linux-kernel - Re: [PATCH next] cpuset: fix warning when attaching tasks with offline CPUs

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Message-ID: <4afb9640-a61f-4807-a211-5d2a3cfbdda9@huaweicloud.com>
Date: Tue, 15 Jul 2025 10:31:06 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Waiman Long <llong@...hat.com>, tj@...nel.org, hannes@...xchg.org,
 mkoutny@...e.com, peterz@...radead.org
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
 lujialin4@...wei.com, chenridong@...wei.com
Subject: Re: [PATCH next] cpuset: fix warning when attaching tasks with
 offline CPUs



On 2025/7/15 3:46, Waiman Long wrote:
> On 7/13/25 11:23 PM, Chen Ridong wrote:
>> From: Chen Ridong <chenridong@...wei.com>
>>
>> A kernel warning was observed in the cpuset migration path:
>>
>>      WARNING: CPU: 3 PID: 123 at kernel/cgroup/cpuset.c:3130
>>      cgroup_migrate_execute+0x8df/0xf30
>>      Call Trace:
>>       cgroup_transfer_tasks+0x2f3/0x3b0
>>       cpuset_migrate_tasks_workfn+0x146/0x3b0
>>       process_one_work+0x5ba/0xda0
>>       worker_thread+0x788/0x1220
>>
>> The issue can be reliably reproduced with:
>>
>>      # Setup test cpuset
>>      mkdir /sys/fs/cgroup/cpuset/test
>>      echo 2-3 > /sys/fs/cgroup/cpuset/test/cpuset.cpus
>>      echo 0 > /sys/fs/cgroup/cpuset/test/cpuset.mems
>>
>>      # Start test process
>>      sleep 100 &
>>      pid=$!
>>      echo $pid > /sys/fs/cgroup/cpuset/test/cgroup.procs
>>      taskset -p 0xC $pid  # Bind to CPUs 2-3
>>
>>      # Take CPUs offline
>>      echo 0 > /sys/devices/system/cpu/cpu3/online
>>      echo 0 > /sys/devices/system/cpu/cpu2/online
>>
>> Root cause analysis:
>> When tasks are migrated to top_cpuset due to CPUs going offline,
>> cpuset_attach_task() sets the CPU affinity using cpus_attach which
>> is initialized from cpu_possible_mask. This mask may include offline
>> CPUs. When __set_cpus_allowed_ptr() computes the intersection between:
>> 1. cpus_attach (possible CPUs, may include offline)
>> 2. p->user_cpus_ptr (original user-set mask)
>> The resulting new_mask may contain only offline CPUs, causing the
>> operation to fail.
>>
>> The fix changes cpus_attach initialization to use cpu_active_mask
>> instead of cpu_possible_mask, ensuring we only consider online CPUs
>> when setting the new affinity. This prevents the scenario where
>> the intersection would result in an invalid CPU set.
>>
>> Fixes: da019032819a ("sched: Enforce user requested affinity")
>> Reported-by: Yang Lijin <yanglijin@...wei.com>
>> Signed-off-by: Chen Ridong <chenridong@...wei.com>
>> ---
>>   kernel/cgroup/cpuset.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
>> index f74d04429a29..5401adbdffa6 100644
>> --- a/kernel/cgroup/cpuset.c
>> +++ b/kernel/cgroup/cpuset.c
>> @@ -3121,7 +3121,7 @@ static void cpuset_attach_task(struct cpuset *cs, struct task_struct *task)
>>       if (cs != &top_cpuset)
>>           guarantee_active_cpus(task, cpus_attach);
>>       else
>> -        cpumask_andnot(cpus_attach, task_cpu_possible_mask(task),
>> +        cpumask_andnot(cpus_attach, cpu_active_mask,
>>                      subpartitions_cpus);
>>       /*
>>        * can_attach beforehand should guarantee that this doesn't
> 
> Offline CPUs are explicitly included for tasks in top_cpuset. Can you try the following patch to see
> if it works?
> 

Thank you very much. I tried this patch and it worked. I will resend the new patch.

Best regards
Ridong

> Thanks,
> Longman
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 3bc4301466f3..acd70120228c 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -3114,6 +3114,10 @@ static void cpuset_cancel_attach(struct cgroup_taskset *tset)
>  static cpumask_var_t cpus_attach;
>  static nodemask_t cpuset_attach_nodemask_to;
> 
> +/*
> + * Note that tasks in the top cpuset won't get update to their cpumasks when
> + * a hotplug event happens. So we include offline CPUs as well.
> + */
>  static void cpuset_attach_task(struct cpuset *cs, struct task_struct *task)
>  {
>         lockdep_assert_held(&cpuset_mutex);
> @@ -3127,7 +3131,16 @@ static void cpuset_attach_task(struct cpuset *cs, struct task_struct *task)
>          * can_attach beforehand should guarantee that this doesn't
>          * fail.  TODO: have a better way to handle failure here
>          */
> -       WARN_ON_ONCE(set_cpus_allowed_ptr(task, cpus_attach));
> +       if (unlikely(set_cpus_allowed_ptr(task, cpus_attach))) {
> +               /*
> +                * Since offline CPUs are included for top_cpuset,
> +                * set_cpus_allowed_ptr() can fail if user_cpus_ptr contains
> +                * only offline CPUs. Take out the offline CPUs and retry.
> +                */
> +               if (cs == &top_cpuset)
> +                       cpumask_and(cpus_attach, cpus_attach, cpu_active_mask);
> +               WARN_ON_ONCE(set_cpus_allowed_ptr(task, cpus_attach));
> +       }
> 
>         cpuset_change_task_nodemask(task, &cpuset_attach_nodemask_to);
>         cpuset1_update_task_spread_flags(cs, task);