Re: [PATCH-next 1/6] cgroup/cpuset: Find furthest isolated ancestor for invalidated isolated partition
From: Ridong Chen
Date: Fri Oct 23 2026 - 02:25:03 EST
On 10/22/2026 11:16 AM, Waiman Long wrote:
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@xxxxxxxxxx>
---
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)
Nit.
iancestor is confusing.
Can we renmae it furthest_isolated_ancestor?
{
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) {
Reviewed-by: Ridong Chen <ridong.chen@xxxxxxxxx>
Thanks.
--
Best regards
Ridong