netdev - Re: 32 core net-next stack/netfilter "scaling"

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Date:	Mon, 09 Feb 2009 15:57:05 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	Eric Dumazet <dada1@...mosbay.com>
CC:	Rick Jones <rick.jones2@...com>,
	Netfilter Developers <netfilter-devel@...r.kernel.org>,
	Linux Network Development list <netdev@...r.kernel.org>,
	Stephen Hemminger <shemminger@...tta.com>
Subject: Re: 32 core net-next stack/netfilter "scaling"

Eric Dumazet wrote:
> Eric Dumazet a écrit :
>> Patrick McHardy a écrit :
>>> That looks more complicated since it requires to take multiple locks
>>> occasionally (f.i. hash insertion, potentially helper-related and
>>> expectation-related stuff), and there is the unconfirmed_list, where
>>> fine-grained locking can't really be used without changing it to
>>> a hash.
>>>
>> Yes its more complicated, but look what we did in 2.6.29 for tcp/udp
>>  sockets, using RCU to have lockless lookups.
>> Yes, we still take a lock when doing an insert or delete at socket
>> bind/unbind time.
>>
>> We could keep a central nf_conntrack_lock to guard insertions/deletes
>> from hash and unconfirmed_list.
>>
>> But *normal* packets that only need to change state of one particular
>> connection could use RCU (without spinlock) to locate the conntrack,
>> then lock the found conntrack to perform all state changes.
> 
> Well... RCU is already used by conntrack :)
> 
> Maybe only __nf_ct_refresh_acct() needs not taking nf_conntrack_lock

The lock is currently used to avoid timer update races.
Martin has two old patches two remove it, but they require
changes to the timer code to support a mod_timer variant
that doesn't enable an inactive timer:

http://people.netfilter.org/gandalf/performance/patches/mod_timer_noact
http://people.netfilter.org/gandalf/performance/patches/__nf_ct_refresh_acct-locking
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