Re: [PATCH v1] memcg: fix soft lockup in the OOM process

[Chen Ridong <chenridong@huaweicloud.com>]

On 2024/12/19 15:57, Michal Hocko wrote:
> On Thu 19-12-24 09:27:52, Chen Ridong wrote:
>>
>>
>> On 2024/12/18 18:22, Michal Hocko wrote:
>>> On Wed 18-12-24 17:00:38, Chen Ridong wrote:
>>>>
>>>>
>>>> On 2024/12/18 15:56, Michal Hocko wrote:
>>>>> On Wed 18-12-24 15:44:34, Chen Ridong wrote:
>>>>>>
>>>>>>
>>>>>> On 2024/12/17 20:54, Michal Hocko wrote:
>>>>>>> On Tue 17-12-24 12:18:28, Chen Ridong wrote:
>>>>>>> [...]
>>>>>>>> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
>>>>>>>> index 1c485beb0b93..14260381cccc 100644
>>>>>>>> --- a/mm/oom_kill.c
>>>>>>>> +++ b/mm/oom_kill.c
>>>>>>>> @@ -390,6 +390,7 @@ static int dump_task(struct task_struct *p, void *arg)
>>>>>>>>  	if (!is_memcg_oom(oc) && !oom_cpuset_eligible(p, oc))
>>>>>>>>  		return 0;
>>>>>>>>  
>>>>>>>> +	cond_resched();
>>>>>>>>  	task = find_lock_task_mm(p);
>>>>>>>>  	if (!task) {
>>>>>>>>  		/*
>>>>>>>
>>>>>>> This is called from RCU read lock for the global OOM killer path and I
>>>>>>> do not think you can schedule there. I do not remember specifics of task
>>>>>>> traversal for crgoup path but I guess that you might need to silence the
>>>>>>> soft lockup detector instead or come up with a different iteration
>>>>>>> scheme.
>>>>>>
>>>>>> Thank you, Michal.
>>>>>>
>>>>>> I made a mistake. I added cond_resched in the mem_cgroup_scan_tasks
>>>>>> function below the fn, but after reconsideration, it may cause
>>>>>> unnecessary scheduling for other callers of mem_cgroup_scan_tasks.
>>>>>> Therefore, I moved it into the dump_task function. However, I missed the
>>>>>> RCU lock from the global OOM.
>>>>>>
>>>>>> I think we can use touch_nmi_watchdog in place of cond_resched, which
>>>>>> can silence the soft lockup detector. Do you think that is acceptable?
>>>>>
>>>>> It is certainly a way to go. Not the best one at that though. Maybe we
>>>>> need different solution for the global and for the memcg OOMs. During
>>>>> the global OOM we rarely care about latency as the whole system is
>>>>> likely to struggle. Memcg ooms are much more likely. Having that many
>>>>> tasks in a memcg certainly requires a further partitioning so if
>>>>> configured properly the OOM latency shouldn't be visible much. But I am
>>>>> wondering whether the cgroup task iteration could use cond_resched while
>>>>> the global one would touch_nmi_watchdog for every N iterations. I might
>>>>> be missing something but I do not see any locking required outside of
>>>>> css_task_iter_*.
>>>>
>>>> Do you mean like that:
>>>
>>> I've had something like this (untested) in mind
>>> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
>>> index 7b3503d12aaf..37abc94abd2e 100644
>>> --- a/mm/memcontrol.c
>>> +++ b/mm/memcontrol.c
>>> @@ -1167,10 +1167,14 @@ void mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
>>>  	for_each_mem_cgroup_tree(iter, memcg) {
>>>  		struct css_task_iter it;
>>>  		struct task_struct *task;
>>> +		unsigned int i = 0
>>>  
>>>  		css_task_iter_start(&iter->css, CSS_TASK_ITER_PROCS, &it);
>>> -		while (!ret && (task = css_task_iter_next(&it)))
>>> +		while (!ret && (task = css_task_iter_next(&it))) {
>>>  			ret = fn(task, arg);
>>> +			if (++i % 1000)
>>> +				cond_resched();
>>> +		}
>>>  		css_task_iter_end(&it);
>>>  		if (ret) {
>>>  			mem_cgroup_iter_break(memcg, iter);
>>
>> Thank you for your patience.
>>
>> I had this idea in mind as well.
>> However, there are two considerations that led me to reconsider it:
>>
>> 1. I wasn't convinced about how we should call cond_resched every N
>> iterations. Should it be 1000 or 10000?
> 
> Sure, there will likely not be any _right_ value. This is mostly to
> mitigate the overhead of cond_resched which is not completely free.
> Having a system with 1000 tasks is not completely uncommon and we do not
> really need cond_resched now.
> 
> More importantly we can expect cond_resched will eventually go away with
> the PREEMPT_LAZY (or what is the current name of that) so I wouldn't
> overthink this.
> 
>> 2. I don't think all callers of mem_cgroup_scan_tasks need cond_resched.
>> Only fn is expensive (e.g., dump_tasks), and it needs cond_resched. At
>> least, I have not encountered any other issue except except when fn is
>> dump_tasks.
> 
> See above. I wouldn't really overthink this.

Thanks, I tested and sent v2.

Best regards,
Ridong