netdev - Re: [PATCH net-next] openvswitch: do not propagate headroom updates to internal port

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Message-Id: <20171202.211726.1769384755580628445.davem@davemloft.net>
Date:   Sat, 02 Dec 2017 21:17:26 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     pabeni@...hat.com
Cc:     netdev@...r.kernel.org, pshelar@...ira.com
Subject: Re: [PATCH net-next] openvswitch: do not propagate headroom
 updates to internal port

From: Paolo Abeni <pabeni@...hat.com>
Date: Thu, 30 Nov 2017 15:35:33 +0100

> After commit 3a927bc7cf9d ("ovs: propagate per dp max headroom to
> all vports") the need_headroom for the internal vport is updated
> accordingly to the max needed headroom in its datapath.
> 
> That avoids the pskb_expand_head() costs when sending/forwarding
> packets towards tunnel devices, at least for some scenarios.
> 
> We still require such copy when using the ovs-preferred configuration
> for vxlan tunnels:
> 
>     br_int
>   /       \
> tap      vxlan
>            (remote_ip:X)
> 
> br_phy
>      \
>     NIC
> 
> where the route towards the IP 'X' is via 'br_phy'.
> 
> When forwarding traffic from the tap towards the vxlan device, we
> will call pskb_expand_head() in vxlan_build_skb() because
> br-phy->needed_headroom is equal to tun->needed_headroom.
> 
> With this change we avoid updating the internal vport needed_headroom,
> so that in the above scenario no head copy is needed, giving 5%
> performance improvement in UDP throughput test.
> 
> As a trade-off, packets sent from the internal port towards a tunnel
> device will now experience the head copy overhead. The rationale is
> that the latter use-case is less relevant performance-wise.
> 
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>

Applied, thanks.