RE: [PATCH net-next v2] xen-netback: Rework rx_work_todo

[Paul Durrant]

> -----Original Message-----
> From: Zoltan Kiss
> Sent: 20 January 2014 12:23
> To: Ian Campbell; Wei Liu; xen-devel@xxxxxxxxxxxxxxxxxxxx;
> netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Jonathan Davies
> Cc: Paul Durrant
> Subject: Re: [PATCH net-next v2] xen-netback: Rework rx_work_todo
> 
> Any reviews on this one? It fixes an important lockup situation, so
> either this or some other fix should go in soon.
> 
> On 15/01/14 17:11, Zoltan Kiss 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:
> > - [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 doesn't return true anymore
> >
> > Also, if interrupt sent but there is still no room in the ring, it take quite a
> > long time until xenvif_rx_action realize it. This patch ditch that two variable,
> > and rework rx_work_todo. If the thread finds it can't fit more skb's into the
> > ring, it saves the last slot estimation into rx_last_skb_slots, otherwise it's
> > kept as 0. Then rx_work_todo will check if:
> > - there is something to send to the ring (like before)
> > - there is space for the topmost packet in the queue
> >
> > I think that's more natural and optimal thing to test than two bool which are
> > set somewhere else.
> >
> > Signed-off-by: Zoltan Kiss <zoltan.kiss@xxxxxxxxxx>
> > ---
> >   drivers/net/xen-netback/common.h    |    6 +-----
> >   drivers/net/xen-netback/interface.c |    1 -
> >   drivers/net/xen-netback/netback.c   |   16 ++++++----------
> >   3 files changed, 7 insertions(+), 16 deletions(-)
> >
> > diff --git a/drivers/net/xen-netback/common.h b/drivers/net/xen-
> netback/common.h
> > index 4c76bcb..ae413a2 100644
> > --- a/drivers/net/xen-netback/common.h
> > +++ b/drivers/net/xen-netback/common.h
> > @@ -143,11 +143,7 @@ struct xenvif {
> >   	char rx_irq_name[IFNAMSIZ+4]; /* DEVNAME-rx */
> >   	struct xen_netif_rx_back_ring rx;
> >   	struct sk_buff_head rx_queue;
> > -	bool rx_queue_stopped;
> > -	/* Set when the RX interrupt is triggered by the frontend.
> > -	 * The worker thread may need to wake the queue.
> > -	 */
> > -	bool rx_event;
> > +	RING_IDX rx_last_skb_slots;
> >
> >   	/* This array is allocated seperately as it is large */
> >   	struct gnttab_copy *grant_copy_op;
> > diff --git a/drivers/net/xen-netback/interface.c b/drivers/net/xen-
> netback/interface.c
> > index b9de31e..7669d49 100644
> > --- a/drivers/net/xen-netback/interface.c
> > +++ b/drivers/net/xen-netback/interface.c
> > @@ -100,7 +100,6 @@ static irqreturn_t xenvif_rx_interrupt(int irq, void
> *dev_id)
> >   {
> >   	struct xenvif *vif = dev_id;
> >
> > -	vif->rx_event = true;
> >   	xenvif_kick_thread(vif);
> >
> >   	return IRQ_HANDLED;
> > diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-
> netback/netback.c
> > index 2738563..bb241d0 100644
> > --- a/drivers/net/xen-netback/netback.c
> > +++ b/drivers/net/xen-netback/netback.c
> > @@ -477,7 +477,6 @@ static void xenvif_rx_action(struct xenvif *vif)
> >   	unsigned long offset;
> >   	struct skb_cb_overlay *sco;
> >   	bool need_to_notify = false;
> > -	bool ring_full = false;
> >
> >   	struct netrx_pending_operations npo = {
> >   		.copy  = vif->grant_copy_op,
> > @@ -487,7 +486,7 @@ static void xenvif_rx_action(struct xenvif *vif)
> >   	skb_queue_head_init(&rxq);
> >
> >   	while ((skb = skb_dequeue(&vif->rx_queue)) != NULL) {
> > -		int max_slots_needed;
> > +		RING_IDX max_slots_needed;
> >   		int i;
> >
> >   		/* We need a cheap worse case estimate for the number of
> > @@ -510,9 +509,10 @@ static void xenvif_rx_action(struct xenvif *vif)
> >   		if (!xenvif_rx_ring_slots_available(vif, max_slots_needed)) {
> >   			skb_queue_head(&vif->rx_queue, skb);
> >   			need_to_notify = true;
> > -			ring_full = true;
> > +			vif->rx_last_skb_slots = max_slots_needed;
> >   			break;
> > -		}
> > +		} else
> > +			vif->rx_last_skb_slots = 0;
> >
> >   		sco = (struct skb_cb_overlay *)skb->cb;
> >   		sco->meta_slots_used = xenvif_gop_skb(skb, &npo);
> > @@ -523,8 +523,6 @@ static void xenvif_rx_action(struct xenvif *vif)
> >
> >   	BUG_ON(npo.meta_prod > ARRAY_SIZE(vif->meta));
> >
> > -	vif->rx_queue_stopped = !npo.copy_prod && ring_full;
> > -
> >   	if (!npo.copy_prod)
> >   		goto done;
> >
> > @@ -1727,8 +1725,8 @@ static struct xen_netif_rx_response
> *make_rx_response(struct xenvif *vif,
> >
> >   static inline int rx_work_todo(struct xenvif *vif)
> >   {
> > -	return (!skb_queue_empty(&vif->rx_queue) && !vif-
> >rx_queue_stopped) ||
> > -		vif->rx_event;
> > +	return !skb_queue_empty(&vif->rx_queue) &&
> > +	       xenvif_rx_ring_slots_available(vif, vif->rx_last_skb_slots);
> >   }
> >
> >   static inline int tx_work_todo(struct xenvif *vif)
> > @@ -1814,8 +1812,6 @@ int xenvif_kthread(void *data)
> >   		if (!skb_queue_empty(&vif->rx_queue))
> >   			xenvif_rx_action(vif);
> >
> > -		vif->rx_event = false;
> > -

The minimal patch is to simply move this line up above the previous if clause, but I'm happy with your patch as it stands so

Reviewed-by: Paul Durrant <paul.durrant@xxxxxxxxxx>

> >   		if (skb_queue_empty(&vif->rx_queue) &&
> >   		    netif_queue_stopped(vif->dev))
> >   			xenvif_start_queue(vif);
> >

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