[PATCH-next 1/6] cgroup/cpuset: Find furthest isolated ancestor for invalidated isolated partition

[Waiman Long <longman@redhat.com>]

Commit 46b885416c27 ("cgroup/cpuset: Fix a missing isolated_cpus
update in update_cpumasks_hier()") searches for the closet partition
root ancestor to determine if the isolated CPUs in a to be invalidated
isolation partition should be removed from isolated_cpus or not.

As reported by sashiko, this method may not work when there is a deeply
nested isolated partition hierarchy. For instance, Consider a nested
hierarchy A0 -> A1 -> A2 -> A3 where all four are PRS_ISOLATED:

  A0: effective_xcpus = 0-4
  A1: effective_xcpus = 1-4
  A2: effective_xcpus = 2-4
  A3: effective_xcpus = 3-4

When A1 shrinks its exclusive_cpus from 1-4 to 1-2, A2 will update it
effective_xcpus to 2. A3 will find its closet partition root ancestor
A2 and decides that its effective_xcpus 3-4 should be removed from
isolated_cpus which is incorrect.

Address this corner case by searching for the furthest isolated partition
root ancestor instead to decide if its isolated CPUs should be returned
to isolated_cpus.

Fixes: 46b885416c27 ("cgroup/cpuset: Fix a missing isolated_cpus update in update_cpumasks_hier()")
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/cgroup/cpuset.c | 36 +++++++++++++++++-------------------
 1 file changed, 17 insertions(+), 19 deletions(-)

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 585a982249ad..7ead8c3b7974 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2156,22 +2156,20 @@ static void compute_partition_effective_cpumask(struct cpuset *cs,
 }
 
 /*
- * ancestor_partition - Find and return the closest valid partition root ancestor
+ * iancestor_partition - Return furthest isolated partition root ancestor available
  * @cs: the current cpuset
- *
- * Return: The closet partition root ancestor cpuset
+ * Return: The furthest isolated partition root ancestor available or NULL if
+ *	   not found.
  */
-static struct cpuset *ancestor_partition(struct cpuset *cs)
+static struct cpuset *iancestor_partition(struct cpuset *cs)
 {
 	struct cpuset *ancestor = parent_cs(cs);
+	struct cpuset *iancestor = NULL;
 
-	/*
-	 * The top_cpuset is always a valid partition root which will stop
-	 * the loop if it is reached.
-	 */
-	while (!is_partition_valid(ancestor))
-		ancestor = parent_cs(ancestor);
-	return ancestor;
+	for (; ancestor; ancestor = parent_cs(ancestor))
+		if (ancestor->partition_root_state == PRS_ISOLATED)
+			iancestor = ancestor;
+	return iancestor;
 }
 
 /*
@@ -2283,23 +2281,23 @@ static void update_cpumasks_hier(struct cpuset *cs, struct tmpmasks *tmp,
 				 * For a to be invalidated isolated partition,
 				 * its isolated CPUs may or may not need to be
 				 * removed from isolated_cpus depending on if
-				 * its closest partition root ancestor is a root
-				 * or isolated partition. With a PRS_ISOLATED
-				 * ancestor, the CPUs are checked against its
-				 * effective xcpus to see if any of them needed
-				 * to be removed.
+				 * there is a furthest isolated partition root
+				 * ancestor available. When such a PRS_ISOLATED
+				 * ancestor is available, the CPUs are checked
+				 * against its effective xcpus to see if any of
+				 * them needed to be removed.
 				 */
 				if (is_partition_valid(cp))
 					new_prs = -old_prs;
 
 				if (old_prs == PRS_ISOLATED) {
-					struct cpuset *ancestor = ancestor_partition(cp);
+					struct cpuset *iancestor = iancestor_partition(cp);
 
-					if (ancestor->partition_root_state == PRS_ROOT)
+					if (!iancestor)
 						del_cpus = cp->effective_xcpus;
 					else if (cpumask_andnot(tmp->delmask,
 								cp->effective_xcpus,
-								ancestor->effective_xcpus))
+								iancestor->effective_xcpus))
 						del_cpus = tmp->delmask;
 
 					if (del_cpus) {
-- 
2.55.0