netdev - Re: [RFT 3/4] netfilter: use sequence number synchronization for counters

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Date:	Wed, 28 Jan 2009 17:15:25 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	Eric Dumazet <dada1@...mosbay.com>
CC:	Stephen Hemminger <shemminger@...tta.com>,
	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: [RFT 3/4] netfilter: use sequence number synchronization for
 counters

Eric Dumazet wrote:
> Stephen Hemminger a écrit :
>>>>   --- a/net/ipv4/netfilter/ip_tables.c    2009-01-27 
>>>> 14:48:41.567879095 -0800
>>>> +++ b/net/ipv4/netfilter/ip_tables.c    2009-01-27 
>>>> 15:45:05.766673246 -0800
>>>> @@ -366,7 +366,9 @@ ipt_do_table(struct sk_buff *skb,
>>>>              if (IPT_MATCH_ITERATE(e, do_match, skb, &mtpar) != 0)
>>>>                  goto no_match;
>>>>  
>>>> +            write_seqcount_begin(&e->seq);
>>>>              ADD_COUNTER(e->counters, ntohs(ip->tot_len), 1);
>>>> +            write_seqcount_end(&e->seq);
>>>>         
>>> Its not very good to do it like this, (one seqcount_t per rule per cpu)
>>>     
>>
>> If we use one count per table, that solves it, but it becomes a hot
>> spot, and on an active machine will never settle.
>>
>>   
> One seqcount per table and per cpu.
> Only one cpu (the owner) will need to change the seqcount (one increment 
> when entering ipt_do_table(), one increment when leaving)

That would also make sure the counters add up, right?

> This location is only read by the thread doing the "iptables -L". We 
> dont care it spends a few cycles, it's already a big cruncher.

Indeed.
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