netdev - Re: [PATCH] skbuff: remove pointless conditional before kfree

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Date:	Tue, 28 Aug 2012 07:12:34 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Wei Yongjun <weiyj.lk@...il.com>
Cc:	davem@...emloft.net, yongjun_wei@...ndmicro.com.cn,
	netdev@...r.kernel.org
Subject: Re: [PATCH] skbuff: remove pointless conditional before kfree_skb()

On Tue, 2012-08-28 at 21:10 +0800, Wei Yongjun wrote:
> From: Wei Yongjun <yongjun_wei@...ndmicro.com.cn>
> 
> Remove pointless conditional before kfree_skb().
> 
> Signed-off-by: Wei Yongjun <yongjun_wei@...ndmicro.com.cn>
> ---
>  include/linux/skbuff.h | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 7632c87..0b846d9 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -2464,8 +2464,7 @@ static inline void nf_conntrack_get_reasm(struct sk_buff *skb)
>  }
>  static inline void nf_conntrack_put_reasm(struct sk_buff *skb)
>  {
> -	if (skb)
> -		kfree_skb(skb);
> +	kfree_skb(skb);
>  }
>  #endif
>  #ifdef CONFIG_BRIDGE_NETFILTER
> 


Its not exactly pointless.

Its a tradeoff between kernel code size, and ability for cpu to predict
a branch in kfree_skb()

This test is in hot path, and therefore this patch can potentially have
a performance impact.

I really think most kfree_skb() calls are done with a non NULL param,
so the branch prediction is good.

But after this patch, things are totally different.

Therefore, I am against it.



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