netdev - Re: [PATCH net-next] net: increase LL_MAX_HEADER if HYPERV

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Date:	Tue, 03 Nov 2015 08:37:24 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	kys@...rosoft.com, haiyangz@...rosoft.com, edumazet@...gle.com,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: increase LL_MAX_HEADER if HYPERV_NET is
 enabled

On Tue, 2015-11-03 at 10:33 -0500, David Miller wrote:
> From: KY Srinivasan <kys@...rosoft.com>
> Date: Tue, 3 Nov 2015 07:59:36 +0000
> 
> > I have implemented the scheme we had discussed a few weeks ago. In
> > this new implementation our driver is NOT requesting addition
> > headroom - rndis header and the per packet state is being maintained
> > outside of the skb. What I am seeing is that when I have
> > LL_MAX_HEADER set to 220 bytes, even though our driver is not using
> > the additional head room, I see about a 10% boost in the peak
> > performance (about 34 Gbps on a 40Gbps interface). However, when I
> > set the LL_MAX_HEADER value to the current default, the peak
> > performance drops back to what we currently have (around 31
> > Gbps). In both these cases, there is no reallocation of skb since no
> > additional headroom is being requested and yet there is a
> > significant difference in performance.  I trying to figure out why
> > this is the case, your insights will be greatly appreciated.
> 
> It probably has something to do with cache line or data alignment.

This also might be because of a slight change in skb->truesize, and/or
a change of amount of payload in skb->head

(Increasing LL_MAX_HEADER is reducing amount of payload in skb->head)

Can't you run perf tool to get some precise profiling ?


Another red flag in you driver xmit is :

return (ret == -EAGAIN) ? NETDEV_TX_BUSY : NETDEV_TX_OK;


extract from include/linux/netdevice.h
 * netdev_tx_t (*ndo_start_xmit)(struct sk_buff *skb,
 *                               struct net_device *dev);
 *      Called when a packet needs to be transmitted.
 *      Returns NETDEV_TX_OK.  Can return NETDEV_TX_BUSY, but you should stop
 *      the queue before that can happen; it's for obsolete devices and weird
 *      corner cases, but the stack really does a non-trivial amount
 *      of useless work if you return NETDEV_TX_BUSY.



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