Re: [PATCH v3] sched/numa: add statistics of numa balance task migration

["Chen, Yu C" <yu.c.chen@intel.com>]

Hi Venkat,

On 5/6/2025 1:25 AM, Venkat Rao Bagalkote wrote:
> 
> On 05/05/25 8:33 pm, Chen, Yu C wrote:
>> On 5/5/2025 2:43 PM, Jain, Ayush wrote:
>>>
>>> Hello,
>>>
>>> Hitting Kernel Panic on latest-next while running rcutorture tests
>>>
>>> 37ff6e9a2ce3 ("Add linux-next specific files for 20250502")
>>>
>>> reverting this patch fixes it
>>> 3b2339eeb032 ("sched-numa-add-statistics-of-numa-balance-task- 
>>> migration-v3")
>>> https://web.git.kernel.org/pub/scm/linux/kernel/git/next/linux- 
>>> next.git/commit/kernel/sched/core.c? 
>>> id=3b2339eeb032627e9329daf70a4ba8cd62c9cc8d
>>>
>>> by looking at RIP pointer
>>>
>>> $ ./scripts/faddr2line vmlinux __migrate_swap_task+0x2e/0x180
>>> __migrate_swap_task+0x2e/0x180:
>>> count_memcg_events_mm at include/linux/memcontrol.h:987
>>> (inlined by) count_memcg_events_mm at include/linux/memcontrol.h:978
>>> (inlined by) __migrate_swap_task at kernel/sched/core.c:3356
>>>
>>> memcg = mem_cgroup_from_task(rcu_dereference(mm->owner));
>>> mm->owner -> NULL
>>>
>>> Attaching kernel logs below:
>>>
>>> [ 1070.635450] rcu-torture: rcu_torture_read_exit: End of episode
>>> [ 1074.047617] BUG: kernel NULL pointer dereference, address:
>>> 0000000000000498
>>
>> Thanks Ayush,
>>
>> According to this address,
>>    4c 8b af 50 09 00 00    mov    0x950(%rdi),%r13  <--- r13 = p->mm;
>>    49 8b bd 98 04 00 00    mov    0x498(%r13),%rdi  <--- p->mm->owner
>> It seems that this task to be swapped has NULL mm_struct.
>>
>> Does the following help?
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 96db6947bc92..0cb8cc4d551d 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -3353,7 +3353,8 @@ void set_task_cpu(struct task_struct *p, 
>> unsigned int new_cpu)
>>  static void __migrate_swap_task(struct task_struct *p, int cpu)
>>  {
>>         __schedstat_inc(p->stats.numa_task_swapped);
>> -       count_memcg_event_mm(p->mm, NUMA_TASK_SWAP);
>> +       if (p->mm)
>> +               count_memcg_event_mm(p->mm, NUMA_TASK_SWAP);
>>
>>         if (task_on_rq_queued(p)) {
>>                 struct rq *src_rq, *dst_rq;
>>
> 
> Hello Chenyu,
> 
> 
> This issue is reported even on IBM Power servers.
> 
> Proposed fix works fine. Hence,
> 
> 
> Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
> 

Thank you for the test. May I know if you could help
check if v4 works without any NULL pointer exception?

https://lore.kernel.org/lkml/cover.1746611892.git.yu.c.chen@intel.com/

thanks,
Chenyu