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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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