netdev - RE: [PATCH net] xen-netback: fix abuse of napi budget

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Date:	Tue, 10 Dec 2013 11:03:22 +0000
From:	Paul Durrant <Paul.Durrant@...rix.com>
To:	Wei Liu <wei.liu2@...rix.com>
CC:	Wei Liu <wei.liu2@...rix.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Ian Campbell <Ian.Campbell@...rix.com>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH net] xen-netback: fix abuse of napi budget

> -----Original Message-----
> From: Wei Liu [mailto:wei.liu2@...rix.com]
> Sent: 10 December 2013 10:55
> To: Paul Durrant
> Cc: Wei Liu; David Vrabel; Ian Campbell; xen-devel@...ts.xen.org;
> netdev@...r.kernel.org
> Subject: Re: [PATCH net] xen-netback: fix abuse of napi budget
> 
> On Tue, Dec 10, 2013 at 10:48:13AM +0000, Paul Durrant wrote:
> > > -----Original Message-----
> > > From: Wei Liu [mailto:wei.liu2@...rix.com]
> > > Sent: 10 December 2013 10:45
> > > To: David Vrabel
> > > Cc: Ian Campbell; Paul Durrant; xen-devel@...ts.xen.org;
> > > netdev@...r.kernel.org; Wei Liu
> > > Subject: Re: [PATCH net] xen-netback: fix abuse of napi budget
> > >
> > > On Tue, Dec 10, 2013 at 10:37:36AM +0000, Wei Liu wrote:
> > > > On Tue, Dec 10, 2013 at 10:30:13AM +0000, David Vrabel wrote:
> > > > > On 10/12/13 10:25, Ian Campbell wrote:
> > > > > > On Tue, 2013-12-10 at 10:16 +0000, Paul Durrant wrote:
> > > > > >> netback seemed to be somewhat confused about the napi budget
> > > parameter and
> > > > > >> basically ignored it. This patch fixes that, properly limiting the work
> > > done
> > > > > >> in each poll.
> > > > > >
> > > > > > What do you mean "ignored", xenvif_tx_submit seems to be
> tracking
> > > and
> > > > > > testing work_done against the budget.
> > > > >
> > > > > I have seen this warning in net_rx_action() trigger.
> > > > >
> > > > > 	WARN_ON_ONCE(work > weight);
> > > > >
> > > > > Which means netback wasn't limiting the work done.
> > > > >
> > > >
> > > > But in the original code work_done is returned by xenvif_tx_submit
> which
> > > > has guard against that situation, right?
> > > >
> > >
> > > And now I think I spot a bug...
> > >
> > >   work_done = xenvif_tx_submit(vif, nr_gops);
> > >
> > > The second argument should really be "budget". :-(
> > >
> >
> > Yep - that's basically the problem.
> >
> 
> So size-wise the attached patch is smaller. Now the only problem is that
> is it better to move flow control earlier.
> 

Yes, but I think that patch is dangerous as I explained to Ian. If we don't limit early then tx_queue can grow uncontrollably if the frontend continues to throw more data into the ring than we actually ship out on each napi poll. I will re-submit with a more elaborate description.

  Paul

> Wei.
> 
> ---8<---
> From 11db4a9cd7267a621725c48f0e0a99c1d6d31866 Mon Sep 17 00:00:00
> 2001
> From: Wei Liu <wei.liu2@...rix.com>
> Date: Tue, 10 Dec 2013 10:49:59 +0000
> Subject: [PATCH] xen-netback: correct typo nr_gops -> budget
> 
> work_done should be limited by budget not nr_gops. Otherwise we trigger
> "WARN_ON_ONCE(work > weight)" in net/dev/core:net_rx_action.
> 
> Signed-off-by: Wei Liu <wei.liu2@...rix.com>
> ---
>  drivers/net/xen-netback/netback.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-
> netback/netback.c
> index acf1392..b11f65d 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -1707,7 +1707,7 @@ int xenvif_tx_action(struct xenvif *vif, int budget)
> 
>  	gnttab_batch_copy(vif->tx_copy_ops, nr_gops);
> 
> -	work_done = xenvif_tx_submit(vif, nr_gops);
> +	work_done = xenvif_tx_submit(vif, budget);
> 
>  	return work_done;
>  }
> --
> 1.7.10.4

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