netdev - Re: [PATCH 1/4 net-next] net: allow skb->head to be a page fragment

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Date:	Sat, 28 Apr 2012 01:27:24 -0700
From:	Alexander Duyck <alexander.duyck@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Alexander Duyck <alexander.h.duyck@...el.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	Neal Cardwell <ncardwell@...gle.com>,
	Tom Herbert <therbert@...gle.com>,
	Maciej Żenczykowski <maze@...gle.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Ben Hutchings <bhutchings@...arflare.com>,
	Matt Carlson <mcarlson@...adcom.com>,
	Michael Chan <mchan@...adcom.com>,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH 1/4 net-next] net: allow skb->head to be a page fragment

On Sat, Apr 28, 2012 at 12:16 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Fri, 2012-04-27 at 17:27 -0700, Alexander Duyck wrote:
>> On 04/27/2012 03:33 AM, Eric Dumazet wrote:
>> > From: Eric Dumazet <edumazet@...gle.com>
>> >
>
>> > +struct sk_buff *build_skb(void *data, unsigned int frag_size)
>> >  {
>> >     struct skb_shared_info *shinfo;
>> >     struct sk_buff *skb;
>> > -   unsigned int size;
>> > +   unsigned int size = frag_size ? : ksize(data);
>> >
>> >     skb = kmem_cache_alloc(skbuff_head_cache, GFP_ATOMIC);
>> >     if (!skb)
>> >             return NULL;
>> >
>> > -   size = ksize(data) - SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
>> > +   size -= SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
>> >
>> >     memset(skb, 0, offsetof(struct sk_buff, tail));
>> >     skb->truesize = SKB_TRUESIZE(size);
>> > +   skb->head_frag = frag_size != 0;
>> >     atomic_set(&skb->users, 1);
>> >     skb->head = data;
>> >     skb->data = data;
>>
>> This doesn't seem right to me.  You are only counting the piece of the
>> page that got filled with data and the piece that will get overwritten
>> with the shared info.  What about the rest of the page?  It looks like
>> in the tg3 patch you have the driver using a half page.  Based on this
>> function I suspect the resultant truesize would be something like 64 +
>> 256 + 320 for an ack.  Shouldn't your truesize in that case be 2048 + 256?
>
> Re-reading your mail, I think you missed fact that tg3 driver currently
> uses a kmalloc(64+1500+14+SKB_DATA_ALIGN(sizeof(struct
> skb_shared_info)) , so basically does a kmalloc(2048). But this is done
> before NIC fills the frame in it, so we cant know what will be the frame
> length...
>
> So build_skb() does later a ksize(data) and this gives 2048, even for a
> small ACK packet ...
>
> So the spirit of this patch is not to change any truesize.
>
> tg3 for example splits a PAGE_SIZE into 2048 bytes frags (2 frags on x86
> for example). Its done about the same in Intel IGB driver (IGB assumes
> PAGE_SIZE is 4096 since it uses PAGE_SIZE/2)
>
> The only thing that is changed here is where skb->head is allocated
> from : kmalloc() caches or a frag from a page (and one reference to
> page->_count)
>
> Hope this clears your concern ?

I had misread what was happening here.  I didn't fully understand what
build_skb was doing before reading the patch and that is what led me
astray.

I was thinking there would be a bigger gain with a change like this
but essentially all you are doing is making the packets more mergable
resulting in you being able to drop the 256 bytes for the sk_buff in
either GRO or when it gets queued for the socket.

What caught my eye about all this is that I could probably cut a lot
of work out of the ixgbe driver and FCoE path by modifying things so
that FCoE packets could be received as a page, and then treated as a
linear skb without needing calls to skb_linearize.

Thanks,

Alex
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