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

[Libo Chen <libo.chen@oracle.com>]

On 5/5/25 11:27, Chen, Yu C wrote:
> Hi Michal,
> 
> On 5/6/2025 1:46 AM, Michal Koutný wrote:
>> On Mon, May 05, 2025 at 11:03:10PM +0800, "Chen, Yu C" <yu.c.chen@intel.com> wrote:
>>> 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.
>>
>> So it's likely a kernel thread. Does it make sense to NUMA balance
>> those? (I naïvely think it doesn't, please correct me.) ...
>>
> 
> I agree kernel threads are not supposed to be covered by
> NUMA balance, because currently NUMA balance only considers
> user pages via VMAs, and one question below:
> 
>>>   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);
>>
>> ... proper fix should likely guard this earlier, like the guard in
>> task_numa_fault() but for the other swapped task.
> I see. For task swapping in task_numa_compare(),
> it is triggered when there are no idle CPUs in task A's
> preferred node.
> In this case, we choose a task B on A's preferred node,
> and swap B with A. This helps improve A's Numa locality
> without introducing the load imbalance between Nodes.
> 
> But B's Numa node preference is not mandatory in
> current implementation IIUC, because B's load is mainly

hmm, that's doesn't seem to be right, can we choose B that
is not a kthread from A's preferred node?

> considered. That is to say, is it legit to swap a
> Numa sensitive task A with a non-Numa sensitive kernel
> thread B? If not, I think we can add kernel thread
> check in task swap like the guard in
> task_tick_numa()/task_numa_fault().
> 


> thanks,
> Chenyu
> 
>>
>> Michal
>