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Message-ID: <0d057037-f1ef-4c8c-a843-42ad78581816@redhat.com>
Date: Tue, 16 Dec 2025 17:03:36 -0500
From: Waiman Long <llong@...hat.com>
To: Chen Ridong <chenridong@...weicloud.com>, tj@...nel.org,
hannes@...xchg.org, mkoutny@...e.com
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
lujialin4@...wei.com, chenridong@...wei.com
Subject: Re: [PATCH -next] cpuset: fix warning when disabling remote partition
On 12/8/25 9:56 PM, Chen Ridong wrote:
> On 2025/11/27 11:04, Chen Ridong wrote:
>> From: Chen Ridong <chenridong@...wei.com>
>>
>> A warning was triggered as follows:
>>
>> WARNING: kernel/cgroup/cpuset.c:1651 at remote_partition_disable+0xf7/0x110
>> RIP: 0010:remote_partition_disable+0xf7/0x110
>> RSP: 0018:ffffc90001947d88 EFLAGS: 00000206
>> RAX: 0000000000007fff RBX: ffff888103b6e000 RCX: 0000000000006f40
>> RDX: 0000000000006f00 RSI: ffffc90001947da8 RDI: ffff888103b6e000
>> RBP: ffff888103b6e000 R08: 0000000000000000 R09: 0000000000000000
>> R10: 0000000000000001 R11: ffff88810b2e2728 R12: ffffc90001947da8
>> R13: 0000000000000000 R14: ffffc90001947da8 R15: ffff8881081f1c00
>> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> CR2: 00007f55c8bbe0b2 CR3: 000000010b14c000 CR4: 00000000000006f0
>> Call Trace:
>> <TASK>
>> update_prstate+0x2d3/0x580
>> cpuset_partition_write+0x94/0xf0
>> kernfs_fop_write_iter+0x147/0x200
>> vfs_write+0x35d/0x500
>> ksys_write+0x66/0xe0
>> do_syscall_64+0x6b/0x390
>> entry_SYSCALL_64_after_hwframe+0x4b/0x53
>> RIP: 0033:0x7f55c8cd4887
Sorry for the late reply. I was in the Linux Plumbers Conference last
week and so didn't time to fully review it.
>>
>> Reproduction steps (on a 16-CPU machine):
>>
>> # cd /sys/fs/cgroup/
>> # mkdir A1
>> # echo +cpuset > A1/cgroup.subtree_control
>> # echo "0-14" > A1/cpuset.cpus.exclusive
>> # mkdir A1/A2
>> # echo "0-14" > A1/A2/cpuset.cpus.exclusive
>> # echo "root" > A1/A2/cpuset.cpus.partition
>> # echo 0 > /sys/devices/system/cpu/cpu15/online
>> # echo member > A1/A2/cpuset.cpus.partition
>>
>> When CPU 15 is offlined, subpartitions_cpus gets cleared because no CPUs
>> remain available for the top_cpuset, forcing partitions to share CPUs with
>> the top_cpuset. In this scenario, disabling the remote partition triggers
>> a warning stating that effective_xcpus is not a subset of
>> subpartitions_cpus. Partitions should be invalidated in this case to
>> inform users that the partition is now invalid(cpus are shared with
>> top_cpuset).
This is real corner case as such a scenario should rarely happen in a
real production environment.
>>
>> To fix this issue:
>> 1. Only emit the warning only if subpartitions_cpus is not empty and the
>> effective_xcpus is not a subset of subpartitions_cpus.
>> 2. During the CPU hotplug process, invalidate partitions if
>> subpartitions_cpus is empty.
>>
>> Fixes: 4449b1ce46bf ("cgroup/cpuset: Remove remote_partition_check() & make update_cpumasks_hier() handle remote partition")
>> Signed-off-by: Chen Ridong <chenridong@...wei.com>
>> ---
>> kernel/cgroup/cpuset.c | 21 ++++++++++++++++-----
>> 1 file changed, 16 insertions(+), 5 deletions(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index fea577b4016a..fbe539d66d9b 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -1648,7 +1648,14 @@ static int remote_partition_enable(struct cpuset *cs, int new_prs,
>> static void remote_partition_disable(struct cpuset *cs, struct tmpmasks *tmp)
>> {
>> WARN_ON_ONCE(!is_remote_partition(cs));
>> - WARN_ON_ONCE(!cpumask_subset(cs->effective_xcpus, subpartitions_cpus));
>> + /*
>> + * When a CPU is offlined, top_cpuset may end up with no available CPUs,
>> + * which should clear subpartitions_cpus. We should not emit a warning for this
>> + * scenario: the hierarchy is updated from top to bottom, so subpartitions_cpus
>> + * may already be cleared when disabling the partition.
>> + */
>> + WARN_ON_ONCE(!cpumask_subset(cs->effective_xcpus, subpartitions_cpus) &&
>> + !cpumask_empty(subpartitions_cpus));
>>
>> spin_lock_irq(&callback_lock);
>> cs->remote_partition = false;
>> @@ -3956,8 +3963,9 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
>> if (remote || (is_partition_valid(cs) && is_partition_valid(parent)))
>> compute_partition_effective_cpumask(cs, &new_cpus);
>>
>> - if (remote && cpumask_empty(&new_cpus) &&
>> - partition_is_populated(cs, NULL)) {
>> + if (remote && (cpumask_empty(subpartitions_cpus) ||
>> + (cpumask_empty(&new_cpus) &&
>> + partition_is_populated(cs, NULL)))) {
>> cs->prs_err = PERR_HOTPLUG;
>> remote_partition_disable(cs, tmp);
>> compute_effective_cpumask(&new_cpus, cs, parent);
>> @@ -3970,9 +3978,12 @@ static void cpuset_hotplug_update_tasks(struct cpuset *cs, struct tmpmasks *tmp)
>> * 1) empty effective cpus but not valid empty partition.
>> * 2) parent is invalid or doesn't grant any cpus to child
>> * partitions.
>> + * 3) subpartitions_cpus is empty.
>> */
>> - if (is_local_partition(cs) && (!is_partition_valid(parent) ||
>> - tasks_nocpu_error(parent, cs, &new_cpus)))
>> + if (is_local_partition(cs) &&
>> + (!is_partition_valid(parent) ||
>> + tasks_nocpu_error(parent, cs, &new_cpus) ||
>> + cpumask_empty(subpartitions_cpus)))
>> partcmd = partcmd_invalidate;
>> /*
>> * On the other hand, an invalid partition root may be transitioned
> Friendly ping.
Reviewed-by: Waiman Long <longman@...hat.com
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