Subject: Re: [Xen-devel] [PATCHv1 net] xen-netback: require fewer guest Rx slots when not using GSO

From: Konrad Rzeszutek Wilk <[email-masked]>
Date: Wed, 9 Sep 2015 15:34:16 -0400

On Tue, Sep 08, 2015 at 02:25:14PM +0100, David Vrabel wrote:
> Commit f48da8b14d04ca87ffcffe68829afd45f926ec6a (xen-netback: fix
> unlimited guest Rx internal queue and carrier flapping) introduced a
> regression.
> 
> The PV frontend in IPXE only places 4 requests on the guest Rx ring.

in IPXE ? What is that?

Is that MiniOS with PXE built in?

Does it always place 4 requests? For every operation?

> Since netback required at least (MAX_SKB_FRAGS + 1) slots, IPXE could
> not receive any packets.
> 
> a) If GSO is not enabled on the VIF, fewer guest Rx slots are required
>    for the largest possible packet.  Calculate the required slots

s/slots/slot number/
>    based on the maximum GSO size or the MTU.
> 
>    This calculation of the number of required slots relies on
>    1650d5455bd2 (xen-netback: always fully coalesce guest Rx packets)
>    which present in 4.0-rc1 and later.
> 
> b) Reduce the Rx stall detection to checking for at least one

s/to/by/
>    available Rx request.  This is fine since we're predominately
>    concerned with detecting interfaces which are down and thus have
>    zero available Rx requests.

s/have zero available Rx requests/no available Rx requests/?

> 
> Signed-off-by: David Vrabel <[email-masked]>
> ---
>  drivers/net/xen-netback/common.h  | 10 ----------
>  drivers/net/xen-netback/netback.c | 23 ++++++++++++++++-------
>  2 files changed, 16 insertions(+), 17 deletions(-)
> 
> Note that this can only be backported as-is on top of
> 1650d5455bd2dc6b5ee134bd6fc1a3236c266b5b (xen-netback: always fully
> coalesce guest Rx packets) which is in 4.0 and later.
> 
> diff --git a/drivers/net/xen-netback/common.h b/drivers/net/xen-netback/common.h
> index 8a495b3..25990b1 100644
> --- a/drivers/net/xen-netback/common.h
> +++ b/drivers/net/xen-netback/common.h
> @@ -200,11 +200,6 @@ struct xenvif_queue { /* Per-queue data for xenvif */
>  	struct xenvif_stats stats;
>  };
>  
> -/* Maximum number of Rx slots a to-guest packet may use, including the
> - * slot needed for GSO meta-data.
> - */
> -#define XEN_NETBK_RX_SLOTS_MAX (MAX_SKB_FRAGS + 1)
> -
>  enum state_bit_shift {
>  	/* This bit marks that the vif is connected */
>  	VIF_STATUS_CONNECTED,
> @@ -306,11 +301,6 @@ int xenvif_dealloc_kthread(void *data);
>  
>  void xenvif_rx_queue_tail(struct xenvif_queue *queue, struct sk_buff *skb);
>  
> -/* Determine whether the needed number of slots (req) are available,
> - * and set req_event if not.
> - */
> -bool xenvif_rx_ring_slots_available(struct xenvif_queue *queue, int needed);
> -
>  void xenvif_carrier_on(struct xenvif *vif);
>  
>  /* Callback from stack when TX packet can be released */
> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
> index 3f44b52..e791930 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -149,9 +149,20 @@ static inline pending_ring_idx_t pending_index(unsigned i)
>  	return i & (MAX_PENDING_REQS-1);
>  }
>  
> -bool xenvif_rx_ring_slots_available(struct xenvif_queue *queue, int needed)
> +static int xenvif_rx_ring_slots_needed(struct xenvif *vif)
> +{
> +	if (vif->gso_mask)
> +		return DIV_ROUND_UP(vif->dev->gso_max_size, PAGE_SIZE) + 1;
> +	else
> +		return DIV_ROUND_UP(vif->dev->mtu, PAGE_SIZE);
> +}
> +
> +static bool xenvif_rx_ring_slots_available(struct xenvif_queue *queue)
>  {
>  	RING_IDX prod, cons;
> +	int needed;
> +
> +	needed = xenvif_rx_ring_slots_needed(queue->vif);
>  
>  	do {
>  		prod = queue->rx.sring->req_prod;
> @@ -513,7 +524,7 @@ static void xenvif_rx_action(struct xenvif_queue *queue)
>  
>  	skb_queue_head_init(&rxq);
>  
> -	while (xenvif_rx_ring_slots_available(queue, XEN_NETBK_RX_SLOTS_MAX)
> +	while (xenvif_rx_ring_slots_available(queue)
>  	       && (skb = xenvif_rx_dequeue(queue)) != NULL) {
>  		queue->last_rx_time = jiffies;
>  
> @@ -1839,8 +1850,7 @@ static bool xenvif_rx_queue_stalled(struct xenvif_queue *queue)
>  	prod = queue->rx.sring->req_prod;
>  	cons = queue->rx.req_cons;
>  
> -	return !queue->stalled
> -		&& prod - cons < XEN_NETBK_RX_SLOTS_MAX
> +	return !queue->stalled && prod - cons < 1
>  		&& time_after(jiffies,
>  			      queue->last_rx_time + queue->vif->stall_timeout);
>  }
> @@ -1852,14 +1862,13 @@ static bool xenvif_rx_queue_ready(struct xenvif_queue *queue)
>  	prod = queue->rx.sring->req_prod;
>  	cons = queue->rx.req_cons;
>  
> -	return queue->stalled
> -		&& prod - cons >= XEN_NETBK_RX_SLOTS_MAX;
> +	return queue->stalled && prod - cons >= 1;
>  }
>  
>  static bool xenvif_have_rx_work(struct xenvif_queue *queue)
>  {
>  	return (!skb_queue_empty(&queue->rx_queue)
> -		&& xenvif_rx_ring_slots_available(queue, XEN_NETBK_RX_SLOTS_MAX))
> +		&& xenvif_rx_ring_slots_available(queue))
>  		|| (queue->vif->stall_timeout &&
>  		    (xenvif_rx_queue_stalled(queue)
>  		     || xenvif_rx_queue_ready(queue)))
> -- 
> 2.1.4
> 
> 
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