netdev - Re: [PATCH RFC 00/14] shrink skb cb to 44 bytes

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Date:	Mon, 02 Mar 2015 20:02:05 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	fw@...len.de, netdev@...r.kernel.org, johannes@...solutions.net,
	linux-wireless@...r.kernel.org
Subject: Re: [PATCH RFC 00/14] shrink skb cb to 44 bytes

On Mon, 2015-03-02 at 17:17 -0500, David Miller wrote:
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Mon, 02 Mar 2015 11:49:23 -0800
> 
> > Size of skb->cb[] is not the major factor. Trying to gain 4 or 8 bytes
> > is not going to improve performance a lot.
> > 
> > The real problem is that we clear it in skb_alloc()/build_skb(), instead
> > of each layer doing so at demand, and only the part that matters for
> > this layer.
> > 
> > Basically, skb->cb[] could be 80 or 160 bytes instead of 48, and we
> > should not care, as long as no layer does a stupid/lazy 
> > 
> > memset(skb->cb, 0, sizeof(skb->cb))
> > 
> > Presumably skb_clone() could be extended to receive the length of
> > skb->cb[] that current layer cares about.
> 
> Regardless, I think Florian's work has value.

Of course. I hope my answer was not implying the contrary !

48 -> 44 cb change, with the 8 bytes alignment and various __packed
tricks that might confuse compilers on some arches, will be hard to
quantify in term of performances on all arches.

About the GRO layout change, reason why 'struct sk_buff *last;' is at
the end of struct napi_gro_cb is that this field is not used in fast
path.

Note : We could try to use one bit in skb to advertise zero shinfo(skb).

Many skbs have a zeroed shinfo() (but shinfo->dataref == 1) , and
dereferencing skb_shinfo adds a cache line miss. 

-> We could avoid memset(shinfo, 0, offsetof(struct skb_shared_info,
dataref)) & atomic_set(&shinfo->dataref, 1); 

 in alloc_skb() and friends completely.

Unfortunately this kind of change would be quite invasive...

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