[PATCH-next v4 4/6] cgroup/cpuset: Made cpuset_attach_old_cs track task group leaders

[Waiman Long <longman@redhat.com>]

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 moved 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 cpuset_attach_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 be necessary
no matter which child cpuset is selected as cpuset_attach_old_cs.

IOW, cpuset_attach_old_cs should be updated to match the latest task
group leader in cpuset_can_attach().

Suggested-by: Ridong Chen <ridong.chen@linux.dev>
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 0f93f3d84494..0bb63a9cda0b 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1111,6 +1111,20 @@ static void update_sibling_cpumasks(struct cpuset *parent, struct cpuset *cs,
 /*
  * cpuset_can_attach() and cpuset_attach() specific internal data
  * Protected by cpuset_mutex
+ *
+ * The cpuset_attach_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
+ *    moved 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 cpuset_attach_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 be necessary no
+ * matter which child cpuset is selected as cpuset_attach_old_cs.
  */
 static struct cpuset *cpuset_attach_old_cs;
 static bool attach_cpus_updated;
@@ -3091,6 +3105,10 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
 		if (ret)
 			goto out_unlock;
 
+		/* Update cpuset_attach_old_cs to the latest group leader */
+		if (task == task->group_leader)
+			cpuset_attach_old_cs = task_cs(task);
+
 		if (setsched_check) {
 			ret = security_task_setscheduler(task);
 			if (ret)
-- 
2.54.0