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

[Chen Ridong <chenridong@huaweicloud.com>]

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:

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index d9061bd55436..9d197a731841 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -5023,7 +5023,7 @@ struct task_struct *css_task_iter_next(struct
css_task_iter *it)
        }

        spin_unlock_irqrestore(&css_set_lock, irqflags);
-
+       cond_resched();
        return it->cur_task;
 }

@@ -433,8 +433,10 @@ static void dump_tasks(struct oom_control *oc)
                struct task_struct *p;

                rcu_read_lock();
-               for_each_process(p)
+               for_each_process(p) {
+                       touch_nmi_watchdog();
                        dump_task(p, oc);
+               }
                rcu_read_unlock();
        }


The 'css_task_iter_*' functions are used in many places. We should be
very careful when adding cond_resched within these functions. I don't
see any RCU or spinlock usage outside of css_task_iter_*, except for
mutex locks, such as in cgroup_do_freeze.

And perhaps Tj will have some opinions on this?

Best regards,
Ridong