netdev - Re: [PATCH] net: speedup dst

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Message-ID: <491D3F18.5030505@cosmosbay.com>
Date:	Fri, 14 Nov 2008 10:04:24 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	David Miller <davem@...emloft.net>
CC:	netdev@...r.kernel.org, shemminger@...tta.com,
	Alexey Dobriyan <adobriyan@...il.com>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Subject: Re: [PATCH] net: speedup dst_release()

David Miller a écrit :
> From: Eric Dumazet <dada1@...mosbay.com>
> Date: Fri, 14 Nov 2008 09:09:31 +0100
> 
>> During tbench/oprofile sessions, I found that dst_release() was in third position.
>  ...
>> Instead of first checking the refcount value, then decrement it,
>> we use atomic_dec_return() to help CPU to make the right memory transaction
>> (ie getting the cache line in exclusive mode)
>  ...
>> Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
> 
> This looks great, applied, thanks Eric.
> 

Thanks David


I think I understood some regressions here on 32bits 

offsetof(struct dst_entry, __refcnt) is 0x7c again !!!

This is really really bad for performance

I believe this comes from a patch from Alexey Dobriyan
(commit def8b4faff5ca349beafbbfeb2c51f3602a6ef3a
net: reduce structures when XFRM=n)


This kills effort from Zhang Yanmin (and me...)

(commit f1dd9c379cac7d5a76259e7dffcd5f8edc697d17
[NET]: Fix tbench regression in 2.6.25-rc1)


Really we must find something so that this damned __refcnt is starting at 0x80


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