netdev - Re: question about size of sk_buff and skb_shared

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Date:   Tue, 18 Apr 2017 17:15:35 -0700
From:   Code Soldier1 <codesoldier1@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev@...r.kernel.org
Subject: Re: question about size of sk_buff and skb_shared_info

I am not suggesting that we do not do cache alignment. That is
required for performance. I have gone through this exercise because I
need to add a field to sk_buff and I want to do that without causing
any adverse effects.

Now that we have discovered that there are 40 bytes that can be used
without any adverse effect, may I increase skb->cb by 8 bytes ?

If not then may I increase skb_shared_info -- However that would have
to be by 64bytes.


On Tue, Apr 18, 2017 at 4:55 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Please do not top post on netdev
>
> On Tue, 2017-04-18 at 16:26 -0700, Code Soldier1 wrote:
>> Eric,
>>
>> This alignment flag is passed to the cache constructor and the
>> allocation is indeed cache aligned. However, since the allocated size
>> is not a multiple of the alignment, wont memory be wasted ?. We can
>> get 40 extra bytes without any side effects since they are on the same
>> cache line ?
>
> We _want_ to align skb to cache lines.
>
> Fact that few bytes might be wasted is unfortunate, but negligible.
>
> BTW, if you do kmalloc(1025), kmalloc() wastes 1023 bytes.
>
>
>>
>> kmem_cache_create() code does an ALIGN() to round up the size.
>>
>>         kasan_cache_create(cachep, &size, &flags);
>>
>>         size = ALIGN(size, cachep->align);
>>
>> This is the size used in calculate_slab_order() to calculate num. I am
>> assuming that in the non debug case gfp_order will be 0.
>>
>> Perhaps I am missing something.
>>
>>
>> On Tue, Apr 18, 2017 at 12:00 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > On Tue, 2017-04-18 at 10:34 -0700, Code Soldier1 wrote:
>> >> Hi Folks,
>> >>
>> >> I am sure there is a reason for the current sizes of these structures,
>> >> However the reason is not obvious to me. So please help me understand.
>> >>
>> >> Currently the size of sk_buff on an x86_64 system is 232 bytes -- Why
>> >> is that. I expected it to be a multiple of 32/64 as they are the most
>> >> common cache lines. Since the alignment calculation will align the
>> >> structure with the hw cache line, it seems like we might be wasting
>> >> space ?
>> >>
>> >> skb_shared_info on the other hand is perfectly aligned with a size of 320 bytes.
>> >>
>> >> Thanks,
>> >>
>> >
>> > The alignment is there.
>> > Look at skb_init() code, using SLAB_HWCACHE_ALIGN
>> >
>> >
>> >
>> >
>> >
>>
>>
>>
>
>



-- 
CS1