Re: [Xen-devel] [PATCH net-next] xen-netback: improve guest-receive-side flow control

[annie li <annie.li@xxxxxxxxxx>]

On 2013/11/29 0:27, Wei Liu wrote:
> On Thu, Nov 28, 2013 at 04:05:52PM +0000, Paul Durrant wrote:
> > > -----Original Message-----
> > > From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
> > > Sent: 28 November 2013 15:51
> > > To: Paul Durrant
> > > Cc: xen-devel@xxxxxxxxxxxxx; Wei Liu; Ian Campbell; David Vrabel
> > > Subject: Re: [PATCH net-next] xen-netback: improve guest-receive-side flow
> > > control
> > >
> > > On Thu, Nov 28, 2013 at 01:11:12PM +0000, Paul Durrant wrote:
> > > > The flow control code relies on a double pass of tke skb, firstly to count
> > > > the number of ring slots needed to supply sufficient grant references, and
> > > > another to actually consume those references and build the grant copy
> > > > operations. It transpires that, in some scenarios, the initial count and the
> > > > number of references consumed differs and, after this happens a number
> > > of
> > > > times, flow control is completely broken.
Can you describe it in more details? What kind of scenarios does this 
issue happen? I assume current xenvif_count_skb_slots() returns accurate 
value now.
> > > The removal of xenvif_count_skb_slots() is OK because you're now
> > > estimating minimum slots, but the actual number of slots consumed needs
> > > to be acurate. Any reason you don't want to use xenvif_gop_skb()?
> > I can't. xenvif_gop_skb() pulls requests off the ring and if there aren't 
> > enough there it will crash'n'burn as far as I can tell. So, I need an upper 
> > bound on the number of slots before I call it.
> I get what you mean. We can at least use some of the snippet in it
> without pulling requests. See above.
I prefer accurate value since it would help to save more slots. But I am 
not sure what the flow control issue is, does using similar mechanism of 
xenvif_count_skb_slots here would cause this issue too?
Thanks
Annie

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