Re: [PATCH] cgroup/cpuset: Fix partition recovery with unavailable CPUs

[Guopeng Zhang <guopeng.zhang@linux.dev>]

在 2026/10/22 11:31, Waiman Long 写道:
> On 10/20/26 9:33 AM, Guopeng Zhang wrote:
>> From: Guopeng Zhang <zhangguopeng@kylinos.cn>
>>
>> An invalid local partition can become valid again even when its
>> exclusive CPUs are no longer available from its parent.
>>
>> This can be reproduced on a cgroup v2 system with CPUs 0-7 online:
>>
>>      cd /sys/fs/cgroup
>>      echo +cpuset > cgroup.subtree_control
>>      mkdir A
>>      echo 1-4 > A/cpuset.cpus
>>      echo 1-4 > A/cpuset.cpus.exclusive
>>      echo root > A/cpuset.cpus.partition
>>      echo +cpuset > A/cgroup.subtree_control
>>      mkdir A/B
>>      echo 3-4 > A/B/cpuset.cpus
>>      echo 3-4 > A/B/cpuset.cpus.exclusive
>>      echo root > A/B/cpuset.cpus.partition
>>      echo 1-2 > A/cpuset.cpus.exclusive
>>      cat A/B/cpuset.cpus.partition
>>      root
>>      cat A/B/cpuset.cpus.exclusive.effective
>>      cat A/cpuset.cpus.effective
>>      1-2
>>      cat A/B/cpuset.cpus.effective
>>      1-2
>>
>> B is changed back to a valid root even though its exclusive CPUs are
>> no longer available and cpuset.cpus.exclusive.effective is empty. B
>> should remain invalid until A can provide its exclusive CPUs again.
>>
>> Shrinking A's exclusive CPUs invalidates B in
>> compute_partition_effective_cpumask(). The subsequent walk in
>> update_cpumasks_hier() tries to recover B, but
>> update_parent_effective_cpumask() skips the recovery checks when xcpus
>> is not a subset of parent->effective_xcpus. Since part_error is still
>> zero, B is changed back to a valid partition.
>>
>> Set PERR_INVCPUS when the subset check fails so that the partition stays
>> invalid. New test cases are added to test_cpuset_prs.sh for removing and
>> restoring the parent's exclusive CPUs.
>>
>> Fixes: 0c7f293efc87 ("cgroup/cpuset: Add cpuset.cpus.exclusive.effective for v2")
>> Signed-off-by: Guopeng Zhang <zhangguopeng@kylinos.cn>
>> ---
>>   kernel/cgroup/cpuset.c                            | 8 ++++++--
>>   tools/testing/selftests/cgroup/test_cpuset_prs.sh | 4 ++++
>>   2 files changed, 10 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index 585a982249ad..be80763953ed 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -1986,12 +1986,16 @@ 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) && !cpumask_empty(xcpus)) {
>>               struct cgroup_subsys_state *css;
>>               struct cpuset *child;
>>               bool exclusive = true;
>>   +            if (!cpumask_subset(xcpus, parent->effective_xcpus)) {
>> +                part_error = PERR_INVCPUS;
>> +                goto write_error;
>> +            }
>> +
>>               /*
>>                * Convert invalid partition to valid has to pass the
>>                * cpu exclusivity test as well as the housekeeping
>> diff --git a/tools/testing/selftests/cgroup/test_cpuset_prs.sh b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
>> index d04aad697b96..f1d517647ade 100755
>> --- a/tools/testing/selftests/cgroup/test_cpuset_prs.sh
>> +++ b/tools/testing/selftests/cgroup/test_cpuset_prs.sh
>> @@ -427,6 +427,10 @@ 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"
> 
> Patch 2 of my series should fix this problem too. I do borrow your test cases in my patch. I hope you don't mind about it.
> 
Thanks for addressing this issue as well.
Of course, I don't mind. Glad the test cases are useful!

Thanks,
Guopeng