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

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Message-ID: <52D6A0B9.2030204@citrix.com>
Date:	Wed, 15 Jan 2014 14:52:41 +0000
From:	Zoltan Kiss <zoltan.kiss@...rix.com>
To:	Wei Liu <wei.liu2@...rix.com>
CC:	<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 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.

Zoli
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