[PATCH v2 0/2] cgroup/cpuset: Miscellaneous fixes and cleanups

[PATCH v2 0/2] cgroup/cpuset: Miscellaneous fixes and cleanups

[Waiman Long]

 v2:
  - Update patch 1 as suggested by Ridong Chen and add new test cases.
  - Minor update to patch 2 code and comment log.

Patch 1 updates compute_effective_cpumask() and adds new
compute_effective_nodemask() helper to make sure that effective_cpus
and effective_mems will inherit parent's versions for v2 if
cpuset.cpus/cpuset.mems is empty.

Patch 2 makes cpuset_update_tasks_nodemask() to perform memory rebind
and migration only for thread group leader like cpuset_attach().

Waiman Long (2):
  cgroup/cpuset: Avoid unnecessary cpus & mems update in
    cpuset_hotplug_update_tasks()
  cgroup/cpuset: Rebind/migrate mm only for threadgroup leader in
    cpuset_update_tasks_nodemask()

 Documentation/admin-guide/cgroup-v2.rst       |  7 +++
 kernel/cgroup/cpuset.c                        | 49 ++++++++++++-------
 .../selftests/cgroup/test_cpuset_prs.sh       | 11 ++++-
 3 files changed, 46 insertions(+), 21 deletions(-)

-- 
2.54.0

[PATCH v2 1/2] cgroup/cpuset: Avoid unnecessary cpus & mems update in cpuset_hotplug_update_tasks()

[Waiman Long]

As reported by sashiko [1], cpuset_hotplug_update_tasks() may perform
unnecessary task iteration and updating of tasks' CPU and node masks
when mems_allowed and/or cpus_allowed are not set in cpuset v2. It is
due to the fact that the temporary new_cpus and new_mems masks do not
inherit parent's effective_cpus/mems when they are empty which is the
expected behavior for cpuset v2 since commit 4ec22e9c5a90 ("cpuset:
Enable cpuset controller in default hierarchy").

Fix that and avoid unnecessay work by enhancing
compute_effective_cpumask() to add the empty cpumask check
and inheriting the parent's versions if empty when in v2. A new
compute_effective_nodemask() helper is also added to perform similar
function for new effective_mems.

Add new test_cpuset_prs.sh test cases to confirm that effective_cpus
will inherit the parent's version if cpuset.cpus is empty.

[1] https://sashiko.dev/#/patchset/20260621032816.1806773-1-longman%40redhat.com

Suggested-by: Ridong Chen <ridong.chen@linux.dev>
Fixes: 4ec22e9c5a90 ("cpuset: Enable cpuset controller in default hierarchy")
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset.c                        | 45 +++++++++++--------
 .../selftests/cgroup/test_cpuset_prs.sh       | 11 ++++-
 2 files changed, 35 insertions(+), 21 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index aff86acea701..044ddbf66f8e 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1094,12 +1094,35 @@ void cpuset_update_tasks_cpumask(struct cpuset *cs, struct cpumask *new_cpus)
  * @cs: the cpuset the need to recompute the new effective_cpus mask
  * @parent: the parent cpuset
  *
+ * For v2, the parent's effective_cpus is inherited if cpumask empty.
  * The result is valid only if the given cpuset isn't a partition root.
  */
 static void compute_effective_cpumask(struct cpumask *new_cpus,
 				      struct cpuset *cs, struct cpuset *parent)
 {
-	cpumask_and(new_cpus, cs->cpus_allowed, parent->effective_cpus);
+	bool has_cpus;
+
+	has_cpus = cpumask_and(new_cpus, cs->cpus_allowed, parent->effective_cpus);
+	if (!has_cpus && is_in_v2_mode())
+		cpumask_copy(new_cpus, parent->effective_cpus);
+}
+
+/**
+ * compute_effective_nodemask - Compute the effective nodemask of the cpuset
+ * @new_cpus: the temp variable for the new effective_mems mask
+ * @cs: the cpuset the need to recompute the new effective_mems mask
+ * @parent: the parent cpuset
+ *
+ * For v2, the parent's effective_mems is inherited if nodemask empty.
+ */
+static void compute_effective_nodemask(nodemask_t *new_mems,
+				       struct cpuset *cs, struct cpuset *parent)
+{
+	bool has_mems;
+
+	has_mems = nodes_and(*new_mems, cs->mems_allowed, parent->effective_mems);
+	if (!has_mems && is_in_v2_mode())
+		nodes_copy(*new_mems, parent->effective_mems);
 }
 
 /*
@@ -2148,15 +2171,6 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp,
 			goto update_parent_effective;
 		}
 
-		/*
-		 * If it becomes empty, inherit the effective mask of the
-		 * parent, which is guaranteed to have some CPUs unless
-		 * it is a partition root that has explicitly distributed
-		 * out all its CPUs.
-		 */
-		if (is_in_v2_mode() && !remote && cpumask_empty(tmp->new_cpus))
-			cpumask_copy(tmp->new_cpus, parent->effective_cpus);
-
 		/*
 		 * Skip the whole subtree if
 		 * 1) the cpumask remains the same,
@@ -2704,14 +2718,7 @@ static void update_nodemasks_hier(struct cpuset *cs, nodemask_t *new_mems)
 	cpuset_for_each_descendant_pre(cp, pos_css, cs) {
 		struct cpuset *parent = parent_cs(cp);
 
-		bool has_mems = nodes_and(*new_mems, cp->mems_allowed, parent->effective_mems);
-
-		/*
-		 * If it becomes empty, inherit the effective mask of the
-		 * parent, which is guaranteed to have some MEMs.
-		 */
-		if (is_in_v2_mode() && !has_mems)
-			*new_mems = parent->effective_mems;
+		compute_effective_nodemask(new_mems, cp, parent);
 
 		/* Skip the whole subtree if the nodemask remains the same. */
 		if (nodes_equal(*new_mems, cp->effective_mems)) {
@@ -3923,7 +3930,7 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
 
 	parent = parent_cs(cs);
 	compute_effective_cpumask(&new_cpus, cs, parent);
-	nodes_and(new_mems, cs->mems_allowed, parent->effective_mems);
+	compute_effective_nodemask(&new_mems, cs, parent);
 
 	if (!tmp || !cs->partition_root_state)
 		goto update_tasks;
diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
index 0d41aa0d343d..ca9bc38fdb95 100755
--- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
+++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
@@ -495,13 +495,20 @@ REMOTE_TEST_MATRIX=(
 	# Narrowing cpuset.cpus to previously sibling-excluded CPUs should
 	# not return CPUs that were never actually owned.
 	"  C1-4:P1   .   C1-2:P1  C1-3:P2  .       .  \
-	      .      .     .         C3    .       .     p1:4|c11:1-2|c12:3 \
+	      .      .     .       C3      .       .     p1:4|c11:1-2|c12:3 \
 							 p1:P1|c11:P1|c12:P2 3"
 	# Expanding cpuset.cpus to include a previously sibling-excluded CPU
 	# after the sibling has become a member should correctly request it.
 	"  C1-4:P1   .   C1-2:P1  C1-3:P2  .       .  \
-	      .      .      P0     C2-3    .       .     p1:1,4|c11:1|c12:2-3 \
+	      .      .     P0      C2-3    .       .     p1:1,4|c11:1|c12:2-3 \
 							 p1:P1|c11:P0|c12:P2 2-3"
+	# Cpusets with empty cpuset.cpus should inherit parent's effective_cpus
+	"  C1-4:P1 C5-6   C1-2     .       C5      .  \
+	      .      P1    P1      .       .       .     p1:3-4|p2:5-6|c11:1-2|c12:3-4|c21:5|c22:5-6 \
+							 p1:P1|p2:P1|c11:P1"
+	"  C1-4:P1 C5-6   C1-2     .       C5      .  \
+	      .      P1    P1      .      O5=0     .     p1:3-4|p2:6|c11:1-2|c12:3-4|c21:6|c22:6 \
+							 p1:P1|p2:P1|c11:P1"
 )
 
 #
-- 
2.54.0

[PATCH v2 2/2] cgroup/cpuset: Rebind/migrate mm only for threadgroup leader in cpuset_update_tasks_nodemask()

[Waiman Long]

As reported by sashiko [1], cpuset_update_tasks_nodemask() will do
mpol_rebind_mm() and possibly cpuset_migrate_mm() for all threads of
a multithreaded process. Since commit 3df9ca0a2b8b ("cpuset: migrate
memory only for threadgroup leaders"), cpuset_attach() had been updated
to rebind and migrate memory only for threadgroup leaders to mark the
group leader as the owner of the mm_struct.

To be consistent and avoid unnecessary performance overhead for heavily
multithreaded processes, follow the cpuset_attach() example and perform
memory rebind and migration only for threadgroup leaders.

Also add a paragraph in cgroup-v2.rst under cpuset.mems that the
threadgroup leader is the memory owner of that threadgroup. Therefore
the non-leading threads shouldn't be in other cgroups whose "cpuset.mems"
doesn't fully overlap that of the group leader.

[1] https://sashiko.dev/#/patchset/20260621032816.1806773-1-longman%40redhat.com

Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
---
 Documentation/admin-guide/cgroup-v2.rst | 7 +++++++
 kernel/cgroup/cpuset.c                  | 4 ++++
 2 files changed, 11 insertions(+)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 993446ab66d0..f9c353174a7e 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -2527,6 +2527,13 @@ Cpuset Interface Files
 	a need to change "cpuset.mems" with active tasks, it shouldn't
 	be done frequently.
 
+	For a multithreaded process, the threadgroup leader is
+	considered the owner of the group's memory. Memory policy
+	rebinding and migration will only happen with respect to the
+	threadgroup leader. To avoid unexpected result, non-leading
+	threads shouldn't be put into another cgroup whose "cpuset.mems"
+	doesn't fully overlap that of the threadgroup leader.
+
   cpuset.mems.effective
 	A read-only multiple values file which exists on all
 	cpuset-enabled cgroups.
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 044ddbf66f8e..055ae54a040a 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2673,6 +2673,10 @@ void cpuset_update_tasks_nodemask(struct cpuset *cs)
 
 		cpuset_change_task_nodemask(task, &newmems);
 
+		/* Rebind and migrate mm only for thread group leader */
+		if (!thread_group_leader(task))
+			continue;
+
 		mm = get_task_mm(task);
 		if (!mm)
 			continue;
-- 
2.54.0

Re: [PATCH v2 1/2] cgroup/cpuset: Avoid unnecessary cpus & mems update in cpuset_hotplug_update_tasks()

[Ridong Chen]

On 6/24/2026 7:04 AM, Waiman Long wrote:
> As reported by sashiko [1], cpuset_hotplug_update_tasks() may perform
> unnecessary task iteration and updating of tasks' CPU and node masks
> when mems_allowed and/or cpus_allowed are not set in cpuset v2. It is
> due to the fact that the temporary new_cpus and new_mems masks do not
> inherit parent's effective_cpus/mems when they are empty which is the
> expected behavior for cpuset v2 since commit 4ec22e9c5a90 ("cpuset:
> Enable cpuset controller in default hierarchy").
> 
> Fix that and avoid unnecessay work by enhancing
> compute_effective_cpumask() to add the empty cpumask check
> and inheriting the parent's versions if empty when in v2. A new
> compute_effective_nodemask() helper is also added to perform similar
> function for new effective_mems.
> 
> Add new test_cpuset_prs.sh test cases to confirm that effective_cpus
> will inherit the parent's version if cpuset.cpus is empty.
> 
> [1] https://sashiko.dev/#/patchset/20260621032816.1806773-1-longman%40redhat.com
> 
> Suggested-by: Ridong Chen <ridong.chen@linux.dev>
> Fixes: 4ec22e9c5a90 ("cpuset: Enable cpuset controller in default hierarchy")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>   kernel/cgroup/cpuset.c                        | 45 +++++++++++--------
>   .../selftests/cgroup/test_cpuset_prs.sh       | 11 ++++-
>   2 files changed, 35 insertions(+), 21 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index aff86acea701..044ddbf66f8e 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1094,12 +1094,35 @@ void cpuset_update_tasks_cpumask(struct cpuset *cs, struct cpumask *new_cpus)
>    * @cs: the cpuset the need to recompute the new effective_cpus mask
>    * @parent: the parent cpuset
>    *
> + * For v2, the parent's effective_cpus is inherited if cpumask empty.
>    * The result is valid only if the given cpuset isn't a partition root.
>    */
>   static void compute_effective_cpumask(struct cpumask *new_cpus,
>   				      struct cpuset *cs, struct cpuset *parent)
>   {
> -	cpumask_and(new_cpus, cs->cpus_allowed, parent->effective_cpus);
> +	bool has_cpus;
> +
> +	has_cpus = cpumask_and(new_cpus, cs->cpus_allowed, parent->effective_cpus);
> +	if (!has_cpus && is_in_v2_mode())
> +		cpumask_copy(new_cpus, parent->effective_cpus);
> +}
> +
> +/**
> + * compute_effective_nodemask - Compute the effective nodemask of the cpuset
> + * @new_cpus: the temp variable for the new effective_mems mask
> + * @cs: the cpuset the need to recompute the new effective_mems mask
> + * @parent: the parent cpuset
> + *
> + * For v2, the parent's effective_mems is inherited if nodemask empty.
> + */
> +static void compute_effective_nodemask(nodemask_t *new_mems,
> +				       struct cpuset *cs, struct cpuset *parent)
> +{
> +	bool has_mems;
> +
> +	has_mems = nodes_and(*new_mems, cs->mems_allowed, parent->effective_mems);
> +	if (!has_mems && is_in_v2_mode())
> +		nodes_copy(*new_mems, parent->effective_mems);
>   }
>   
>   /*
> @@ -2148,15 +2171,6 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp,
>   			goto update_parent_effective;
>   		}
>   
> -		/*
> -		 * If it becomes empty, inherit the effective mask of the
> -		 * parent, which is guaranteed to have some CPUs unless
> -		 * it is a partition root that has explicitly distributed
> -		 * out all its CPUs.
> -		 */
> -		if (is_in_v2_mode() && !remote && cpumask_empty(tmp->new_cpus))
> -			cpumask_copy(tmp->new_cpus, parent->effective_cpus);
> -
>   		/*
>   		 * Skip the whole subtree if
>   		 * 1) the cpumask remains the same,
> @@ -2704,14 +2718,7 @@ static void update_nodemasks_hier(struct cpuset *cs, nodemask_t *new_mems)
>   	cpuset_for_each_descendant_pre(cp, pos_css, cs) {
>   		struct cpuset *parent = parent_cs(cp);
>   
> -		bool has_mems = nodes_and(*new_mems, cp->mems_allowed, parent->effective_mems);
> -
> -		/*
> -		 * If it becomes empty, inherit the effective mask of the
> -		 * parent, which is guaranteed to have some MEMs.
> -		 */
> -		if (is_in_v2_mode() && !has_mems)
> -			*new_mems = parent->effective_mems;
> +		compute_effective_nodemask(new_mems, cp, parent);
>   
>   		/* Skip the whole subtree if the nodemask remains the same. */
>   		if (nodes_equal(*new_mems, cp->effective_mems)) {
> @@ -3923,7 +3930,7 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
>   
>   	parent = parent_cs(cs);
>   	compute_effective_cpumask(&new_cpus, cs, parent);
> -	nodes_and(new_mems, cs->mems_allowed, parent->effective_mems);
> +	compute_effective_nodemask(&new_mems, cs, parent);
>   
>   	if (!tmp || !cs->partition_root_state)
>   		goto update_tasks;
> diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> index 0d41aa0d343d..ca9bc38fdb95 100755
> --- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> +++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
> @@ -495,13 +495,20 @@ REMOTE_TEST_MATRIX=(
>   	# Narrowing cpuset.cpus to previously sibling-excluded CPUs should
>   	# not return CPUs that were never actually owned.
>   	"  C1-4:P1   .   C1-2:P1  C1-3:P2  .       .  \
> -	      .      .     .         C3    .       .     p1:4|c11:1-2|c12:3 \
> +	      .      .     .       C3      .       .     p1:4|c11:1-2|c12:3 \
>   							 p1:P1|c11:P1|c12:P2 3"
>   	# Expanding cpuset.cpus to include a previously sibling-excluded CPU
>   	# after the sibling has become a member should correctly request it.
>   	"  C1-4:P1   .   C1-2:P1  C1-3:P2  .       .  \
> -	      .      .      P0     C2-3    .       .     p1:1,4|c11:1|c12:2-3 \
> +	      .      .     P0      C2-3    .       .     p1:1,4|c11:1|c12:2-3 \
>   							 p1:P1|c11:P0|c12:P2 2-3"
> +	# Cpusets with empty cpuset.cpus should inherit parent's effective_cpus
> +	"  C1-4:P1 C5-6   C1-2     .       C5      .  \
> +	      .      P1    P1      .       .       .     p1:3-4|p2:5-6|c11:1-2|c12:3-4|c21:5|c22:5-6 \
> +							 p1:P1|p2:P1|c11:P1"
> +	"  C1-4:P1 C5-6   C1-2     .       C5      .  \
> +	      .      P1    P1      .      O5=0     .     p1:3-4|p2:6|c11:1-2|c12:3-4|c21:6|c22:6 \
> +							 p1:P1|p2:P1|c11:P1"
>   )
>   
>   #

LGTM.

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

-- 
Best regards
Ridong

Re: [PATCH v2 2/2] cgroup/cpuset: Rebind/migrate mm only for threadgroup leader in cpuset_update_tasks_nodemask()

[Manuel Ebner]

Hi

On Tue, 2026-06-23 at 19:04 -0400, Waiman Long wrote:
> [...]
> Also add a paragraph in cgroup-v2.rst under cpuset.mems that the
> threadgroup leader is the memory owner of that threadgroup. Therefore
> the non-leading threads shouldn't be in other cgroups whose "cpuset.mems"
> doesn't fully overlap that of the group leader.

This sentence is long and complex, split into two if possible. I couldn't
figure out how to do so.

> [...]
> --- a/Documentation/admin-guide/cgroup-v2.rst
> +++ b/Documentation/admin-guide/cgroup-v2.rst
> @@ -2527,6 +2527,13 @@ Cpuset Interface Files
>  	a need to change "cpuset.mems" with active tasks, it shouldn't
>  	be done frequently.
>  
> +	For a multithreaded process, the threadgroup leader is
> +	considered the owner of the group's memory. Memory policy
> +	rebinding and migration will only happen with respect to the
> +	threadgroup leader. To avoid unexpected result, non-leading

/result/results/
or 
To avoid an unexpected result, 

> +	threads shouldn't be put into another cgroup whose "cpuset.mems"
> +	doesn't fully overlap that of the threadgroup leader.

maybe
/threadgroup/threadgroups/


Thanks
 Manuel

Re: [PATCH v2 1/2] cgroup/cpuset: Avoid unnecessary cpus & mems update in cpuset_hotplug_update_tasks()

[Manuel Ebner]

On Tue, 2026-06-23 at 19:04 -0400, Waiman Long wrote:
> As reported by sashiko [1], cpuset_hotplug_update_tasks() may perform
> unnecessary task iteration and updating of tasks' CPU and node masks
> when mems_allowed and/or cpus_allowed are not set in cpuset v2. It is
> due to the fact that the temporary new_cpus and new_mems masks do not
> inherit parent's effective_cpus/mems when they are empty which is the
> expected behavior for cpuset v2 since commit 4ec22e9c5a90 ("cpuset:
> Enable cpuset controller in default hierarchy").
> 
> Fix that and avoid unnecessay work by enhancing
> compute_effective_cpumask() to add the empty cpumask check
> and inheriting the parent's versions if empty when in v2. A new
> compute_effective_nodemask() helper is also added to perform similar
> function for new effective_mems.

perform a similar function
or
perform similar functions

> [...]
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index aff86acea701..044ddbf66f8e 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1094,12 +1094,35 @@ void cpuset_update_tasks_cpumask(struct cpuset *cs, struct
> cpumask *new_cpus)
>   * @cs: the cpuset the need to recompute the new effective_cpus mask
>   * @parent: the parent cpuset
>   *
> + * For v2, the parent's effective_cpus is inherited if cpumask empty.

+ * For v2, the parent's effective_cpus is inherited if cpumask is empty.

Thanks
 Manuel

Re: [PATCH v2 0/2] cgroup/cpuset: Miscellaneous fixes and cleanups

[Tejun Heo]

> Waiman Long (2):
>   cgroup/cpuset: Avoid unnecessary cpus & mems update in
>     cpuset_hotplug_update_tasks()
>   cgroup/cpuset: Rebind/migrate mm only for threadgroup leader in
>     cpuset_update_tasks_nodemask()

Applied 1-2 to cgroup/for-7.3. I folded in a few minor fixups: a
changelog typo, the compute_effective_nodemask() kerneldoc parameter
name (new_cpus to new_mems), and the comment and doc grammar nits Manuel
noted. Also added Ridong's Reviewed-by to patch 1.

Thanks.

--
tejun