Re: [PATCH] mm/oom_kill: kill current in OOM when binding to cpu-less nodes

[Jinjiang Tu <tujinjiang@huawei.com>]

[-- Attachment #1: Type: text/plain, Size: 2930 bytes --]


在 2025/9/4 22:43, Joshua Hahn 写道:
> On Thu, 4 Sep 2025 16:36:28 +0200 Michal Hocko<mhocko@suse.com> wrote:
>
>> On Thu 04-09-25 07:26:25, Joshua Hahn wrote:
>>> On Thu, 4 Sep 2025 21:44:31 +0800 Jinjiang Tu<tujinjiang@huawei.com> wrote:
>>>
>>> Hello Jinjiang,
>>>
>>> I hope you are doing well, thank you for this patchset!
>>>
>>>> out_of_memory() selects tasks without considering mempolicy. Assuming a
>>>> cpu-less NUMA Node, ordinary process that don't set mempolicy don't
>>>> allocate memory from this cpu-less Node, unless other NUMA Nodes are below
>>>> low watermark. If a task binds to this cpu-less Node and triggers OOM, many
>>>> tasks may be killed wrongly that don't occupy memory from this Node.
>>> I am wondeirng whether you have seen this happen in practice, or if this is
>>> just based on inspecting the code. I have a feeling that the case you are
>>> concerned about may already be covered in select_bad_process.
>>>
>>> out_of_memory(oc)
>>>      select_bad_process(oc)
>>>          oom_evaluate_task(p, oc)
>>> 	    oom_cpuset_eligible(task, oc)
>>> 	
>>> 	        [...snip...]
>>>
>>> 		for_each_thread(start, tsk) {
>>> 		    if (mask) {
>>> 		        ret = mempolicy_in_oom_domain(tsk, mask);
>>> 		    } else {
>>> 		        ret = cpuset_mems_allowed_intersects(current, tsk)
>>> 		    }
>>> 		}
>>>
>>> While iterating through the list of candidate processes, we check whether
>>> oc->nodemask exists, and if not, we check if the nodemasks intersects. It seems
>>> like these are the two checks that you add in the helper function.
>>>
>>> With that said, I might be missing something obvious -- please feel to
>>> correct me if I am misunderstanding your patch or if I'm missing something
>>> in the existing oom target selection : -)
>> The thing with mempolicy_in_oom_domain is that it doesn't really do what
>> you might be thinking it is doing ;) as it will true also for tasks
>> without any NUMA affinity because those intersect with the given mask by
>> definition as they can allocate from any node. So they are eligible and
>> that is what Jinjiang Tu is considered about I believe.
> Hello Michal! Thank you for your insights : -)
>
> Looking back, I made the mistake of thinking that we cared about the
> !oc->nodemask case, where Jinjiang's patch cares about the oc->nodemask == True
> case. So I was checking that cpuset_mems_allowed_intersects was the same as
> nodes_intersects, whereas I should have been checking if mempolicy_in_oom_domain
> is correct.

Most tasks don't mbind to specific nodes. In our use case, as described in the reply
to Michal, ordinary tasks are unlikely to allocate from these cpu-less NUMA Nodes.

>
> Looking into it, everything you said is correct and I think I defintely
> overlooked what the patch was trying to do. Thank you for clarifying these
> points for me!
>
> I hope you have a great day,
> Joshua
>
>> -- 
>> Michal Hocko
>> SUSE Labs

[-- Attachment #2: Type: text/html, Size: 4013 bytes --]