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

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Date:	Mon, 02 Mar 2015 23:05:34 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
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

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Mon, 02 Mar 2015 20:02:05 -0800

> 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.

Understood.

While reviewing this I noticed that the jiffies timestamp in GRO cb
could really be u32 if we want instead of full "unsigned long".

> 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...

Right, all these kinds of things touch everything.
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