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

From: Zoltan Kiss <[email-masked]>
Date: Mon, 20 Jan 2014 12:23:28 +0000

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 <[email-masked]>
> ---
>   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;
> -
>   		if (skb_queue_empty(&vif->rx_queue) &&
>   		    netif_queue_stopped(vif->dev))
>   			xenvif_start_queue(vif);
>