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

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Message-ID: <497FFCAE.70503@cosmosbay.com>
Date:	Wed, 28 Jan 2009 07:35:26 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Stephen Hemminger <shemminger@...tta.com>
CC:	David Miller <davem@...emloft.net>,
	Patrick McHardy <kaber@...sh.net>, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org
Subject: Re: [RFT 3/4] netfilter: use sequence number synchronization for
 counters

Stephen Hemminger a écrit :
> On Wed, 28 Jan 2009 07:17:04 +0100
> Eric Dumazet <dada1@...mosbay.com> wrote:
>
>   
>> Stephen Hemminger a écrit :
>>     
>>> Change how synchronization is done on the iptables counters. Use seqcount
>>> wrapper instead of depending on reader/writer lock.
>>>
>>> Signed-off-by: Stephen Hemminger <shemminger@...tta.com>
>>>
>>>
>>>   
>>> --- 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)

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.

I dont understand your concern, what do you mean by "never settle" ?

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