fail the
attach operation in these unexpected cases as we will have to enhance the
code to support this if such use cases are valid and not coding errors.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset-internal.h |   1 +
 kernel/cgroup/cpuset.c          | 115 ++++++++++++++++++++------------
 2 files changed, 72 insertions(+), 44 deletions(-)

diff --git a/kernel/cgroup/cpuset-internal.h b/kernel/cgroup/cpuset-internal.h
index 6636cf5ce326..8cbfacc5f315 100644
--- a/kernel/cgroup/cpuset-internal.h
+++ b/kernel/cgroup/cpuset-internal.h
@@ -149,6 +149,7 @@ struct cpuset {
 	 * For linking impacted cpusets during an attach operation.
 	 */
 	struct llist_node attach_node;
+	bool attach_source;
 
 	/* partition root state */
 	int partition_root_state;
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index e9e97c6765f0..09b3ad52c639 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -366,10 +366,12 @@ static struct {
 	bool cpus_updated;
 	bool mems_updated;
 	bool task_work_queued;
+	bool many_dest_cs;	/* Have many destination cpusets */
 	struct cpuset *old_cs;	/* Source cpuset */
 	nodemask_t nodemask_to;
 } attach_ctx;
 static LLIST_HEAD(src_cs_head);
+static LLIST_HEAD(dst_cs_head);
 
 static inline void check_insane_mems_config(nodemask_t *nodes)
 {
@@ -3025,8 +3027,23 @@ static int cpuset_can_attach_check(struct cpuset *cs, struct cpuset *oldcs,
 	if (!oldcs)
 		return 0;
 
-	if (!llist_on_list(&oldcs->attach_node))
+	/*
+	 * The same cpuset cannot be both a source and a destination.
+	 * The current code does not support that, print a warning and
+	 * fail the attach if so.
+	 */
+	if (WARN_ON_ONCE((!oldcs->attach_source &&
+			  llist_on_list(&oldcs->attach_node)) ||
+			  cs->attach_source))
+		return -EINVAL;
+
+	if (!llist_on_list(&oldcs->attach_node)) {
 		llist_add(&oldcs->attach_node, &src_cs_head);
+		oldcs->attach_source = true;
+	}
+
+	if (!llist_on_list(&cs->attach_node))
+		llist_add(&cs->attach_node, &dst_cs_head);
 
 	cpus_updated = !cpumask_equal(cs->effective_cpus, oldcs->effective_cpus);
 	mems_updated = !nodes_equal(cs->effective_mems, oldcs->effective_mems);
@@ -3056,35 +3073,31 @@ static int cpuset_can_attach_check(struct cpuset *cs, struct cpuset *oldcs,
 	return 0;
 }
 
-static int cpuset_reserve_dl_bw(struct cpuset *cs)
+static int cpuset_reserve_dl_bw(void)
 {
+	struct cpuset *cs;
 	int cpu, ret;
 
-	if (!cs->sum_migrate_dl_bw)
-		return 0;
+	llist_for_each_entry(cs, dst_cs_head.first, attach_node) {
+		if (!cs->sum_migrate_dl_bw)
+			continue;
 
-	cpu = cpumask_any_and(cpu_active_mask, cs->effective_cpus);
-	if (unlikely(cpu >= nr_cpu_ids))
-		return -EINVAL;
+		cpu = cpumask_any_and(cpu_active_mask, cs->effective_cpus);
+		if (unlikely(cpu >= nr_cpu_ids))
+			return -EINVAL;
 
-	ret = dl_bw_alloc(cpu, cs->sum_migrate_dl_bw);
-	if (ret)
-		return ret;
+		ret = dl_bw_alloc(cpu, cs->sum_migrate_dl_bw);
+		if (ret)
+			return ret;
 
-	cs->dl_bw_cpu = cpu;
+		cs->dl_bw_cpu = cpu;
+	}
 	return 0;
 }
 
-static void reset_migrate_dl_data(struct cpuset *cs)
-{
-	cs->nr_migrate_dl_tasks = 0;
-	cs->sum_migrate_dl_bw = 0;
-	cs->dl_bw_cpu = -1;
-}
-
 /*
  * Clear and optionally apply (@cancel is false) the attach related data in the
- * source cpusets.
+ * source or destination cpuset.
  */
 static void clear_attach_data(struct llist_head *head, bool cancel)
 {
@@ -3096,8 +3109,13 @@ static void clear_attach_data(struct llist_head *head, bool cancel)
 		if (cs->nr_migrate_dl_tasks) {
 			if (!cancel)
 				cs->nr_deadline_tasks += cs->nr_migrate_dl_tasks;
+			else if (cs->dl_bw_cpu >= 0) /* && cacnel */
+				dl_bw_free(cs->dl_bw_cpu, cs->sum_migrate_dl_bw);
 			cs->nr_migrate_dl_tasks = 0;
+			cs->sum_migrate_dl_bw = 0;
+			cs->dl_bw_cpu = -1;
 		}
+		cs->attach_source = false;
 	}
 }
 
@@ -3118,6 +3136,7 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 	mutex_lock(&cpuset_mutex);
 	attach_ctx.cpus_updated = false;
 	attach_ctx.mems_updated = false;
+	attach_ctx.many_dest_cs = false;
 
 	/* Check to see if task is allowed in the cpuset */
 	ret = cpuset_can_attach_check(cs, oldcs, &setsched_check);
@@ -3142,9 +3161,13 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 	 * selected as attach_ctx.old_cs.
 	 */
 	cgroup_taskset_for_each(task, css, tset) {
+		struct cpuset *new_cs = css_cs(css);
 		struct cpuset *new_oldcs = task_cs(task);
 
-		if (new_oldcs != oldcs) {
+		if ((new_oldcs != oldcs) || (new_cs != cs)) {
+			if (new_cs != cs)
+				attach_ctx.many_dest_cs = true;
+			cs = new_cs;
 			oldcs = new_oldcs;
 			ret = cpuset_can_attach_check(cs, oldcs, &setsched_check);
 			if (ret)
@@ -3178,12 +3201,28 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 		}
 	}
 
-	ret = cpuset_reserve_dl_bw(cs);
+	/*
+	 * The only case where there are multiple destination cpusets for
+	 * task migration is when enabling a v2 cpuset controllers where
+	 * tasks will be migrated to multiple child cpusets from a parent
+	 * cpuset with the same effective CPUs and memory nodes. IOW,
+	 * both attach_cpus_updated and attach_mems_updated should be false.
+	 * If not, it is a condition that the current code cannot handled.
+	 * Print a warning and abort the attach operation as further code
+	 * change will be needed.
+	 */
+	if (WARN_ON_ONCE(attach_ctx.many_dest_cs && (!cpuset_v2() ||
+			 attach_ctx.cpus_updated || attach_ctx.mems_updated))) {
+		ret = -EINVAL;
+		goto out_unlock;
+	}
+
+	ret = cpuset_reserve_dl_bw();
 
 out_unlock:
 	if (ret) {
-		reset_migrate_dl_data(cs);
 		clear_attach_data(&src_cs_head, true);
+		clear_attach_data(&dst_cs_head, true);
 	} else {
 		attach_ctx.in_progress++;
 	}
@@ -3194,22 +3233,10 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 
 static void cpuset_cancel_attach(struct cgroup_taskset *tset)
 {
-	struct cgroup_subsys_state *css;
-	struct cpuset *cs;
-
-	cgroup_taskset_first(tset, &css);
-	cs = css_cs(css);
-
 	mutex_lock(&cpuset_mutex);
 	dec_attach_in_progress_locked();
 	clear_attach_data(&src_cs_head, true);
-
-	if (cs->dl_bw_cpu >= 0)
-		dl_bw_free(cs->dl_bw_cpu, cs->sum_migrate_dl_bw);
-
-	if (cs->nr_migrate_dl_tasks)
-		reset_migrate_dl_data(cs);
-
+	clear_attach_data(&dst_cs_head, true);
 	mutex_unlock(&cpuset_mutex);
 }
 
@@ -3293,25 +3320,25 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 	 * In the default hierarchy, enabling cpuset in the child cgroups
 	 * will trigger a cpuset_attach() call with no change in effective cpus
 	 * and mems. In that case, we can optimize out by skipping the task
-	 * iteration and update.
+	 * iteration and updatebut the destination cpuset list is iterated to
+	 * set old_mems_allowed.
 	 */
-	if (cpuset_v2() && !attach_ctx.cpus_updated && !attach_ctx.mems_updated)
+	if (cpuset_v2() && !attach_ctx.cpus_updated && !attach_ctx.mems_updated) {
+		llist_for_each_entry(cs, dst_cs_head.first, attach_node)
+			cs->old_mems_allowed = attach_ctx.nodemask_to;
 		goto out;
+	}
 
+	/* Task iteration shouldn't happen with attach_ctx.many_dest_cs set */
 	cgroup_taskset_for_each(task, css, tset)
 		cpuset_attach_task(cs, task);
 
-out:
 	if (attach_ctx.task_work_queued)
 		schedule_flush_migrate_mm();
 	cs->old_mems_allowed = attach_ctx.nodemask_to;
-
-	if (cs->nr_migrate_dl_tasks) {
-		cs->nr_deadline_tasks += cs->nr_migrate_dl_tasks;
-		reset_migrate_dl_data(cs);
-	}
-
+out:
 	clear_attach_data(&src_cs_head, false);
+	clear_attach_data(&dst_cs_head, false);
 	dec_attach_in_progress_locked();
 
 	mutex_unlock(&cpuset_mutex);
-- 
2.54.0

[PATCH v8 11/11] selftests/cgroup: Add test for cpuset affinity on controller disable

[Waiman Long]

From: Michal Koutný <mkoutny@suse.com>

Add a new selftest that exposes a bug in cpuset_attach() where thread
CPU affinity is not properly updated when the cpuset controller is
disabled in a threaded cgroup hierarchy.

The test creates a threaded cgroup hierarchy with two child cgroups
(A and B) having different cpuset.cpus constraints:
- Parent: cpuset.cpus=0-1
- Child A: cpuset.cpus=0-1
- Child B: cpuset.cpus=1 (restricted to CPU 1 only)

A multithreaded process is created with threads placed in different
cgroups. When the cpuset controller is disabled on the parent, thread
affinities should be updated to match the parent's cpuset.

Expected behavior:
- thread_a affinity: {0-1} before and after (unchanged)
- thread_b affinity: {1} before, {0-1} after (expanded)

Current buggy behavior:
- thread_b affinity remains {1} after controller disable

Assisted-by: Claude:claude-sonnet-4-5
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Acked-by: Waiman Long <longman@redhat.com>
---
 tools/testing/selftests/cgroup/test_cpuset.c | 243 +++++++++++++++++++
 1 file changed, 243 insertions(+)

diff --git a/tools/testing/selftests/cgroup/test_cpuset.c b/tools/testing/selftests/cgroup/test_cpuset.c
index c5cf8b56ceb8..8b4c4a9dd78b 100644
--- a/tools/testing/selftests/cgroup/test_cpuset.c
+++ b/tools/testing/selftests/cgroup/test_cpuset.c
@@ -1,7 +1,13 @@
 // SPDX-License-Identifier: GPL-2.0
 
+#define _GNU_SOURCE
+#include <assert.h>
 #include <linux/limits.h>
+#include <pthread.h>
+#include <sched.h>
 #include <signal.h>
+#include <sys/syscall.h>
+#include <unistd.h>
 
 #include "kselftest.h"
 #include "cgroup_util.h"
@@ -232,6 +238,242 @@ static int test_cpuset_perms_subtree(const char *root)
 	return ret;
 }
 
+static int get_cpu_affinity(cpu_set_t *mask)
+{
+	CPU_ZERO(mask);
+	return sched_getaffinity(0, sizeof(*mask), mask);
+}
+
+static int cpu_set_equal(cpu_set_t *dst, unsigned long mask)
+{
+	cpu_set_t expected;
+
+	CPU_ZERO(&expected);
+	assert(sizeof(mask) < CPU_SETSIZE);
+
+	for (int cpu = 0; cpu < sizeof(mask); ++cpu)
+		if ((1UL << cpu) & mask)
+			CPU_SET(cpu, &expected);
+
+	return CPU_EQUAL(&expected, dst);
+}
+
+enum test_phase {
+	AFFINITY_SETUP,
+	AFFINITY_THREAD_A_READY,
+	AFFINITY_THREADS_READY,
+	AFFINITY_CONTROLLER_DISABLED,
+	AFFINITY_COMPLETE,
+	AFFINITY_ERROR
+};
+
+struct thread_args {
+	const char *cgroup;
+	cpu_set_t *affinity_before;
+	cpu_set_t *affinity_after;
+	enum test_phase ready_phase;
+};
+
+static pthread_mutex_t test_mutex = PTHREAD_MUTEX_INITIALIZER;
+static pthread_cond_t test_cond = PTHREAD_COND_INITIALIZER;
+static enum test_phase test_phase;
+
+static void *affinity_thread_fn(void *arg)
+{
+	struct thread_args *args = (struct thread_args *)arg;
+
+	if (cg_enter_current_thread(args->cgroup))
+		goto fail;
+
+	if (get_cpu_affinity(args->affinity_before) != 0)
+		goto fail;
+
+	pthread_mutex_lock(&test_mutex);
+	if (test_phase < args->ready_phase)
+		test_phase = args->ready_phase;
+	pthread_cond_broadcast(&test_cond);
+
+	while (test_phase < AFFINITY_CONTROLLER_DISABLED)
+		pthread_cond_wait(&test_cond, &test_mutex);
+	pthread_mutex_unlock(&test_mutex);
+
+	if (get_cpu_affinity(args->affinity_after) != 0)
+		goto fail;
+
+
+	return NULL;
+
+fail:
+	pthread_mutex_lock(&test_mutex);
+	test_phase = AFFINITY_ERROR;
+	pthread_cond_broadcast(&test_cond);
+	pthread_mutex_unlock(&test_mutex);
+	return NULL;
+}
+
+/*
+ * Test that disabling cpuset controller properly updates thread affinity.
+ *
+ * This test exposes a bug in cpuset_attach() where threads in child cgroups
+ * don't get their affinity updated when the cpuset controller is disabled.
+ *
+ * Setup:
+ * - Create parent cgroup with cpuset.cpus=0-1
+ * - Create child A with cpuset.cpus=0-1
+ * - Create child B with cpuset.cpus=1
+ * - Place multithreaded process: group leader + thread_a in A, thread_b in B
+ * - Disable cpuset controller on parent
+ *
+ * Expected: thread_b's affinity should expand from {1} to {0-1}
+ * Buggy: thread_b's affinity remains {1}
+ */
+static int test_cpuset_affinity_on_controller_disable(const char *root)
+{
+	char *parent = NULL, *child_a = NULL, *child_b = NULL;
+	pthread_t thread_a, thread_b;
+	int thread_a_created = 0, thread_b_created = 0;
+	cpu_set_t affinity_a_before, affinity_a_after;
+	cpu_set_t affinity_b_before, affinity_b_after;
+	int ret = KSFT_FAIL;
+
+	parent = cg_name(root, "cpuset_affinity_test");
+	if (!parent)
+		goto cleanup;
+	if (cg_create(parent))
+		goto cleanup;
+	if (cg_write(parent, "cgroup.type", "threaded"))
+		goto cleanup;
+
+	child_a = cg_name(parent, "A");
+	if (!child_a)
+		goto cleanup;
+	if (cg_create(child_a))
+		goto cleanup;
+	if (cg_write(child_a, "cgroup.type", "threaded"))
+		goto cleanup;
+
+	child_b = cg_name(parent, "B");
+	if (!child_b)
+		goto cleanup;
+	if (cg_create(child_b))
+		goto cleanup;
+	if (cg_write(child_b, "cgroup.type", "threaded"))
+		goto cleanup;
+
+	/* Now enable cpuset controller in parent */
+	if (cg_write(parent, "cgroup.subtree_control", "+cpuset")) {
+		ret = KSFT_SKIP;
+		goto cleanup;
+	}
+
+	/* Set CPU affinity constraints */
+	if (cg_write(parent, "cpuset.cpus", "0-1"))
+		goto cleanup;
+	if (cg_write(child_a, "cpuset.cpus", "0-1"))
+		goto cleanup;
+	if (cg_write(child_b, "cpuset.cpus", "1"))
+		goto cleanup;
+
+	/* Move group leader (main thread) to child A */
+	if (cg_enter_current(child_a))
+		goto cleanup;
+
+	/* Create threads - they will move themselves to their respective cgroups */
+	test_phase = AFFINITY_SETUP;
+
+	struct thread_args args_a = {
+		.cgroup = child_a,
+		.affinity_before = &affinity_a_before,
+		.affinity_after = &affinity_a_after,
+		.ready_phase = AFFINITY_THREAD_A_READY,
+	};
+	if (pthread_create(&thread_a, NULL, affinity_thread_fn, &args_a))
+		goto cleanup;
+	thread_a_created = 1;
+
+	struct thread_args args_b = {
+		.cgroup = child_b,
+		.affinity_before = &affinity_b_before,
+		.affinity_after = &affinity_b_after,
+		.ready_phase = AFFINITY_THREADS_READY,
+	};
+	if (pthread_create(&thread_b, NULL, affinity_thread_fn, &args_b))
+		goto cleanup_threads;
+	thread_b_created = 1;
+
+	pthread_mutex_lock(&test_mutex);
+	while (test_phase < AFFINITY_THREADS_READY)
+		pthread_cond_wait(&test_cond, &test_mutex);
+
+	/* If a thread failed during setup, bail out */
+	if (test_phase == AFFINITY_ERROR) {
+		pthread_mutex_unlock(&test_mutex);
+		goto cleanup_threads;
+	}
+	pthread_mutex_unlock(&test_mutex);
+
+	if (!cpu_set_equal(&affinity_a_before, 0x3)) {
+		ksft_print_msg("FAIL: thread_a initial affinity incorrect\n");
+		goto cleanup_threads;
+	}
+
+	if (!cpu_set_equal(&affinity_b_before, 0x2)) {
+		ksft_print_msg("FAIL: thread_b initial affinity incorrect\n");
+		goto cleanup_threads;
+	}
+
+	/* Disable cpuset controller - this should trigger affinity update */
+	if (cg_write(parent, "cgroup.subtree_control", "-cpuset"))
+		goto cleanup_threads;
+
+	/* Signal threads to save their final affinity and exit */
+	pthread_mutex_lock(&test_mutex);
+	test_phase = AFFINITY_CONTROLLER_DISABLED;
+	pthread_cond_broadcast(&test_cond);
+	pthread_mutex_unlock(&test_mutex);
+
+	pthread_join(thread_a, NULL);
+	pthread_join(thread_b, NULL);
+
+	/* Verify thread affinities AFTER disabling controller */
+	if (!cpu_set_equal(&affinity_a_after, 0x3)) {
+		ksft_print_msg("FAIL: thread_a final affinity incorrect\n");
+		goto cleanup;
+	}
+
+	if (!cpu_set_equal(&affinity_b_after, 0x3)) {
+		ksft_print_msg("FAIL: thread_b affinity did not expand to {0-1}\n");
+		goto cleanup;
+	}
+
+	ret = KSFT_PASS;
+	goto cleanup;
+
+cleanup_threads:
+	pthread_mutex_lock(&test_mutex);
+	test_phase = AFFINITY_COMPLETE;
+	pthread_cond_broadcast(&test_cond);
+	pthread_mutex_unlock(&test_mutex);
+
+	if (thread_a_created)
+		pthread_join(thread_a, NULL);
+	if (thread_b_created)
+		pthread_join(thread_b, NULL);
+
+cleanup:
+	/* Move back to root before cleanup */
+	cg_enter_current(root);
+
+	cg_destroy(child_b);
+	free(child_b);
+	cg_destroy(child_a);
+	free(child_a);
+	cg_destroy(parent);
+	free(parent);
+
+	return ret;
+}
+
 
 #define T(x) { x, #x }
 struct cpuset_test {
@@ -241,6 +483,7 @@ struct cpuset_test {
 	T(test_cpuset_perms_object_allow),
 	T(test_cpuset_perms_object_deny),
 	T(test_cpuset_perms_subtree),
+	T(test_cpuset_affinity_on_controller_disable),
 };
 #undef T
 
-- 
2.54.0