netdev - Re: [PATCH] net: make sure struct dst_entry refcount is aligned on 64 bytes

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Message-ID: <20081114113558.GA9692@x200.localdomain>
Date:	Fri, 14 Nov 2008 14:35:58 +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: make sure struct dst_entry refcount is aligned on
	64 bytes

On Fri, Nov 14, 2008 at 11:47:01AM +0100, Eric Dumazet wrote:
> Alexey Dobriyan a écrit :
>> 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.
>
> Well, your patch is a good thing, we only need to make adjustments.
>
>>
>>> 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
>>
>> Make it last member?
>
> Yes, it will help tbench, but not machines that stress IP route cache
>
> (dst_use() must dirty the three fields "refcnt, __use , lastuse" )
>
> Also, 'next' pointer should be in the same cache line, to speedup route
> cache lookups.

Knowledge taken.

> Next problem is that offsets depend on architecture being 32 or 64 bits.
>
> On 64bit, offsetof(struct dst_entry, __refcnt) is 0xb0 : not very good...

I think all these constraints can be satisfied with clever rearranging of dst_entry.
Let me come up with alternative patch which still reduces dst slab size.
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