netdev - Re: [PATCH net-next] net-gro: restore frag0 optimization

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Date:	Mon, 31 Mar 2014 16:27:47 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	mschmidt@...hat.com, netdev@...r.kernel.org, edumazet@...gle.com,
	hkchu@...gle.com
Subject: Re: [PATCH net-next] net-gro: restore frag0 optimization

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Sat, 29 Mar 2014 21:28:21 -0700

> From: Eric Dumazet <edumazet@...gle.com>
> 
> Main difference between napi_frags_skb() and napi_gro_receive() is that
> the later is called while ethernet header was already pulled by the NIC
> driver (eth_type_trans() was called before napi_gro_receive())
> 
> Jerry Chu in commit 299603e8370a ("net-gro: Prepare GRO stack for the
> upcoming tunneling support") tried to remove this difference by calling
> eth_type_trans() from napi_frags_skb() instead of doing this later from
> napi_frags_finish()
> 
> Goal was that napi_gro_complete() could call 
> ptype->callbacks.gro_complete(skb, 0)  (offset of first network header =
> 0)
> 
> Also, xxx_gro_receive() handlers all use off = skb_gro_offset(skb) to
> point to their own header, for the current skb and ones held in gro_list
> 
> Problem is this cleanup work defeated the frag0 optimization:
> It turns out the consecutive pskb_may_pull() calls are too expensive.
> 
> This patch brings back the frag0 stuff in napi_frags_skb().
> 
> As all skb have their mac header in skb head, we no longer need
> skb_gro_mac_header()
> 
> Reported-by: Michal Schmidt <mschmidt@...hat.com>
> Fixes: 299603e8370a ("net-gro: Prepare GRO stack for the upcoming tunneling support")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Jerry Chu <hkchu@...gle.com>
> ---
> David, this patch is definitely risky, I targeted net-next for staging
> it. I tested it with mlx4 NIC.
> 
> BTW, I believe the BUG_ON(skb->end - skb->tail < grow); could trigger
> if the headers are bigger than what the driver allocated for skb->head.
> 
> After encapsulation support added by Jerry, this might happen ?
> 
> We probably need to tweak skb_gro_reset_offset() to cap frag0_len to
> force calling slow path. I'll cook a patch.

Ok, applied, thanks for taking care of this Eric.

We can send to -stable after it's gotten a lot of testing and
auditing.
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