netdev - Re: [PATCH] net: speedup dst

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Message-ID: <20081114093613.GA2834@x200.localdomain>
Date:	Fri, 14 Nov 2008 12:36:13 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	shemminger@...tta.com,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
Subject: Re: [PATCH] net: speedup dst_release()

On Fri, Nov 14, 2008 at 10:04:24AM +0100, Eric Dumazet wrote:
> 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)

Ick.

> 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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