netdev - Re: [PATCH net-next] gro: small napi_get

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Message-ID: <1386311188.30495.238.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 05 Dec 2013 22:26:28 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Joe Perches <joe@...ches.com>
Cc:	David Miller <davem@...emloft.net>, netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] gro: small napi_get_frags() optim

On Thu, 2013-12-05 at 22:11 -0800, Joe Perches wrote:

> Thanks, I agree this patch is appropriate as skb is unlikely
> to be NULL in normal cases.

We do not care if skb is NULL or not.

> 
> But just for thought: isn't the test expense essentially
> free when skb is in a register vs the member set cost?
> 

Test is not 'free', even if predicted.

Study the generated assembly code if you are interested, you'll see this
makes a difference.

> Also, netdev_alloc_skb_ip_align is static inline and it has
> a test of
> 	if (NET_IP_ALIGN && skb)
> just before the return so this "if (skb)" test could already
> be elided by a good compiler but this change requires an
> unconditional set.

NET_IP_ALIGN is 0 on x86, even a 'bad' compiler zaps the whole thing.



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