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

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Date:	Tue, 3 Nov 2015 21:00:12 +0000
From:	Haiyang Zhang <haiyangz@...rosoft.com>
To:	David Miller <davem@...emloft.net>
CC:	"eric.dumazet@...il.com" <eric.dumazet@...il.com>,
	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: David Miller [mailto:davem@...emloft.net]
> Sent: Tuesday, November 3, 2015 2:50 PM
> To: Haiyang Zhang <haiyangz@...rosoft.com>
> Cc: eric.dumazet@...il.com; KY Srinivasan <kys@...rosoft.com>;
> edumazet@...gle.com; netdev@...r.kernel.org
> Subject: Re: [PATCH net-next] net: increase LL_MAX_HEADER if HYPERV_NET
> is enabled
> 
> From: Haiyang Zhang <haiyangz@...rosoft.com>
> Date: Tue, 3 Nov 2015 18:49:05 +0000
> 
> >> -----Original Message-----
> >> From: David Miller [mailto:davem@...emloft.net]
> >> Sent: Tuesday, November 3, 2015 1:20 PM
> >> To: Haiyang Zhang <haiyangz@...rosoft.com>
> >> Cc: eric.dumazet@...il.com; KY Srinivasan <kys@...rosoft.com>;
> >> edumazet@...gle.com; netdev@...r.kernel.org
> >> Subject: Re: [PATCH net-next] net: increase LL_MAX_HEADER if
> >> HYPERV_NET is enabled
> >>
> >> From: Haiyang Zhang <haiyangz@...rosoft.com>
> >> Date: Tue, 3 Nov 2015 17:34:47 +0000
> >>
> >> > But we still keep this busy return in our code, just for "weird corner
> cases".
> >>
> >> The queue_stopped condition must be precise.
> >
> > The only case we return NETDEV_TX_BUSY is when the outgoing ring
> > buffer is full, which almost never happens because we stop the xmit queue
> if ring is <10% available.
> 
> I don't think you understand.
> 
> You must perform the queue stop operation such that it is impossible for
> your
> ->ndo_start_xmit() method to be invoked in a way such that you cannot
> ->transmit
> the SKB given to you immediately.

We already did the queue stop operation in the netvsc_send_pkt() in file netvsc.c:
		if (ring_avail < RING_AVAIL_PERCENT_LOWATER) {
			netif_tx_stop_queue(netdev_get_tx_queue(ndev, q_idx));
This flow control mechanism stops the tx queue when the outgoing buffer on vmbus 
is <10% available. So we can always transmit immediately when start_xmit is called.

The case of returning NETDEV_TX_BUSY is not expected to happen normally.

Thanks,
- Haiyang

> 
> It's quite tiring to keep trying to explain this over and over repeatedly.
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