Re: [PATCH] rcu: fix the OOM problem of huge IP abnormal packet traffic

[Ding Tianhong]

Hi Davidï

The Patch "rcu: Fix soft lockup for rcu_nocb_kthread" has been added to several stable treeï it may introduced an issue in certain special scenarios,
The Patch "softirq: Let ksoftirqd do its job" could fix this issue, so I hope you could add this patch to stable tree, thanks.

Ding

On 2017/1/10 13:51, Paul E. McKenney wrote:
> On Tue, Jan 10, 2017 at 11:20:40AM +0800, Ding Tianhong wrote:
>>
>>
>> On 2017/1/4 21:48, Paul E. McKenney wrote:
>>> On Wed, Jan 04, 2017 at 03:02:30PM +0800, Ding Tianhong wrote:
>>>>
>>>>
>>>> On 2017/1/4 8:57, Paul E. McKenney wrote:
>>>>> On Wed, Dec 28, 2016 at 04:13:15PM -0800, Paul E. McKenney wrote:
>>>>>> On Wed, Dec 28, 2016 at 01:58:06PM +0800, Ding Tianhong wrote:
>>>>>>> Hi, Paul:
>>>>>>>
>>>>>>> I try to debug this problem and found this solution could work well for both problem scene.
>>>>>>>
>>>>>>>
>>>>>>> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
>>>>>>> index 85c5a88..dbc14a7 100644
>>>>>>> --- a/kernel/rcu/tree_plugin.h
>>>>>>> +++ b/kernel/rcu/tree_plugin.h
>>>>>>> @@ -2172,7 +2172,7 @@ static int rcu_nocb_kthread(void *arg)
>>>>>>>                         if (__rcu_reclaim(rdp->rsp->name, list))
>>>>>>>                                 cl++;
>>>>>>>                         c++;
>>>>>>> -                   local_bh_enable();
>>>>>>> +                 _local_bh_enable();
>>>>>>>                         cond_resched_rcu_qs();
>>>>>>>                         list = next;
>>>>>>>                 }
>>>>>>>
>>>>>>>
>>>>>>> The cond_resched_rcu_qs() would process the softirq if the softirq is pending, so no need to use
>>>>>>> local_bh_enable() to process the softirq twice here, and it will avoid OOM when huge packets arrives,
>>>>>>> what do you think about it? Please give me some suggestion.
>>>>>>
>>>>>> From what I can see, there is absolutely no guarantee that
>>>>>> cond_resched_rcu_qs() will do local_bh_enable(), and thus no guarantee
>>>>>> that it will process any pending softirqs -- and that is not part of
>>>>>> its job in any case.  So I cannot recommend the above patch.
>>>>>>
>>>>>> On efficient handling of large invalid packets (that is still the issue,
>>>>>> right?), I must defer to Dave and Eric.
>>>>>
>>>>> On the perhaps unlikely off-chance that there is a fix for this outside
>>>>> of networking, what symptoms are you seeing without this fix in place?
>>>>> Still RCU CPU stall warnings?  Soft lockups?  Something else?
>>>>>
>>>>> 								Thanx, Paul
>>>>>
>>>>
>>>> Hi Paul:
>>>>
>>>> I was still try to test and fix this by another way, but could explain more about this problem.
>>>>
>>>> when the huge packets coming, the packets was abnormal and will be freed by dst_release->call_rcu(dst_destroy_rcu),
>>>> so the rcuos kthread will handle the dst_destroy_rcu to free them, but when the rcuos was looping ,I fould the local_bh_enable() will
>>>> call do_softirq to receive a certain number of packets which is abnormal and need to be free, but more packets is coming so when cond_resched_rcu_qs run,
>>>> it will do the ksoftirqd and do softirq again, so rcuos kthread need free more, it looks more and more worse and lead to OOM because many more packets need to
>>>> be freed.
>>>> So I think the do_softirq in the local_bh_enable is not need here, the cond_resched_rcu_qs() will handle the do_softirq once, it is enough.
>>>>
>>>> and recently I found that the Eric has upstream a new patch named (softirq: Let ksoftirqd do its job) may fix this, and still test it, not get any results yet.
>>>
>>> OK, I don't see any reasonable way that the RCU callback-offload tasks
>>> (rcuos) can figure out whether or not they should let softirqs happen --
>>> unconditionally suppressing them might help your workload, but would
>>> break workloads needing low networking latency, of which there are many.
>>>
>>> So please let me know now things go with Eric's patch.
>>>
>> Hi Paul:
>>
>> Good news, the Eric's patch could fix this problem, it means that if the softirqd kthread is running, we should not take too much
>> time in the softirq process, this behavior equivalent that we remove the do_softirq in the local_bh_enable(), but this solution looks more
>> perfect, we need to inform the lts kernel maintainer to applied this patch which is not looks like a bugfix.
> 
> Here is hoping!  ;-)
> 
> 							Thanx, Paul
> 
> 
> .
>