netdev - Re: [RFC] Could we avoid touching dst->refcount in some cases ?

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Date:	Tue, 25 Nov 2008 05:43:32 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	David Miller <davem@...emloft.net>
CC:	andi@...stfloor.org, netdev@...r.kernel.org
Subject: Re: [RFC] Could we avoid touching dst->refcount in some cases ?

David Miller a écrit :
> From: Eric Dumazet <dada1@...mosbay.com>
> Date: Mon, 24 Nov 2008 11:14:29 +0100
> 
>> So maybe we could make ip_append_data() (or its callers) a
>> litle bit smarter, avoiding increment/decrement if possible.
> 
> These ideas are interesting but hard to make work.
> 
> I think the receive path has more chance of getting gains
> from this, to be honest.
> 
> One third (effectively) of TCP stream packets are ACKs and
> freed immediately.  This means that the looked up route does
> not escape the packet receive path.  So we could elide the
> counter increment in that case.
> 
> In fact, once we queue even TCP data, there is no need for
> that cached skb->dst route any longer.
> 
> So pretty much all TCP packets could avoid the dst refcounting
> on receive.

Very interesting. So we could try the following path :

1) First try to release dst when queueing skb to various queues
(UDP, TCP, ...) while its hot. Reader wont have to release it
while its cold.

2) Check if we can handle the input path without any refcount
   dirtying ?

To make the transition easy, we could use a bit on skb to mark
dst being not refcounted (ie no dst_release() should be done on it)

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