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Date: Thu, 5 Jun 2014 08:12:33 +0000
From: Paul Durrant <Paul.Durrant@...rix.com>
To: Zoltan Kiss <zoltan.kiss@...rix.com>,
"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
Ian Campbell <Ian.Campbell@...rix.com>,
"Wei Liu" <wei.liu2@...rix.com>,
"linux@...elenboom.it" <linux@...elenboom.it>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
David Vrabel <david.vrabel@...rix.com>,
"davem@...emloft.net" <davem@...emloft.net>
Subject: RE: [PATCH net v2] xen-netback: Fix handling of skbs requiring too
many slots
> -----Original Message-----
> From: Zoltan Kiss
> Sent: 04 June 2014 19:59
> To: xen-devel@...ts.xenproject.org; Ian Campbell; Wei Liu; Paul Durrant;
> linux@...elenboom.it
> Cc: netdev@...r.kernel.org; David Vrabel; davem@...emloft.net; Zoltan
> Kiss
> Subject: [PATCH net v2] xen-netback: Fix handling of skbs requiring too many
> slots
>
> A recent commit (a02eb4 "xen-netback: worse-case estimate in
> xenvif_rx_action is
> underestimating") capped the slot estimation to MAX_SKB_FRAGS, but that
> triggers
> the next BUG_ON a few lines down, as the packet consumes more slots than
> estimated.
> This patch introduces full_coalesce on the skb callback buffer, which is used
> in
> start_new_rx_buffer() to decide whether netback needs coalescing more
> aggresively. By doing that, no packet should need more than
> (XEN_NETIF_MAX_TX_SIZE + 1) / PAGE_SIZE data slots (excluding the
> optional GSO
> slot, it doesn't carry data, therefore irrelevant in this case), as the provided
> buffers are fully utilized.
>
> Signed-off-by: Zoltan Kiss <zoltan.kiss@...rix.com>
> Cc: Paul Durrant <paul.durrant@...rix.com>
Reviewed-by: Paul Durrant <paul.durrant@...il.com>
> Cc: Wei Liu <wei.liu2@...rix.com>
> Cc: Ian Campbell <ian.campbell@...rix.com>
> Cc: David Vrabel <david.vrabel@...rix.com>
> ---
> v2:
> - clarify that GSO slot doesn't count here
> - fix style
>
> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-
> netback/netback.c
> index 64ab1d1..6a21588 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -163,7 +163,8 @@ bool xenvif_rx_ring_slots_available(struct xenvif *vif,
> int needed)
> * adding 'size' bytes to a buffer which currently contains 'offset'
> * bytes.
> */
> -static bool start_new_rx_buffer(int offset, unsigned long size, int head)
> +static bool start_new_rx_buffer(int offset, unsigned long size, int head,
> + bool full_coalesce)
> {
> /* simple case: we have completely filled the current buffer. */
> if (offset == MAX_BUFFER_OFFSET)
> @@ -175,6 +176,7 @@ static bool start_new_rx_buffer(int offset, unsigned
> long size, int head)
> * (i) this frag would fit completely in the next buffer
> * and (ii) there is already some data in the current buffer
> * and (iii) this is not the head buffer.
> + * and (iv) there is no need to fully utilize the buffers
> *
> * Where:
> * - (i) stops us splitting a frag into two copies
> @@ -185,6 +187,8 @@ static bool start_new_rx_buffer(int offset, unsigned
> long size, int head)
> * by (ii) but is explicitly checked because
> * netfront relies on the first buffer being
> * non-empty and can crash otherwise.
> + * - (iv) is needed for skbs which can use up more than
> MAX_SKB_FRAGS
> + * slot
> *
> * This means we will effectively linearise small
> * frags but do not needlessly split large buffers
> @@ -192,7 +196,8 @@ static bool start_new_rx_buffer(int offset, unsigned
> long size, int head)
> * own buffers as before.
> */
> BUG_ON(size > MAX_BUFFER_OFFSET);
> - if ((offset + size > MAX_BUFFER_OFFSET) && offset && !head)
> + if ((offset + size > MAX_BUFFER_OFFSET) && offset && !head &&
> + !full_coalesce)
> return true;
>
> return false;
> @@ -227,6 +232,13 @@ static struct xenvif_rx_meta
> *get_next_rx_buffer(struct xenvif *vif,
> return meta;
> }
>
> +struct xenvif_rx_cb {
> + int meta_slots_used;
> + bool full_coalesce;
> +};
> +
> +#define XENVIF_RX_CB(skb) ((struct xenvif_rx_cb *)(skb)->cb)
> +
> /*
> * Set up the grant operations for this fragment. If it's a flipping
> * interface, we also set up the unmap request from here.
> @@ -261,7 +273,10 @@ static void xenvif_gop_frag_copy(struct xenvif *vif,
> struct sk_buff *skb,
> if (bytes > size)
> bytes = size;
>
> - if (start_new_rx_buffer(npo->copy_off, bytes, *head)) {
> + if (start_new_rx_buffer(npo->copy_off,
> + bytes,
> + *head,
> + XENVIF_RX_CB(skb)->full_coalesce))
> {
> /*
> * Netfront requires there to be some data in the
> head
> * buffer.
> @@ -541,12 +556,6 @@ static void xenvif_add_frag_responses(struct xenvif
> *vif, int status,
> }
> }
>
> -struct xenvif_rx_cb {
> - int meta_slots_used;
> -};
> -
> -#define XENVIF_RX_CB(skb) ((struct xenvif_rx_cb *)(skb)->cb)
> -
> void xenvif_kick_thread(struct xenvif *vif)
> {
> wake_up(&vif->wq);
> @@ -602,10 +611,14 @@ static void xenvif_rx_action(struct xenvif *vif)
>
> /* To avoid the estimate becoming too pessimal for some
> * frontends that limit posted rx requests, cap the estimate
> - * at MAX_SKB_FRAGS.
> + * at MAX_SKB_FRAGS. In this case netback will fully coalesce
> + * the skb into the provided slots.
> */
> - if (max_slots_needed > MAX_SKB_FRAGS)
> + if (max_slots_needed > MAX_SKB_FRAGS) {
> max_slots_needed = MAX_SKB_FRAGS;
> + XENVIF_RX_CB(skb)->full_coalesce = true;
> + } else
> + XENVIF_RX_CB(skb)->full_coalesce = false;
>
> /* We may need one more slot for GSO metadata */
> if (skb_is_gso(skb) &&
--
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