[PATCH-next v3 6/9] cgroup/cpuset: Don't allow a child root partition under an isolated parent
From: Waiman Long
Date: Wed Oct 14 2026 - 10:15:07 EST
Although it is not typical for real users to create a complicated nested
partition hierarchy, it is currently allowed to create nested root and
isolated partitions. In addition, disabling or invalidating a partition
should always be allowed especially when the cpuset is being deleted.
With a nested root and isolated partition hierarchy, it is possible
that all active housekeeping CPUs can be exhausted when disabling or
invalidating a root partition and returning its CPUs to the isolated
parent.
One way to avoid this problem is to disallow the creation of a root
partition under an isolated parent. It is indeed a new limitation imposed
on the existing cpuset partition functionality. However an isolated
partition is usually a left node where scheduling and some other kernel
noises are disabled. It doesn't make much sense to create a child root
partition under an isolated parent and remove some CPUs from the set
of isolated CPUs again. Creation of a child isolated partition under a
root or isolated parent is still allowed.
The cgroup-v2 documentation is also updated to reflect this new
limitation. New test cases are added to test_cpuset_prs.sh to verify
this new behavior. Some existing test cases are also updated accordingly.
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
Documentation/admin-guide/cgroup-v2.rst | 4 +++-
kernel/cgroup/cpuset-internal.h | 1 +
kernel/cgroup/cpuset.c | 13 +++++++++++++
tools/testing/selftests/cgroup/test_cpuset_prs.sh | 14 ++++++++------
4 files changed, 25 insertions(+), 7 deletions(-)
diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index bd8c80f3bbc9..2c445829e24d 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -2767,7 +2767,9 @@ Cpuset Interface Files
be retained, but behaves more like a "member".
All possible state transitions among "member", "root" and
- "isolated" are allowed.
+ "isolated" are allowed with the exception that a local child
+ "root" partition is not allowed to be created under an "isolated"
+ parent partition.
On read, the "cpuset.cpus.partition" file can show the following
values.
diff --git a/kernel/cgroup/cpuset-internal.h b/kernel/cgroup/cpuset-internal.h
index 5da9c91295b0..2e44dc8634dc 100644
--- a/kernel/cgroup/cpuset-internal.h
+++ b/kernel/cgroup/cpuset-internal.h
@@ -35,6 +35,7 @@ enum prs_errcode {
PERR_HKEEPING,
PERR_ACCESS,
PERR_REMOTE,
+ PERR_ISOPARENT,
};
/* bits in struct cpuset flags field */
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 40c004386af0..681701ba46e1 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -60,6 +60,7 @@ static const char * const perr_strings[] = {
[PERR_HKEEPING] = "partition config conflicts with housekeeping setup",
[PERR_ACCESS] = "Enable partition not permitted",
[PERR_REMOTE] = "Have remote partition underneath",
+ [PERR_ISOPARENT] = "Root partition is not allowed with isolated parent",
};
/*
@@ -2865,6 +2866,7 @@ static int update_prstate(struct cpuset *cs, int new_prs)
int err = PERR_NONE, old_prs = cs->partition_root_state;
struct cpuset *parent = parent_cs(cs);
struct tmpmasks tmpmask;
+ int parent_prs = parent->partition_root_state;
bool isolcpus_updated = false;
bool disable_partition = false;
@@ -2910,6 +2912,14 @@ static int update_prstate(struct cpuset *cs, int new_prs)
enum partition_cmd cmd = (new_prs == PRS_ROOT)
? partcmd_enable : partcmd_enablei;
+ /*
+ * It is not allowed to create a root partition under
+ * an isolated parent partition.
+ */
+ if ((new_prs == PRS_ROOT) && (parent_prs == PRS_ISOLATED)) {
+ err = PERR_ISOPARENT;
+ goto out;
+ }
err = update_parent_effective_cpumask(cs, cmd, NULL, &tmpmask);
} else {
err = remote_partition_enable(cs, new_prs, &tmpmask);
@@ -2922,6 +2932,9 @@ static int update_prstate(struct cpuset *cs, int new_prs)
if (prstate_housekeeping_conflict(new_prs, cs->effective_xcpus)) {
disable_partition = true;
err = PERR_HKEEPING;
+ } else if (parent_prs == PRS_ISOLATED) {
+ disable_partition = true;
+ err = PERR_ISOPARENT;
} else {
isolcpus_updated = true;
}
diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
index 874a8a161236..85a4f87840b4 100755
--- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
+++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
@@ -288,16 +288,12 @@ TEST_MATRIX=(
A1:P0|A2:P1|A3:P0|B1:P1"
" C0-3 C1-3 C3 C4 X2-3 X2-3:P1 P2 P1 0 A1:0-1|A2:2|A3:3|B1:4 \
A1:P0|A2:P1|A3:P2|B1:P1 2-4|3"
- " C0-4 C1-4 C2-4 . X2-4 X2-4:P2 X4:P1 . 0 A1:0-1|A2:2-3|A3:4 \
- A1:P0|A2:P2|A3:P1 2-4|2-3"
- " C0-4 C1-4 C2-4 . X2-4 X2-4:P2 X3-4:P1 . 0 A1:0-1|A2:2|A3:3-4 \
- A1:P0|A2:P2|A3:P1 2"
" C0-4:X2-4 C1-4:X2-4:P1 C2-4:X4:P2 \
. . X5 . . 0 A1:0-4|A2:1-4|A3:2-4 \
A1:P0|A2:P-1|A3:P-2 ."
- " C0-4:X2-4 C1-4:X2-4:P2 C2-4:X4:P1 \
+ " C0-4:X2-4 C1-4:X2-4:P1 C2-4:X4:P2 \
. . . X1 . 0 A1:0-1|A2:2-4|A3:2-4 \
- A1:P0|A2:P2|A3:P-1 2-4"
+ A1:P0|A2:P1|A3:P-2 ."
" CX1-3:P1 CX1-3 CX1-3 . . . P1 . 0 A1:1-3|A2:1-3|A3:1-3 \
A1:P1|A2:P0|A3:P-1"
@@ -430,6 +426,12 @@ TEST_MATRIX=(
# cpuset.cpus can become empty with task in it as it inherits parent's effective CPUs
" C1-3 C2 . . . T:C . . 0 A1:1-3|A2:1-3"
+ # Creation of root partition under an isolated partition is not allowed
+ " C1-3:P2 C2 . . . P1 . . 0 A1:1-3|A2:2 A1:P2|A2:P-1 1-3"
+ " C1-3:P2 C2 . . . P2 . . 0 A1:1,3|A2:2 A1:P2|A2:P2 1-3"
+ " C1-3:P2 C2:P2 . . . P1 . . 0 A1:1-3|A2:2 A1:P2|A2:P-1 1-3"
+
+ #
# old-A1 old-A2 old-A3 old-B1 new-A1 new-A2 new-A3 new-B1 fail ECPUs Pstate ISOLCPUS
# ------ ------ ------ ------ ------ ------ ------ ------ ---- ----- ------ --------
# Failure cases:
--
2.55.0