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

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Date:	Tue, 3 Nov 2015 17:34:47 +0000
From:	Haiyang Zhang <haiyangz@...rosoft.com>
To:	Eric Dumazet <eric.dumazet@...il.com>,
	David Miller <davem@...emloft.net>
CC:	KY Srinivasan <kys@...rosoft.com>,
	"edumazet@...gle.com" <edumazet@...gle.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH net-next] net: increase LL_MAX_HEADER if HYPERV_NET is
 enabled



> -----Original Message-----
> From: Eric Dumazet [mailto:eric.dumazet@...il.com]
> Sent: Tuesday, November 3, 2015 11:37 AM
> To: David Miller <davem@...emloft.net>
> Cc: KY Srinivasan <kys@...rosoft.com>; Haiyang Zhang
> <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.

We stop xmit when ring buffer is <10% available (netvsc_send_pkt()), so we 
almost never hit empty ring buffer and return NETDEV_TX_BUSY. 
But we still keep this busy return in our code, just for "weird corner cases".

Thanks,
- Haiyang