linux-kernel - Re: [PATCH net-next] xen-netback: Rework rx_work

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Message-ID: <20140115145951.GP5698@zion.uk.xensource.com>
Date:	Wed, 15 Jan 2014 14:59:51 +0000
From:	Wei Liu <wei.liu2@...rix.com>
To:	Zoltan Kiss <zoltan.kiss@...rix.com>
CC:	Wei Liu <wei.liu2@...rix.com>, <ian.campbell@...rix.com>,
	<xen-devel@...ts.xenproject.org>, <netdev@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <jonathan.davies@...rix.com>
Subject: Re: [PATCH net-next] xen-netback: Rework rx_work_todo

On Wed, Jan 15, 2014 at 02:52:41PM +0000, Zoltan Kiss wrote:
> On 15/01/14 14:45, Wei Liu wrote:
> >>>>The recent patch to fix receive side flow control (11b57f) solved the spinning
> >>>>> >>thread problem, however caused an another one. The receive side can stall, if:
> >>>>> >>- xenvif_rx_action sets rx_queue_stopped to false
> >>>>> >>- interrupt happens, and sets rx_event to true
> >>>>> >>- then xenvif_kthread sets rx_event to false
> >>>>> >>
> >>>> >
> >>>> >If you mean "rx_work_todo" returns false.
> >>>> >
> >>>> >In this case
> >>>> >
> >>>> >(!skb_queue_empty(&vif->rx_queue) && !vif->rx_queue_stopped) || vif->rx_event;
> >>>> >
> >>>> >can still be true, can't it?
> >>>Sorry, I should wrote rx_queue_stopped to true
> >>>
> >In this case, if rx_queue_stopped is true, then we're expecting frontend
> >to notify us, right?
> >
> >rx_queue_stopped is set to true if we cannot make any progress to queue
> >packet into the ring. In that situation we can expect frontend will send
> >notification to backend after it goes through the backlog in the ring.
> >That means rx_event is set to true, and rx_work_todo is true again. So
> >the ring is actually not stalled in this case as well. Did I miss
> >something?
> >
> 
> Yes, we expect the guest to notify us, and it does, and we set
> rx_event to true (see second point), but then the thread set it to
> false (see third point). Talking with Paul, another solution could
> be to set rx_event false before calling xenvif_rx_action. But using
> rx_last_skb_slots makes it quicker for the thread to see if it
> doesn't have to do anything.
> 

OK, this is a better explaination. So actually there's no bug in the
original implementation and your patch is sort of an improvement.

Could you send a new version of this patch with relevant information in
commit message? Talking to people offline is faster, but I would like to
have public discussion and relevant information archived in a searchable
form. Thanks.

Wei.

> Zoli
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