Re: [PATCH-next 2/6] cgroup/cpuset: Properly compute effective xcpus in update_parent_effective_cpumask()
From: Waiman Long
Date: Thu Oct 22 2026 - 11:06:31 EST
On 10/22/26 9:45 AM, Guopeng Zhang wrote:
Hi Longman,
在 2026/10/22 11:16, Waiman Long 写道:
Since the introduction of cpuset.cpus.exclusive in commitI re-tested the repro from the link below on top of this series
e2ffe502ba45 ("cgroup/cpuset: Add cpuset.cpus.exclusive for v2"), a
compute_effective_exclusive_cpumask() helper which is later renamed to
compute_excpus() is added to compute the effective xcpus of a partition
root. However update_parent_effective_cpumask() is not updated in all
proper places to use the new helper to check if a given cpuset should
remain a valid partition root. This can cause a cpuset to incorrectly
remain a valid partition root even if no effective xcpus is left.
Fix that problem by using compute_excpus() to compute the effective xcpus
into delmask in all cases except partcmd_disable and partcmd_invalidate
and use it for tasks_nocpu_error() check as well as checking its
emptiness. This enables us to catch invalid partition root more
accurately in update_parent_effective_cpumask().
(applied on cgroup/for-7.4-fixes), and it still fails:
https://lore.kernel.org/all/20261010075947.333549-3-guopeng.zhang@xxxxxxxxx/
cd /sys/fs/cgroup
echo +cpuset > cgroup.subtree_control
mkdir A
echo 0-7 > A/cpuset.cpus
echo 0-7 > A/cpuset.cpus.exclusive
echo root > A/cpuset.cpus.partition
cat A/cpuset.cpus.partition
echo 1 > A/cpuset.cpus.exclusive
cat A/cpuset.cpus.partition
The last command still shows "root invalid (Parent unable to distribute
cpu downstream)".
In this case compute_excpus() returns the stale user mask unchanged.
user_xcpus() is gone from update_parent_effective_cpumask(), but the
dependency just moved one level down: the new code calls
compute_excpus(cs, tmp->delmask), and its first line is
cpumask_and(excpus, user_xcpus(cs), parent->effective_xcpus)
Here user_xcpus(cs) is cs->exclusive_cpus, which still holds the old
0-7 when compute_excpus() runs: update_exclusive_cpumask() publishes
cs->exclusive_cpus only after partition_cpus_change(). Nothing corrects
it either - the sibling trim is skipped as exclusive_cpus is non-empty,
and for a direct child of the root cpuset the intersection with
parent->effective_xcpus is a no-op. So the stale 0-7 mask still reaches
tasks_nocpu_error() and PERR_NOCPUS is set again. Please point it out
if I got the analysis wrong.
I have rebased my fix on top of this series and the partition recovers
correctly with it applied. Should the fix be folded into this patch,
or is there a better way to fix it? Happy to hear your thoughts.
You are right. My patch series won't fix this particular issue. However, converting an invalid partition back to a valid one is done on a best effort basis. There is no guarantee that it will always happen. Just echo "root" to "cpuset.cpus.partition" again will make it valid again. So it is not a real bug from my point of view. It is just a nice to have feature.
Cheers,
Longman
Also, is the test case below worth adding here as well? Note that it
fails until the fix is in:
# An invalid local partition that took all the CPUs from its
# populated parent should recover after its cpuset.cpus.exclusive
# list is changed to a valid one.
"C0-7:X0-7:P1 . . . X1 . . . 0 A1:1|XA1:1 A1:P1 ."
Thanks,
Guopeng
Some test case are added to test_cpuset_prs.sh for invalidating a
partition root due to its effective xcpus becoming empty as well as
making sure that an invalid partition can only be revived when exclusive
CPUs are available agin.
Fixes: e2ffe502ba45 ("cgroup/cpuset: Add cpuset.cpus.exclusive for v2")
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
kernel/cgroup/cpuset.c | 36 +++++++++----------
.../selftests/cgroup/test_cpuset_prs.sh | 6 ++++
2 files changed, 22 insertions(+), 20 deletions(-)
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 7ead8c3b7974..db6e4663f4be 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1798,7 +1798,6 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
int deleting; /* Deleting cpus from parent's effective_cpus */
int old_prs, new_prs;
int part_error = PERR_NONE; /* Partition error? */
- struct cpumask *xcpus = user_xcpus(cs);
int parent_prs = parent->partition_root_state;
bool nocpu;
@@ -1841,30 +1840,29 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
if (!newmask && xcpus_empty(cs))
return PERR_CPUSEMPTY;
- nocpu = tasks_nocpu_error(parent, cs, xcpus);
+ /*
+ * Except for partcmd_disable, use delmask to compute the current
+ * effective xcpus value and use it for tasks_nocpu_error check.
+ */
+ if (cmd != partcmd_disable) {
+ compute_excpus(cs, tmp->delmask);
+ nocpu = tasks_nocpu_error(parent, cs, tmp->delmask);
+ }
if ((cmd == partcmd_enable) || (cmd == partcmd_enablei)) {
- /*
- * Need to call compute_excpus() in case
- * exclusive_cpus not set. Sibling conflict should only happen
- * if exclusive_cpus isn't set.
- */
- xcpus = tmp->delmask;
- if (compute_excpus(cs, xcpus))
- WARN_ON_ONCE(!cpumask_empty(cs->exclusive_cpus));
new_prs = (cmd == partcmd_enable) ? PRS_ROOT : PRS_ISOLATED;
/*
* Enabling partition root is not allowed if its
* effective_xcpus is empty.
*/
- if (cpumask_empty(xcpus))
+ if (cpumask_empty(tmp->delmask))
return PERR_INVCPUS;
- if (prstate_housekeeping_conflict(new_prs, parent_prs, xcpus, NULL))
+ if (prstate_housekeeping_conflict(new_prs, parent_prs, tmp->delmask, NULL))
return PERR_HKEEPING;
- if (tasks_nocpu_error(parent, cs, xcpus))
+ if (nocpu)
return PERR_NOCPUS;
/*
@@ -1876,9 +1874,8 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
*
* Warn if it is not the case.
*/
- cpumask_and(tmp->new_cpus, xcpus, cpu_active_mask);
+ cpumask_and(tmp->new_cpus, tmp->delmask, cpu_active_mask);
WARN_ON_ONCE(!cpumask_subset(tmp->new_cpus, parent->effective_cpus));
-
deleting = true;
} else if (cmd == partcmd_disable) {
/*
@@ -1973,7 +1970,7 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
* A partition error happens when parent has tasks and all
* its effective CPUs will have to be distributed out.
*/
- if (nocpu) {
+ if (nocpu || cpumask_empty(tmp->delmask)) {
part_error = PERR_NOCPUS;
if (is_partition_valid(cs))
adding = cpumask_and(tmp->addmask,
@@ -1986,8 +1983,7 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
cpumask_copy(tmp->addmask, cs->effective_xcpus);
adding = true;
}
- } else if (is_partition_invalid(cs) && !cpumask_empty(xcpus) &&
- cpumask_subset(xcpus, parent->effective_xcpus)) {
+ } else if (is_partition_invalid(cs)) {
struct cgroup_subsys_state *css;
struct cpuset *child;
bool exclusive = true;
@@ -1998,7 +1994,7 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
* check.
*/
if (prstate_housekeeping_conflict(cs->partition_root_state,
- parent_prs, xcpus, NULL)) {
+ parent_prs, tmp->delmask, NULL)) {
part_error = PERR_HKEEPING;
goto write_error;
}
@@ -2014,7 +2010,7 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
rcu_read_unlock();
if (exclusive)
deleting = cpumask_and(tmp->delmask,
- xcpus, parent->effective_cpus);
+ tmp->delmask, parent->effective_cpus);
else
part_error = PERR_NOTEXCL;
}
diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
index d04aad697b96..47791591002f 100755
--- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
+++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
@@ -359,6 +359,8 @@ TEST_MATRIX=(
A1:P0|A2:P1|A3:P2|B1:P1 2-4|3"
" C1-6:P1 C3-6:P1 C5-6:P1 . . P2 . . 0 A1:1-2|A2:3-6|A3:5-6 \
A1:P1|A2:P2|A3:P-1 3-6"
+ " C1-6:P2 C3-6:P2 C3-4:P2 . . C5-6 . . 0 A1:1-4|A2:5-6|A3:5-6 \
+ A1:P2|A2:P2|A3:P-2 1-6"
" C1-4:X2-4 C2-4:X3-4:P1 C3-4:X4:P2 \
. . X5 . . 0 A1:1-4|A2:2-4|A3:3-4 \
A1:P0|A2:P-1|A3:P-2 ."
@@ -427,6 +429,10 @@ TEST_MATRIX=(
# cpuset.cpus is allowed, but those extra CPUs are ignored.
" C2-3:P1 C3:P1 . . . C2-4 . . 0 A1:|A2:2-3 A1:P1|A2:P1"
+ # A child cannot recover until its exclusive CPUs are available again.
+ " CX1-4:P1 CX3-4:P1 . . X1-2 . . . 0 A1:1-2|A2:1-2|XA2: A1:P1|A2:P-1"
+ " CX1-4:P1 CX3-4:P1 . . X1-2:X1-4 . . . 0 A1:1-2|A2:3-4|XA2:3-4 A1:P1|A2:P1"
+
# Taking away all CPUs from parent or itself if there are tasks
# will make the partition invalid.
" C2-3:P1 C3:P1 . . T C2-3 . . 0 A1:2-3|A2:2-3 A1:P1|A2:P-1"