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

[Alexey Dobriyan <adobriyan@gmail.com>]

On Fri, Nov 14, 2008 at 12:43:06PM +0100, Eric Dumazet wrote:
> Alexey Dobriyan a écrit :
>> 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@cosmosbay.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@cosmosbay.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.
>
> You cannot reduce size, and it doesnt matter, since we use dst_entry inside rtable
> and rtable is using SLAB_HWCACHE_ALIGN kmem_cachep : we have many bytes available.
>
> After patch on 32 bits
>
> sizeof(struct rtable)=244   (12 bytes left)
>
> Same for other containers.

Hmm, indeed.

I tried moving __refcnt et al to the very beginning, but it seems to make
things worse (on x86_64, almost within statistical error).

And there is no way to use offset_of() inside struct definition. :-(