Re: [PATCH-next v3 3/3] cgroup/cpuset: Check remote partition underneath before making an invalid partition valid
From: Waiman Long
Date: Thu Nov 12 2026 - 15:58:12 EST
On 11/11/26 10:27 AM, Guopeng Zhang wrote:
在 2026/11/11 10:02, Waiman Long 写道:
When re-evaluating an invalid local partition via partcmd_update,Hi,Longman
update_parent_effective_cpumask() does not check whether a descendant
remote partition is using its exclusive CPUs, promoting the cpuset to a
valid partition root over the remote partition. This condition should only
be possible if parent is the top_cpuset and exclusive_cpus isn't empty.
This leaves an active remote partition underneath a valid
local partition root with overlapping exclusive CPUs, breaking
scheduler domain partitioning and triggering warnings in
compute_partition_effective_cpumask() and generate_sched_domains().
Fix that by checking the effective_xcpus against subpartitions_cpus
when an underneath remote partition is possible to make sure that all
the exclusive CPUs are available before allowing the invalid partition
to be switched back to valid.
Also make sure that effective_xcpus is non-empty before swtiching an
invalid partition back to valid.
Add a test case in test_cpuset_prs.sh to verify that an invalid partition
with a remote partition underneath cannot be switched back to valid
with a hotplug event.
Fixes: 181c8e091aae ("cgroup/cpuset: Introduce remote partition")
Reported-by: Sashiko <sashiko-bot@xxxxxxxxxx>
Link: https://sashiko.dev/#/bug/linux-47715b89-d5fb-4731-b7c9-070773c6ff75
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
kernel/cgroup/cpuset.c | 42 ++++++++++++-------
.../selftests/cgroup/test_cpuset_prs.sh | 4 ++
2 files changed, 30 insertions(+), 16 deletions(-)
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 8b684361c94e..af5fd543fe88 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -2034,11 +2034,7 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
* tasks_nocpu_error() with this very mask, and a failing
* write is routed to partcmd_invalidate instead.
*/
- if (is_partition_invalid(cs)) {
- adding = false;
- deleting = cpumask_and(tmp->delmask,
- newmask, parent->effective_xcpus);
- } else {
+ if (is_partition_valid(cs)) {
cpumask_andnot(tmp->addmask, cs->effective_xcpus, newmask);
adding = cpumask_and(tmp->addmask, tmp->addmask,
parent->effective_xcpus);
@@ -2046,6 +2042,17 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
cpumask_andnot(tmp->delmask, newmask, cs->effective_xcpus);
deleting = cpumask_and(tmp->delmask, tmp->delmask,
parent->effective_xcpus);
+ } else {
+ deleting = cpumask_and(tmp->delmask,
+ newmask, parent->effective_xcpus);
+
+ /* Check if a remote partition may be underneath */
+ if ((parent == &top_cpuset) &&
+ !cpumask_empty(cs->exclusive_cpus) &&
+ cpumask_intersects(tmp->delmask, subpartitions_cpus)) {
+ part_error = PERR_REMOTE;
+ deleting = false;
+ }
}
/*
@@ -2126,34 +2133,37 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
} else if (is_partition_invalid(cs)) {
struct cgroup_subsys_state *css;
struct cpuset *child;
- bool exclusive = true;
/*
* Convert invalid partition to valid has to pass the
- * cpu exclusivity test when cpuset.cpus.exclusive is
- * empty as well as the housekeeping check.
+ * following tests:
+ * 1) housekeeping CPU conflict check
+ * 2) cpu exclusivity test when cpuset.cpus.exclusive is
+ * empty.
+ * 3) if parent is top_cpuset, none of the effective_xcpus
+ * (tmp->delmask) are in subpartitions_cpus, if not a
+ * remote partition may be underneath.
*/
if (prstate_housekeeping_conflict(cs->partition_root_state,
parent_prs, tmp->delmask, NULL)) {
part_error = PERR_HKEEPING;
- goto write_error;
- }
- if (cpumask_empty(cs->exclusive_cpus)) {
+ } else if (cpumask_empty(cs->exclusive_cpus)) {
rcu_read_lock();
cpuset_for_each_child(child, css, parent) {
if (child == cs)
continue;
if (!cpusets_are_exclusive(cs, child)) {
- exclusive = false;
+ part_error = PERR_NOTEXCL;
break;
}
}
rcu_read_unlock();
+ } else if ((parent == &top_cpuset) &&
+ cpumask_intersects(tmp->delmask, subpartitions_cpus)) {
+ part_error = PERR_REMOTE;
}
I think this check can still miss a remote partition during hotplug.
When CPU hotplug leaves no active CPU available to the top partition,
cpuset_handle_hotplug() clears subpartitions_cpus before walking the
descendants. The remote partitions are still valid at that point.
An invalid parent is visited before its remote descendant. Since the
mask is empty, this check allows the parent to become valid and add
its exclusive CPUs back to subpartitions_cpus. When the remote
descendant is visited later, those CPUs are already back in the mask,
so it passes the subset check and stays remote. We are then left with
a valid remote partition below a valid local partition, and
compute_partition_effective_cpumask() warns about the remote child.
Could we check whether there is still a valid remote partition below cs
before allowing cs to become valid? subpartitions_cpus may have been
cleared by hotplug, so the lack of overlap does not tell us that the
remote partition has actually been disabled.
Yes, this is a corner case that I am aware of. Thanks for letting me know about that.
Right now, only invalid local partition has the ability to switch back to valid when the condition is right. Remote partition has no such capability at the moment. So a remote partition should always be invalidated when top_cpuset run out of active CPU. One way to handle this situation is to switch back to chain all remote partition in a linked list. We can then work the remote partition linked list when that happens and determined if we can their effective_xcpus back to subparttitions_cpus or has to disable it before going down to do hierarchical cpuset walk. This will avoid the corner you are talking about. It will be a separate patch. I will update the patch description to add this limitation and a todo item.
Cheers,
Longman