Re: [PATCH-next v2 2/5] cgroup/cpuset: Correctly set cpus_updated in cpuset_hotplug_update_tasks()
From: Waiman Long
Date: Tue Nov 10 2026 - 21:01:43 EST
On 11/10/26 3:40 AM, Ridong Chen wrote:
Yes, you are right. Thanks for the review. Will send out a v3 with the suggested changes.
On 11/10/2026 3:09 AM, Waiman Long wrote:
When cpuset_hotplug_update_tasks() is called with the conditions that
either remote_partition_disable() or update_parent_effective_cpumask()
has to be called, the cs->effective_cpus can be updated such that it
matches what is computed in new_cpus. In such case, cpus_updated can
be incorrectly set to false leading to stale CPU afinity remained in the
tasks not matching the new cpuset value. Fix that by forcing cpus_updated
to true whenever one of these partition helpers is being called.
Fixes: 4b842da276a8a ("cpuset: Make CPU hotplug work with partition")
Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
Link: https://sashiko.dev/#/bug/linux-ab90f7e8-0d60-40ca-83e9-e405075b24d2
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
kernel/cgroup/cpuset.c | 11 +++++++++--
1 file changed, 9 insertions(+), 2 deletions(-)
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 6074327c3477..3955d2d574cf 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -4002,7 +4002,7 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
{
static cpumask_t new_cpus;
static nodemask_t new_mems;
- bool cpus_updated;
+ bool cpus_updated = false;
bool mems_updated;
int partcmd = -1;
struct cpuset *parent;
@@ -4015,12 +4015,18 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
if (!tmp || !cs->partition_root_state)
goto update_tasks;
+ /*
+ * cpus_updated should be set to true when remote_partition_disable()
+ * or update_parent_effective_cpumask() is called as cs->effective_cpus
+ * may have been updated.
+ */
if (is_remote_partition(cs) && (cpumask_empty(subpartitions_cpus) ||
(cpumask_empty(&new_cpus) &&
partition_is_populated(cs, NULL)))) {
WRITE_ONCE(cs->prs_err, PERR_HOTPLUG);
remote_partition_disable(cs, tmp);
compute_effective_cpumask(&new_cpus, cs, parent);
+ cpus_updated = true;
}
/*
@@ -4045,6 +4051,7 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
partcmd = partcmd_update;
if (partcmd >= 0) {
+ cpus_updated = true;
update_parent_effective_cpumask(cs, partcmd, NULL, tmp);
if (is_partition_valid(cs) || (partcmd == partcmd_invalidate)) {
compute_effective_cpumask(&new_cpus, cs, parent);
@@ -4053,7 +4060,7 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
}
update_tasks:
- cpus_updated = !cpumask_equal(&new_cpus, cs->effective_cpus);
+ cpus_updated |= !cpumask_equal(&new_cpus, cs->effective_cpus);
mems_updated = !nodes_equal(new_mems, cs->effective_mems);
if (!cpus_updated && !mems_updated)
goto unlock; /* Hotplug doesn't affect this cpuset */
I think the issue only happens in the remote partition case.
remote_partition_disable() calls reset_partition_data(), which sets
cs->effective_cpus to what compute_effective_cpumask() will return
afterwards, so the cpumask_equal() check misses the change while the
tasks still have the old affinity.
update_parent_effective_cpumask() only updates the effective_cpus of
the parent (through partition_xcpus_add/del()), and cs is skipped in
update_sibling_cpumasks(). So cs->effective_cpus isn't changed there and
the original check still works. If new_cpus is the same as
cs->effective_cpus in that case, forcing cpus_updated just walks all the
tasks in cs and calls set_cpus_allowed_ptr() with their current mask.
Should cpus_updated only be set in the remote partition case?
Also, as the problem comes from remote_partition_disable() in the
hotplug path, should the Fixes tag be the following instead?
Fixes: 181c8e091aae ("cgroup/cpuset: Introduce remote partition")
Cheers,
Longman