[PATCH v5 3/3] cgroup/cpuset: Keep user set cpus affinity

[Waiman Long]

It was found that any change to the current cpuset hierarchy may reset
the cpumask of the tasks in the affected cpusets to the default cpuset
value even if those tasks have cpus affinity explicitly set by the users
before. That is especially easy to trigger under a cgroup v2 environment
where writing "+cpuset" to the root cgroup's cgroup.subtree_control
file will reset the cpus affinity of all the processes in the system.

That is problematic in a nohz_full environment where the tasks running
in the nohz_full CPUs usually have their cpus affinity explicitly set
and will behave incorrectly if cpus affinity changes.

Fix this problem by looking at user_cpus_ptr which will be set if
cpus affinity have been explicitly set before and use it to restrcit
the given cpumask unless there is no overlap. In that case, it will
fallback to the given one.

To handle possible racing with concurrent sched_setaffinity() call, the
user_cpus_ptr is rechecked again after a successful set_cpus_allowed_ptr()
call. If the user_cpus_ptr status changes, the operation is retried
again with the newly assigned user_cpus_ptr.

With that change in place, it was verified that tasks that have its
cpus affinity explicitly set will not be affected by changes made to
the v2 cgroup.subtree_control files.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 kernel/cgroup/cpuset.c | 42 ++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 40 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 58aadfda9b8b..a663848d0459 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -704,6 +704,44 @@ static int validate_change(struct cpuset *cur, struct cpuset *trial)
 	return ret;
 }
 
+/*
+ * Preserve user provided cpumask (if set) as much as possible unless there
+ * is no overlap with the given mask.
+ */
+static int cpuset_set_cpus_allowed_ptr(struct task_struct *p,
+				       const struct cpumask *mask)
+{
+	cpumask_var_t new_mask;
+	int ret;
+
+	if (!READ_ONCE(p->user_cpus_ptr)) {
+		ret = set_cpus_allowed_ptr(p, mask);
+		/*
+		 * If user_cpus_ptr becomes set now, we are racing with
+		 * a concurrent sched_setaffinity(). So use the newly
+		 * set user_cpus_ptr and retry again.
+		 *
+		 * TODO: We cannot detect change in the cpumask pointed to
+		 * by user_cpus_ptr. We will have to add a sequence number
+		 * if such a race needs to be addressed.
+		 */
+		if (ret || !READ_ONCE(p->user_cpus_ptr))
+			return ret;
+	}
+
+	if (!alloc_cpumask_var(&new_mask, GFP_KERNEL))
+		return -ENOMEM;
+
+	if (copy_user_cpus_mask(p, new_mask) &&
+	    cpumask_and(new_mask, new_mask, mask))
+		ret = set_cpus_allowed_ptr(p, new_mask);
+	else
+		ret = set_cpus_allowed_ptr(p, mask);
+
+	free_cpumask_var(new_mask);
+	return ret;
+}
+
 #ifdef CONFIG_SMP
 /*
  * Helper routine for generate_sched_domains().
@@ -1130,7 +1168,7 @@ static void update_tasks_cpumask(struct cpuset *cs)
 
 	css_task_iter_start(&cs->css, 0, &it);
 	while ((task = css_task_iter_next(&it)))
-		set_cpus_allowed_ptr(task, cs->effective_cpus);
+		cpuset_set_cpus_allowed_ptr(task, cs->effective_cpus);
 	css_task_iter_end(&it);
 }
 
@@ -2303,7 +2341,7 @@ static void cpuset_attach(struct cgroup_taskset *tset)
 		 * can_attach beforehand should guarantee that this doesn't
 		 * fail.  TODO: have a better way to handle failure here
 		 */
-		WARN_ON_ONCE(set_cpus_allowed_ptr(task, cpus_attach));
+		WARN_ON_ONCE(cpuset_set_cpus_allowed_ptr(task, cpus_attach));
 
 		cpuset_change_task_nodemask(task, &cpuset_attach_nodemask_to);
 		cpuset_update_task_spread_flag(cs, task);
-- 
2.31.1