Re: [Xen-devel] [PATCH RFC] xen-netback: calculate the number of slots required for large MTU vifs

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Thu, Jul 11, 2013 at 09:52:52PM +0800, annie li wrote:
> 
> On 2013-7-11 14:01, annie li wrote:
> >
> >On 2013-7-11 13:14, Matt Wilson wrote:
> >>On Wed, Jul 10, 2013 at 08:37:03PM +0100, Wei Liu wrote:
> >>>On Wed, Jul 10, 2013 at 09:13:33AM +0100, Wei Liu wrote:
> >>>>On Tue, Jul 09, 2013 at 10:40:59PM +0000, Matt Wilson wrote:
> >>>>>From: Xi Xiong <xixiong@xxxxxxxxxx>
> >>>>>
> >>>>>[ note: I've just cherry picked this onto net-next, and only compile
> >>>>>   tested. This a RFC only. -msw ]
> >>>>>
> >>>>Should probably rebase it on net.git because it is a bug fix. Let's
> >>>>worry about that later...
> >>*nod*
> >>
> >>>>>Currently the number of RX slots required to transmit a SKB to
> >>>>>xen-netfront can be miscalculated when an interface uses a MTU larger
> >>>>>than PAGE_SIZE. If the slot calculation is wrong, xen-netback can
> >>>>>pause the queue indefinitely or reuse slots. The former
> >>>>>manifests as a
> >>>>>loss of connectivity to the guest (which can be restored by lowering
> >>>>>the MTU set on the interface). The latter manifests with "Bad grant
> >>>>>reference" messages from Xen such as:
> >>>>>
> >>>>>(XEN) grant_table.c:1797:d0 Bad grant reference 264241157
> >>>>>
> >>>>>and kernel messages within the guest such as:
> >>>>>
> >>>>>[  180.419567] net eth0: Invalid extra type: 112
> >>>>>[  180.868620] net eth0: rx->offset: 0, size: 4294967295
> >>>>>[  180.868629] net eth0: rx->offset: 0, size: 4294967295
> >>>>>
> >>>>>BUG_ON() assertions can also be hit if RX slots are exhausted while
> >>>>>handling a SKB.
> >>>>>
> >>>>>This patch changes xen_netbk_rx_action() to count the number of RX
> >>>>>slots actually consumed by netbk_gop_skb() instead of
> >>>>>using nr_frags + 1.
> >>>>>This prevents under-counting the number of RX slots consumed when a
> >>>>>SKB has a large linear buffer.
> >>>>>
> >>>>>Additionally, we now store the estimated number of RX slots required
> >>>>>to handle a SKB in the cb overlay. This value is used to determine if
> >>>>>the next SKB in the queue can be processed.
> >>>>>
> >>>>>Finally, the logic in start_new_rx_buffer() can cause RX slots to be
> >>>>>wasted when setting up copy grant table operations for
> >>>>>SKBs with large
> >>>>>linear buffers. For example, a SKB with skb_headlen() equal to 8157
> >>>>>bytes that starts 64 bytes 64 bytes from the start of the page will
> >>>>Duplicated "64 bytes". And this change looks like an improvement not a
> >>>>bug fix. Probably submit a separate patch for this?
> >>Argh, I knew it was in there somewhere (since you pointed it out in
> >>Dubin :-).
> >>
> >>Maybe it could be a separate patch. I think the description is also a
> >>bit confusing. I'll work on rewording it.
> >>
> >>>>>consume three RX slots instead of two. This patch changes the "head"
> >>>>>parameter to netbk_gop_frag_copy() to act as a flag. When set,
> >>>>>start_new_rx_buffer() will always place as much data as possible into
> >>>>>each RX slot.
> >>>>>
> >>>>>Signed-off-by: Xi Xiong <xixiong@xxxxxxxxxx>
> >>>>>Reviewed-by: Matt Wilson <msw@xxxxxxxxxx>
> >>>>>[ msw: minor code cleanups, rewrote commit message, adjusted code
> >>>>>   to count RX slots instead of meta structures ]
> >>>>>Signed-off-by: Matt Wilson <msw@xxxxxxxxxx>
> >>>>>Cc: Annie Li <annie.li@xxxxxxxxxx>
> >>>>>Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> >>>>>Cc: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
> >>>>>Cc: netdev@xxxxxxxxxxxxxxx
> >>>>>Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
> >>>>>---
> >>>>>  drivers/net/xen-netback/netback.c |   51
> >>>>>++++++++++++++++++++++--------------
> >>>>>  1 files changed, 31 insertions(+), 20 deletions(-)
> >>>>>
> >>>>>diff --git a/drivers/net/xen-netback/netback.c
> >>>>>b/drivers/net/xen-netback/netback.c
> >>>>>index 64828de..82dd207 100644
> >>>>>--- a/drivers/net/xen-netback/netback.c
> >>>>>+++ b/drivers/net/xen-netback/netback.c
> >>>>>@@ -110,6 +110,11 @@ union page_ext {
> >>>>>      void *mapping;
> >>>>>  };
> >>>>>  +struct skb_cb_overlay {
> >>>>>+    int meta_slots_used;
> >>>>>+    int peek_slots_count;
> >>>>>+};
> >>>>>+
> >>>>>  struct xen_netbk {
> >>>>>      wait_queue_head_t wq;
> >>>>>      struct task_struct *task;
> >>>>>@@ -370,6 +375,7 @@ unsigned int
> >>>>>xen_netbk_count_skb_slots(struct xenvif *vif, struct
> >>>>>sk_buff *skb)
> >>>>>  {
> >>>>>      unsigned int count;
> >>>>>      int i, copy_off;
> >>>>>+    struct skb_cb_overlay *sco;
> >>>>>        count = DIV_ROUND_UP(skb_headlen(skb), PAGE_SIZE);
> >>>>>  @@ -411,6 +417,9 @@ unsigned int
> >>>>>xen_netbk_count_skb_slots(struct xenvif *vif, struct
> >>>>>sk_buff *skb)
> >>>>>                  offset = 0;
> >>>>>          }
> >>>>>      }
> >>>>>+
> >>>>>+    sco = (struct skb_cb_overlay *) skb->cb;
> >>>>>+    sco->peek_slots_count = count;
> >>>>>      return count;
> >>>>>  }
> >>>>>  @@ -443,13 +452,12 @@ static struct netbk_rx_meta
> >>>>>*get_next_rx_buffer(struct xenvif *vif,
> >>>>>  }
> >>>>>    /*
> >>>>>- * Set up the grant operations for this fragment. If it's a flipping
> >>>>>- * interface, we also set up the unmap request from here.
> >>>>>+ * Set up the grant operations for this fragment.
> >>>>>   */
> >>>>>  static void netbk_gop_frag_copy(struct xenvif *vif,
> >>>>>struct sk_buff *skb,
> >>>>>                  struct netrx_pending_operations *npo,
> >>>>>                  struct page *page, unsigned long size,
> >>>>>-                unsigned long offset, int *head)
> >>>>>+                unsigned long offset, int head, int *first)
> >>>>>  {
> >>>>>      struct gnttab_copy *copy_gop;
> >>>>>      struct netbk_rx_meta *meta;
> >>>>>@@ -479,12 +487,12 @@ static void
> >>>>>netbk_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)) {
> >
> >head is always 1 here when processing the header data, so it is
> >impossible to start a new buffer for header data with larger
> >header size, and get_next_rx_buffer is never called. I assume this
> >would not work well for skb with larger header data size. Have you
> >run network test with this patch?
> 
> Sorry, I forgot the offset == MAX_BUFFER_OFFSET case and
> misunderstand your patch, please ignore my last comments. Your patch
> keeps the original DIV_ROUND_UP and changes the mechanism in
> netbk_gop_frag_copy to make slots same with
> xen_netbk_count_skb_slots. All Roads Lead to Rome!:-)
> 

Actually I'm now in favor of Matt's approach as it a) fix the bug, b)
make efficient use of the ring.

Annie, Ian, what's your opinion?


Wei.

> Thanks
> Annie

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