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Date: Wed, 04 Jun 2014 11:09:15 -0400
From: annie li <annie.li@...cle.com>
To: Zoltan Kiss <zoltan.kiss@...rix.com>
CC: xen-devel@...ts.xenproject.org, ian.campbell@...rix.com,
wei.liu2@...rix.com, paul.durrant@...rix.com, linux@...elenboom.it,
netdev@...r.kernel.org, david.vrabel@...rix.com,
davem@...emloft.net
Subject: Re: [Xen-devel] [PATCH net] xen-netback: Fix handling of skbs requiring
too many slots
On 2014/6/3 16:30, Zoltan Kiss wrote:
> 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 / PAGE_SIZE data slots,
(XEN_NETIF_MAX_TX_SIZE+1) / PAGE_SIZE here?
Thanks
Annie
> as the provided buffers are fully
> utilized.
>
> Signed-off-by: Zoltan Kiss <zoltan.kiss@...rix.com>
> Cc: Paul Durrant <paul.durrant@...rix.com>
> Cc: Wei Liu <wei.liu2@...rix.com>
> Cc: Ian Campbell <ian.campbell@...rix.com>
> Cc: David Vrabel <david.vrabel@...rix.com>
> ---
> diff --git a/drivers/net/xen-netback/netback.c b/drivers/net/xen-netback/netback.c
> index 64ab1d1..65fbcd1 100644
> --- a/drivers/net/xen-netback/netback.c
> +++ b/drivers/net/xen-netback/netback.c
> @@ -163,7 +163,10 @@ 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 +178,7 @@ 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 +189,8 @@ 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 +198,10 @@ 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 +236,13 @@ 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 +277,10 @@ 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 +560,6 @@ 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 +615,14 @@ 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;
I am not so clear hear, when max_slots_needed > MAX_SKB_FRAGS, then
full_coalesce is always true.
>
> /* We may need one more slot for GSO metadata */
> if (skb_is_gso(skb) &&
>
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@...ts.xen.org
> http://lists.xen.org/xen-devel
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