Subject: Re: [PATCH] conntrack: Reduce conntrack count in nf_conntrack_free()

From: Patrick McHardy <[email-masked]>
Date: Tue, 24 Mar 2009 13:43:54 +0100

Eric Dumazet wrote:
>> In a stress situation, you feed more deleted conntracks to call_rcu() than
>> the blimit (10 real freeing per RCU softirq invocation). 
>>
>> So with default qhimark being 10000, this means about 10000 conntracks
>> can sit in RCU (per CPU) before being really freed.
>>
>> Only when hitting 10000, RCU enters a special mode to free all queued items, instead
>> of a small batch of 10
>>
>> To solve your problem we can :
>>
>> 1) reduce qhimark from 10000 to 1000 (for example)
>>    Probably should be done to reduce some spikes in RCU code when freeing
>>    whole 10000 elements...
>> OR
>> 2) change conntrack tunable (max conntrack entries on your machine)
>> OR
>> 3) change net/netfilter/nf_conntrack_core.c to decrement net->ct.count
>>   in nf_conntrack_free() instead of callback.
>>
>> [PATCH] conntrack: Reduce conntrack count in nf_conntrack_free()
>>
>> We use RCU to defer freeing of conntrack structures. In DOS situation, RCU might
>> accumulate about 10.000 elements per CPU in its internal queues. To get accurate
>> conntrack counts (at the expense of slightly more RAM used), we might consider
>> conntrack counter not taking into account "about to be freed elements, waiting
>> in RCU queues". We thus decrement it in nf_conntrack_free(), not in the RCU
>> callback.
>>
>> Signed-off-by: Eric Dumazet <[email-masked]>
>>
>>
>> diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
>> index f4935e3..6478dc7 100644
>> --- a/net/netfilter/nf_conntrack_core.c
>> +++ b/net/netfilter/nf_conntrack_core.c
>> @@ -516,16 +516,17 @@ EXPORT_SYMBOL_GPL(nf_conntrack_alloc);
>>  static void nf_conntrack_free_rcu(struct rcu_head *head)
>>  {
>>  	struct nf_conn *ct = container_of(head, struct nf_conn, rcu);
>> -	struct net *net = nf_ct_net(ct);
>>  
>>  	nf_ct_ext_free(ct);
>>  	kmem_cache_free(nf_conntrack_cachep, ct);
>> -	atomic_dec(&net->ct.count);
>>  }
>>  
>>  void nf_conntrack_free(struct nf_conn *ct)
>>  {
>> +	struct net *net = nf_ct_net(ct);
>> +
>>  	nf_ct_ext_destroy(ct);
>> +	atomic_dec(&net->ct.count);
>>  	call_rcu(&ct->rcu, nf_conntrack_free_rcu);
>>  }
>>  EXPORT_SYMBOL_GPL(nf_conntrack_free);
> 
> I forgot to say this is what we do for 'struct file' freeing as well. We
> decrement nr_files in file_free(), not in file_free_rcu()

While temporarily exceeding the limit by up to 10000 entries is
quite a lot, I guess the important thing is that it can't grow
unbounded, so I think this patch is fine.