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

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Date:	Thu, 05 Dec 2013 22:11:08 -0800
From:	Joe Perches <joe@...ches.com>
To:	Eric Dumazet <eric.dumazet@...il.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 21:44 -0800, Eric Dumazet wrote:
> Remove one useless conditional branch : 
> napi->skb is NULL, so nothing bad can happen.
[]
> diff --git a/net/core/dev.c b/net/core/dev.c
[]
> @@ -3981,8 +3981,7 @@ struct sk_buff *napi_get_frags(struct napi_struct *napi)
>  
>  	if (!skb) {
>  		skb = netdev_alloc_skb_ip_align(napi->dev, GRO_MAX_HEAD);
> -		if (skb)
> -			napi->skb = skb;
> +		napi->skb = skb;

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

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

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.


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