linux-kernel - RE: [PATCH V3 net-next 1/1] hv_netvsc: Use the xmit_more skb flag to optimize signaling the host

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Message-ID: <BY2PR0301MB071195898775621EBF8FA9DEA0D00@BY2PR0301MB0711.namprd03.prod.outlook.com>
Date:	Wed, 6 May 2015 18:28:53 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"olaf@...fle.de" <olaf@...fle.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"jasowang@...hat.com" <jasowang@...hat.com>
Subject: RE: [PATCH V3 net-next 1/1] hv_netvsc: Use the xmit_more skb flag to
 optimize signaling the host



> -----Original Message-----
> From: Eric Dumazet [mailto:eric.dumazet@...il.com]
> Sent: Wednesday, May 6, 2015 11:09 AM
> To: KY Srinivasan
> Cc: davem@...emloft.net; netdev@...r.kernel.org; linux-
> kernel@...r.kernel.org; devel@...uxdriverproject.org; olaf@...fle.de;
> apw@...onical.com; jasowang@...hat.com
> Subject: Re: [PATCH V3 net-next 1/1] hv_netvsc: Use the xmit_more skb flag
> to optimize signaling the host
> 
> On Wed, 2015-05-06 at 11:20 -0700, K. Y. Srinivasan wrote:
> > Based on the information given to this driver (via the xmit_more skb flag),
> > we can defer signaling the host if more packets are on the way. This will
> help
> > make the host more efficient since it can potentially process a larger batch
> of
> > packets. Implement this optimization.
> 
> 
> > diff --git a/drivers/net/hyperv/netvsc_drv.c
> b/drivers/net/hyperv/netvsc_drv.c
> > index 5993c7e..4efaa6e 100644
> > --- a/drivers/net/hyperv/netvsc_drv.c
> > +++ b/drivers/net/hyperv/netvsc_drv.c
> > @@ -435,6 +435,8 @@ check_size:
> >  	packet->page_buf = page_buf;
> >
> >  	packet->q_idx = skb_get_queue_mapping(skb);
> > +	if (netif_tx_queue_stopped(netdev_get_tx_queue(net, packet-
> >q_idx)))
> > +		packet->xmit_more = false;
> 
> Have you tested this condition ever triggers ?
> 
> It should not.
> 
> netvsc_start_xmit() should not be called if the queue is stopped.
> 
> The problem is the following :
> 
> netvsc_start_xmit() is called with the packet that is going to stop the
> queue, filling last slot, but with skb->xmit_more = 1;
> 
> So you need to do something about xmit_more, after calling
> netif_tx_stop_queue(), not before.
Ah! I too was wondering how we could get into this situation. The condition you mention
is already handled in the lower level - if the attempt to put the last packet on vmbus were to
fail because the ring is full, we will notify the host independent of kick_q parameter - look
at the function vmbus_sendpacket_ctl() in drivers/hv/channel.c

I will resend the patch by reverting this to version 2.

Regards,

K. Y
> 
>