Hi David： The Patch "rcu: Fix soft lockup for rcu_nocb_kthread" has been added to several stable tree， 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 > > > . >
