Re: [Xen-devel] [PATCH net-next] xen-netback: Rework rx_work_todo

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, Jan 15, 2014 at 03:08:12PM +0000, Zoltan Kiss wrote:
> On 15/01/14 14:59, Wei Liu wrote:
> >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.
> 
> No, there is a bug in the original implementation:
> - [THREAD] xenvif_rx_action sets rx_queue_stopped to true
> - [INTERRUPT] interrupt happens, and sets rx_event to true
> - [THREAD] then xenvif_kthread sets rx_event to false
> - [THREAD] rx_work_todo never returns true anymore
> 

I see what you mean. The interrupt is "lost", that's why it's stalled.

> I will update the explanation and send in the patch again.
> 

Thanks.

Wei.

> Zoli

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