netdev - Re: [PATCH nf-next-2.6 2/2] conntrack: per

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Message-ID: <4C0F8CD8.6040100@trash.net>
Date:	Wed, 09 Jun 2010 14:45:12 +0200
From:	Patrick McHardy <kaber@...sh.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Changli Gao <xiaosuo@...il.com>,
	Netfilter Developers <netfilter-devel@...r.kernel.org>,
	netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH nf-next-2.6 2/2] conntrack: per_cpu untracking

Eric Dumazet wrote:
> Le mardi 08 juin 2010 à 16:52 +0200, Eric Dumazet a écrit :
>   
>> Le mardi 08 juin 2010 à 16:29 +0200, Patrick McHardy a écrit :
>>     
>>> On 04.06.2010 22:15, Eric Dumazet wrote:
>>>       
>>>> NOTRACK makes all cpus share a cache line on nf_conntrack_untracked
>>>> twice per packet, slowing down performance.
>>>>
>>>> This patch converts it to a per_cpu variable.
>>>>
>>>> We assume same cpu is used for a given packet, entering and exiting the
>>>> NOTRACK state.
>>>>         
>>> That doesn't seem to be a valid assumption, the conntrack entry is
>>> attached to the skb and processing in the output path might get
>>> preempted and rescheduled to a different CPU.
>>>       
>> Thats unfortunate.
>>
>> Ok, only choice then is to not change refcount on the untracked ct, and
>> keep a shared (read only after setup time) untrack structure.
>>
>>
>>     
>
> Oh well, re-reading my patch, I dont see why I said this in Changelog :)
>
> We lazily select the untrack structure in one cpu, then keep the pointer
> to this untrack structure, attached to ct.
>
> The (still atomic) increment / decrement of refcount is done on the
> saved pointer, not on actual per_cpu structure.
>
> So if a packet is rescheduled on a different CPU, second cpu will "only"
> dirty cache line of other cpu, it probably almost never happens...
>   

Indeed, you're right of course.
> Thanks
>
> [PATCH nf-next-2.6 2/2] conntrack: per_cpu untracking
>
> NOTRACK makes all cpus share a cache line on nf_conntrack_untracked
> twice per packet, slowing down performance.
>
> This patch converts it to a per_cpu variable.
>   
Applied, thanks Eric.
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