Re: [PATCH v3 2/3] cgroup/cpuset: Allow top-level partition CPU replacement
From: Ridong Chen
Date: Tue Nov 10 2026 - 07:20:27 EST
On 11/10/2026 1:03 PM, Guopeng Zhang wrote:
From: Guopeng Zhang <zhangguopeng@xxxxxxxxxx>
When replacing a partition's CPUs, tasks_nocpu_error() checks whether
the parent's current effective_cpus is a subset of the new exclusive
CPU mask. The parent mask does not yet include CPUs being returned by
the update, so this can invalidate an otherwise valid partition.
For example, on an 8-CPU system without boot isolation:
echo 2-7 > A/cpuset.cpus
echo isolated > A/cpuset.cpus.partition
echo 0-4 > A/cpuset.cpus
cat A/cpuset.cpus.partition
isolated invalid (Parent unable to distribute cpu downstream)
The update should return CPUs 5-7 to the parent and leave A as a
valid isolated partition.
Skip this subset check for top_cpuset. top_cpuset_sd_nocpu() already
accounts for returned CPUs and ensures that an active boot domain
housekeeping CPU remains.
Add a regression test for this case.
Fixes: 0c7f293efc87 ("cgroup/cpuset: Add cpuset.cpus.exclusive.effective for v2")
Signed-off-by: Guopeng Zhang <zhangguopeng@xxxxxxxxxx>
Hi Guopeng,
Patch 2 depends on patch 1. Skipping the cpumask_subset() check for
top_cpuset is only safe because top_cpuset_sd_nocpu() added by patch 1
now covers it. Without patch 1, patch 2 alone would let a partition
take away all the CPUs of top_cpuset. So patch 2 can't be backported
on its own, and the two have different Fixes tags. I think it would be
better to fold patch 2 into patch 1.
Also, the fix in patch 2 seems incomplete. The same false PERR_NOCPUS
happens when the parent is a local partition, as cpumask_subset()
there doesn't take the CPUs given back by the update into account
either. On an 8-CPU system:
cd /sys/fs/cgroup
echo +cpuset > cgroup.subtree_control
mkdir A1
echo 1-7 > A1/cpuset.cpus
echo root > A1/cpuset.cpus.partition
echo +cpuset > A1/cgroup.subtree_control
mkdir A1/A2
echo 3-7 > A1/A2/cpuset.cpus
echo isolated > A1/A2/cpuset.cpus.partition
sleep 1000 &
echo $! > A1/cgroup.procs
echo 1-4 > A1/A2/cpuset.cpus
cat A1/A2/cpuset.cpus.partition
isolated invalid (Parent unable to distribute cpu downstream)
A2 takes CPUs 1-2 from A1 and gives CPUs 5-7 back, so A1 still has
CPUs for its task. Instead of skipping the check for top_cpuset only,
how about making the check in tasks_nocpu_error() account for the CPUs
given back by cs for all parents? The hotplug caller would then need
to not count cs->effective_xcpus as given back, similar to what patch 3
does for top-level partitions.
---
kernel/cgroup/cpuset.c | 4 +++-
tools/testing/selftests/cgroup/test_cpuset_prs.sh | 3 +++
2 files changed, 6 insertions(+), 1 deletion(-)
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index c405719ccd12..8b6831983803 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1232,8 +1232,10 @@ static bool tasks_nocpu_error(struct cpuset *parent, struct cpuset *cs,
/*
* A populated partition (cs or parent) can't have empty effective_cpus
+ * The top_cpuset check above also accounts for CPUs being returned.
*/
- return (cpumask_subset(parent->effective_cpus, xcpus) &&
+ return (parent != &top_cpuset &&
+ cpumask_subset(parent->effective_cpus, xcpus) &&
partition_is_populated(parent, cs)) ||
(!cpumask_intersects(xcpus, cpu_active_mask) &&
partition_is_populated(cs, NULL));
diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
index 0288af1c2eb3..ee4d57b7e35b 100755
--- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
+++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
@@ -554,6 +554,9 @@ TEST_MATRIX=(
" C2-6:X2-5:P1 CX3-6:P2 \
. . T:X4-6 . . . 0 A1:4-6|A2:4-6 A1:P1|A2:P-2 ."
+ # A CPU replacement must account for CPUs returned to the top cpuset.
+ " C2-${CPULIST#0-}:P2 . . . C0-4 . . . 0 A1:0-4 A1:P2 0-4"
+
#
# old-A1 old-A2 old-A3 old-B1 new-A1 new-A2 new-A3 new-B1 fail ECPUs Pstate ISOLCPUS
# ------ ------ ------ ------ ------ ------ ------ ------ ---- ----- ------ --------
--
Best regards
Ridong