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

[Chen Ridong]

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?


@@ -390,7 +391,7 @@ static int dump_task(struct task_struct *p, void *arg)
        if (!is_memcg_oom(oc) && !oom_cpuset_eligible(p, oc))
                return 0;

+       touch_nmi_watchdog();
        task = find_lock_task_mm(p);

Best regards,
Ridong