From: Anton Blanchard <anton@samba.org>
Date: Sun, 27 Mar 2011 14:57:26 +0000 (+0000)
Subject: net: Always allocate at least 16 skb frags regardless of page size
X-Git-Tag: firefly_0821_release~7613^2~1737^2~7
X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=a715dea3c8e9ef2771c534e05ee1d36f65987e64;p=firefly-linux-kernel-4.4.55.git

net: Always allocate at least 16 skb frags regardless of page size

When analysing performance of the cxgb3 on a ppc64 box I noticed that
we weren't doing much GRO merging. It turns out we are limited by the
number of SKB frags:

#define MAX_SKB_FRAGS (65536/PAGE_SIZE + 2)

With a 4kB page size we have 18 frags, but with a 64kB page size we
only have 3 frags.

I ran a single stream TCP bandwidth test to compare the performance of

Signed-off-by: David S. Miller <davem@davemloft.net>
---

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 24cfa626931e..239083bfea13 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -122,8 +122,14 @@ struct sk_buff_head {
 
 struct sk_buff;
 
-/* To allow 64K frame to be packed as single skb without frag_list */
+/* To allow 64K frame to be packed as single skb without frag_list. Since
+ * GRO uses frags we allocate at least 16 regardless of page size.
+ */
+#if (65536/PAGE_SIZE + 2) < 16
+#define MAX_SKB_FRAGS 16
+#else
 #define MAX_SKB_FRAGS (65536/PAGE_SIZE + 2)
+#endif
 
 typedef struct skb_frag_struct skb_frag_t;