[v1] cpuset: fix warning when attaching tasks with offline CPUs

[PATCH next] cpuset: fix warning when attaching tasks with offline CPUs

Posted by Chen Ridong 6 months, 4 weeks ago

From: Chen Ridong <chenridong@huawei.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@huawei.com>
Signed-off-by: Chen Ridong <chenridong@huawei.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
-- 
2.34.1

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

Posted by Waiman Long 6 months, 4 weeks ago

On 7/13/25 11:23 PM, Chen Ridong wrote:
> From: Chen Ridong <chenridong@huawei.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@huawei.com>
> Signed-off-by: Chen Ridong <chenridong@huawei.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?

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);

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

Posted by Chen Ridong 6 months, 4 weeks ago


On 2025/7/15 3:46, Waiman Long wrote:
> On 7/13/25 11:23 PM, Chen Ridong wrote:
>> From: Chen Ridong <chenridong@huawei.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@huawei.com>
>> Signed-off-by: Chen Ridong <chenridong@huawei.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);

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

Posted by Peter Zijlstra 6 months, 4 weeks ago

On Mon, Jul 14, 2025 at 03:23:11AM +0000, Chen Ridong wrote:
> From: Chen Ridong <chenridong@huawei.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@huawei.com>
> Signed-off-by: Chen Ridong <chenridong@huawei.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);

This breaks things. Any task mask must be a subset of
task_cpu_possible_mask() at all times. It might not be able to run
outside of that mask.

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

Posted by Chen Ridong 6 months, 4 weeks ago


On 2025/7/14 16:41, Peter Zijlstra wrote:
> On Mon, Jul 14, 2025 at 03:23:11AM +0000, Chen Ridong wrote:
>> From: Chen Ridong <chenridong@huawei.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@huawei.com>
>> Signed-off-by: Chen Ridong <chenridong@huawei.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);
> 
> This breaks things. Any task mask must be a subset of
> task_cpu_possible_mask() at all times. It might not be able to run
> outside of that mask.

Hi Peter,

Thanks for your feedback. I'm afraid I don't fully understand what you mean by "breaks things".
Could you please explain in more detail?

To clarify my current understanding: this patch simply changes the cpus_attach initialization from
task_cpu_possible_mask(task) to cpu_active_mask. The intention is that when CPUs are offlined and
tasks get migrated to root cpuset, we shouldn't try to migrate tasks to offline CPUs. And since
cpu_active_mask is a subset of cpu_possible_mask, I thought this would be safe. Did I miss anything?

Best regards
Ridong

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

Posted by Peter Zijlstra 6 months, 4 weeks ago

On Mon, Jul 14, 2025 at 07:30:39PM +0800, Chen Ridong wrote:

> >> 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);
> > 
> > This breaks things. Any task mask must be a subset of
> > task_cpu_possible_mask() at all times. It might not be able to run
> > outside of that mask.
> 
> Hi Peter,
> 
> Thanks for your feedback. I'm afraid I don't fully understand what you
> mean by "breaks things".  Could you please explain in more detail?
> 
> To clarify my current understanding: this patch simply changes the
> cpus_attach initialization from task_cpu_possible_mask(task) to
> cpu_active_mask. The intention is that when CPUs are offlined and
> tasks get migrated to root cpuset, we shouldn't try to migrate tasks
> to offline CPUs. And since cpu_active_mask is a subset of
> cpu_possible_mask, I thought this would be safe. Did I miss anything?

task_cpu_possible_mask() is the mask a task *MUST* stay inside of.

Specifically, this was introduced for ARMv9 where some CPUs drop the
capability to run ARM32 instructions. Trying to schedule an ARM32 task
on a CPU that does not support that instruction set is an immediate and
fatal fail.

Your change results in in something akin to:

  set_cpus_allowed_task(task, cpu_active_mask & ~subpartition_cpus);

Which does not honor the task_cpu_possible_mask() constraint.

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

Posted by Chen Ridong 6 months, 4 weeks ago


On 2025/7/14 19:59, Peter Zijlstra wrote:
> On Mon, Jul 14, 2025 at 07:30:39PM +0800, Chen Ridong wrote:
> 
>>>> 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);
>>>
>>> This breaks things. Any task mask must be a subset of
>>> task_cpu_possible_mask() at all times. It might not be able to run
>>> outside of that mask.
>>
>> Hi Peter,
>>
>> Thanks for your feedback. I'm afraid I don't fully understand what you
>> mean by "breaks things".  Could you please explain in more detail?
>>
>> To clarify my current understanding: this patch simply changes the
>> cpus_attach initialization from task_cpu_possible_mask(task) to
>> cpu_active_mask. The intention is that when CPUs are offlined and
>> tasks get migrated to root cpuset, we shouldn't try to migrate tasks
>> to offline CPUs. And since cpu_active_mask is a subset of
>> cpu_possible_mask, I thought this would be safe. Did I miss anything?
> 
> task_cpu_possible_mask() is the mask a task *MUST* stay inside of.
> 
> Specifically, this was introduced for ARMv9 where some CPUs drop the
> capability to run ARM32 instructions. Trying to schedule an ARM32 task
> on a CPU that does not support that instruction set is an immediate and
> fatal fail.
> 
> Your change results in in something akin to:
> 
>   set_cpus_allowed_task(task, cpu_active_mask & ~subpartition_cpus);
> 
> Which does not honor the task_cpu_possible_mask() constraint.

Thanks for your patience. See now.

Best regards,
Ridong