[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

[Xen-devel] [PATCH v3 net 2/3] xen-netback: worse-case estimate in xenvif_rx_action is underestimating



The worse-case estimate for skb ring slot usage in xenvif_rx_action()
fails to take fragment page_offset into account. The page_offset does,
however, affect the number of times the fragmentation code calls
start_new_rx_buffer() (i.e. consume another slot) and the worse-case
should assume that will always return true. This patch adds the page_offset
into the DIV_ROUND_UP for each frag.

Unfortunately some frontends aggressively limit the number of requests
they post into the shared ring so to avoid an estimate that is 'too'
pessimal it is capped at MAX_SKB_FRAGS.

Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
Cc: Ian Campbell <ian.campbell@xxxxxxxxxx>
Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
Cc: Sander Eikelenboom <linux@xxxxxxxxxxxxxx>
---
 drivers/net/xen-netback/netback.c |   21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/drivers/net/xen-netback/netback.c 
b/drivers/net/xen-netback/netback.c
index 72314c7..573f3e8 100644
--- a/drivers/net/xen-netback/netback.c
+++ b/drivers/net/xen-netback/netback.c
@@ -493,9 +493,28 @@ static void xenvif_rx_action(struct xenvif *vif)
                                                PAGE_SIZE);
                for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
                        unsigned int size;
+                       unsigned int offset;
+
                        size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
-                       max_slots_needed += DIV_ROUND_UP(size, PAGE_SIZE);
+                       offset = skb_shinfo(skb)->frags[i].page_offset;
+
+                       /* For a worse-case estimate we need to factor in
+                        * the fragment page offset as this will affect the
+                        * number of times xenvif_gop_frag_copy() will
+                        * call start_new_rx_buffer().
+                        */
+                       max_slots_needed += DIV_ROUND_UP(offset + size,
+                                                        PAGE_SIZE);
                }
+
+               /* To avoid the estimate becoming too pessimal for some
+                * frontends that limit posted rx requests, cap the estimate
+                * at MAX_SKB_FRAGS.
+                */
+               if (max_slots_needed > MAX_SKB_FRAGS)
+                       max_slots_needed = MAX_SKB_FRAGS;
+
+               /* We may need one more slot for GSO metadata */
                if (skb_is_gso(skb) &&
                   (skb_shinfo(skb)->gso_type & SKB_GSO_TCPV4 ||
                    skb_shinfo(skb)->gso_type & SKB_GSO_TCPV6))
-- 
1.7.10.4


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel


 


Rackspace

Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.