Re: [PATCH nf-next-2.6 2/2] conntrack: per_cpu untracking

Patrick McHardy
2010-06-09T12:45:12Z

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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