[PATCH-next v3 0/3] cgroup/cpuset: Support multiple destination cpusets for cpuset_*attach()

[PATCH-next v3 0/3] cgroup/cpuset: Support multiple destination cpusets for cpuset_*attach()

[Waiman Long]

 v3:
  - Swap the first 2 patches as the original patch 1 can introduce serious bug
    without patch 2. Doing patch 2 first will be less problematic.

 v2: https://lore.kernel.org/lkml/20260712150127.236790-1-longman@redhat.com
  - Make sure that attach_ctx.old_cs won't be set to a source cpuset that is
    also the destination cpuset.

 v1: https://lore.kernel.org/lkml/20260711020540.176740-1-longman@redhat.com

This is a follow-up patch series to [1] to properly handle a special case
for cpuset task migration operation where the source and destination
cpusets are the same.

Patch 1 enables cpuset_*attach() to handle the case where there are many
destination cpusets from enabling cpuset controller. Patch 2 handles
those tasks that have the same source and destination cpuset by skipping
them as they are not migrating with respect to cpuset. Patch 3 adds a
new test case into test_cpuset to test proper handling of cpu affinity
when cpuset controller is disabled.

[1] https://lore.kernel.org/lkml/20260702214757.579012-1-longman@redhat.com

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

Waiman Long (2):
  cgroup/cpuset: Support multiple destination cpusets for
    cpuset_*attach()
  cgroup/cpuset: Handle the special case of non-moving tasks in
    cpuset_can_attach()

 kernel/cgroup/cpuset.c                       | 131 ++++++----
 tools/testing/selftests/cgroup/test_cpuset.c | 243 +++++++++++++++++++
 2 files changed, 321 insertions(+), 53 deletions(-)

-- 
2.55.0

[PATCH-next v3 1/3] 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.

A warning will be printed if there are multiple destination cpusets but
it is not on default hierarchy or when the CPUs or memory nodes change.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset.c | 99 ++++++++++++++++++++++++------------------
 1 file changed, 56 insertions(+), 43 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index cf0d005d2b78..a95cc0040101 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);
 
 /*
  * Wait if task attach is in progress until it is done and then acquire
@@ -3052,6 +3054,9 @@ static int cpuset_can_attach_check(struct cpuset *cs, struct cpuset *oldcs,
 	if (!llist_on_list(&oldcs->attach_node))
 		llist_add(&oldcs->attach_node, &src_cs_head);
 
+	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);
 
@@ -3080,35 +3085,31 @@ 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)
 {
@@ -3120,7 +3121,11 @@ static void clear_attach_data(struct llist_head *head, bool cancel)
 		if (cs->nr_migrate_dl_tasks) {
 			if (!cancel)
 				atomic_add(cs->nr_migrate_dl_tasks, &cs->nr_deadline_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;
 		}
 	}
 }
@@ -3142,6 +3147,7 @@ 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);
@@ -3166,9 +3172,13 @@ 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)
@@ -3202,12 +3212,28 @@ 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 handle.
+	 * Print a warning and abort the attach operation as further code
+	 * change may 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); /* Destination cpuset only */
 		clear_attach_data(&src_cs_head, true);
+		clear_attach_data(&dst_cs_head, true);
 	} else {
 		attach_ctx.in_progress++;
 	}
@@ -3218,22 +3244,10 @@ 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);
 }
 
@@ -3316,25 +3330,24 @@ 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 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;
+	}
 
 	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) {
-		atomic_add(cs->nr_migrate_dl_tasks, &cs->nr_deadline_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);
-- 
2.55.0

[PATCH-next v3 2/3] cgroup/cpuset: Handle the special case of non-moving tasks in cpuset_can_attach()

[Waiman Long]

With cgroup v2 migration of a multithreaded process having threads
in different cgroups of a threaded subtree, it is possible that
cpuset_can_attach() can be called with tasks that are not migrating with
respect to cpuset if cpuset controller is not enabled in some of the
subtree cgroups. IOW, the old cpuset can be the same as the new one. This
can cause problem when we need to track the set of old cpusets and the
new cpusets in singly linked lists as a cpuset cannot be in both lists.

As reported by Tejun, the following is an example threaded subtree with
partial cpuset delegation that can cause this issue to show up.

  P (+cpuset)
  |- R (cpuset)        <- destination
  |  `- C (no cpuset)  -> effective cpuset == R
  `- W (cpuset)

Group leader in R, thread_a in C, thread_b in W; migrate the whole
process into R (echo $PID > R/cgroup.procs). thread_a moves C->R:
its cgroup changes so compare_css_sets() keeps it in the taskset, but
its cpuset css is unchanged (C inherits R's), so task_cs() == cs ==
R. cpuset is in ss_mask because thread_b (W->R) changed. can_attach()
then tags R as a source (thread_a) and the destination (thread_b):

Handle this special case by skipping tasks that are not migrating in
cpuset_can_attach() and avoid calling cpuset_can_attach_check() in this
case. By doing so, the destination cpuset will not be put into source
cpuset linked list.

As the source cpuset cannot be easily determined in cpuset_attach(),
unnecessary work can be performed if a task is not actually
migrating. However, no harm will be done except wasting some
CPU cycles. If it happens that none of the tasks is migrating,
attach_ctx.old_cs will be NULL and task iteration won't be needed.

Reported-by: Tejun Heo <tj@kernel.org>
Closes: https://lore.kernel.org/lkml/e254af713b5345aec3d086771ecf1e71@kernel.org
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset.c | 34 +++++++++++++++++++++++-----------
 1 file changed, 23 insertions(+), 11 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index a95cc0040101..d46590c05173 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -3139,21 +3139,13 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 	bool setsched_check;
 	int ret;
 
-	/* used later by cpuset_attach() */
-	attach_ctx.old_cs = task_cs(cgroup_taskset_first(tset, &css));
-	oldcs = attach_ctx.old_cs;
-	cs = css_cs(css);
-
+	cs = oldcs = NULL;
 	mutex_lock(&cpuset_mutex);
+	attach_ctx.old_cs = NULL;	/* Used later in cpuset_attach_task() */
 	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);
-	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
@@ -3170,21 +3162,33 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 	 * 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.
+	 *
+	 * For a v2 threaded subtree where cpuset isn't enabled in some of the
+	 * cgroups, it is possible that oldcs == cs for some of the tasks.
+	 * In this case, we can skip checking on those tasks as there is no
+	 * actual migration wrt cpuset.
 	 */
 	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) || (new_cs != cs)) {
-			if (new_cs != cs)
+			if (cs && (new_cs != cs))
 				attach_ctx.many_dest_cs = true;
 			cs = new_cs;
 			oldcs = new_oldcs;
+			if (oldcs == cs)
+				continue;
+			if (!attach_ctx.old_cs)
+				attach_ctx.old_cs = oldcs;
 			ret = cpuset_can_attach_check(cs, oldcs, &setsched_check);
 			if (ret)
 				goto out_unlock;
 		}
 
+		if (oldcs == cs)
+			continue;
+
 		ret = task_can_attach(task);
 		if (ret)
 			goto out_unlock;
@@ -3326,6 +3330,14 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 	attach_ctx.task_work_queued = false;
 	guarantee_online_mems(cs, &attach_ctx.nodemask_to);
 
+	/*
+	 * attach_ctx.old_cs can only be NULL if no task is actually migrating.
+	 * This is highly unlikely. If it happens at all, we can skip task
+	 * iteration and setting old_mems_allowed.
+	 */
+	if (unlikely(!attach_ctx.old_cs))
+		goto out;
+
 	/*
 	 * In the default hierarchy, enabling cpuset in the child cgroups
 	 * will trigger a cpuset_attach() call with no change in effective cpus
-- 
2.55.0

[PATCH-next v3 3/3] 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
 
-- 
2.55.0

Re: [PATCH-next v3 0/3] cgroup/cpuset: Support multiple destination cpusets for cpuset_*attach()

[Ridong Chen]

On 7/13/2026 7:55 AM, Waiman Long wrote:
>   v3:
>    - Swap the first 2 patches as the original patch 1 can introduce serious bug
>      without patch 2. Doing patch 2 first will be less problematic.
> 

Would it make sense to squash these two patches into one?

>   v2: https://lore.kernel.org/lkml/20260712150127.236790-1-longman@redhat.com
>    - Make sure that attach_ctx.old_cs won't be set to a source cpuset that is
>      also the destination cpuset.
> 
>   v1: https://lore.kernel.org/lkml/20260711020540.176740-1-longman@redhat.com
> 
> This is a follow-up patch series to [1] to properly handle a special case
> for cpuset task migration operation where the source and destination
> cpusets are the same.
> 
> Patch 1 enables cpuset_*attach() to handle the case where there are many
> destination cpusets from enabling cpuset controller. Patch 2 handles
> those tasks that have the same source and destination cpuset by skipping
> them as they are not migrating with respect to cpuset. Patch 3 adds a
> new test case into test_cpuset to test proper handling of cpu affinity
> when cpuset controller is disabled.
> 
> [1] https://lore.kernel.org/lkml/20260702214757.579012-1-longman@redhat.com
> 
> Michal Koutný (1):
>    selftests/cgroup: Add test for cpuset affinity on controller disable
> 
> Waiman Long (2):
>    cgroup/cpuset: Support multiple destination cpusets for
>      cpuset_*attach()
>    cgroup/cpuset: Handle the special case of non-moving tasks in
>      cpuset_can_attach()
> 
>   kernel/cgroup/cpuset.c                       | 131 ++++++----
>   tools/testing/selftests/cgroup/test_cpuset.c | 243 +++++++++++++++++++
>   2 files changed, 321 insertions(+), 53 deletions(-)
> 

-- 
Best regards
Ridong