netdev - [PATCH 05/10] net: move destructor_arg to the front of sk

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Message-ID: <1334067984-7706-5-git-send-email-ian.campbell@citrix.com>
Date:	Tue, 10 Apr 2012 15:26:19 +0100
From:	Ian Campbell <ian.campbell@...rix.com>
To:	netdev@...r.kernel.org
CC:	David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Wei Liu <wei.liu2@...rix.com>, xen-devel@...ts.xen.org,
	Ian Campbell <ian.campbell@...rix.com>
Subject: [PATCH 05/10] net: move destructor_arg to the front of sk_buff.

As of the previous patch we align the end (rather than the start) of the struct
to a cache line and so, with 32 and 64 byte cache lines and the shinfo size
increase from the next patch, the first 8 bytes of the struct end up on a
different cache line to the rest of it so make sure it is something relatively
unimportant to avoid hitting an extra cache line on hot operations such as
kfree_skb.

Signed-off-by: Ian Campbell <ian.campbell@...rix.com>
Cc: "David S. Miller" <davem@...emloft.net>
Cc: Eric Dumazet <eric.dumazet@...il.com>
---
 include/linux/skbuff.h |   15 ++++++++++-----
 net/core/skbuff.c      |    5 ++++-
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 0ad6a46..f0ae39c 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -265,6 +265,15 @@ struct ubuf_info {
  * the end of the header data, ie. at skb->end.
  */
 struct skb_shared_info {
+	/* Intermediate layers must ensure that destructor_arg
+	 * remains valid until skb destructor */
+	void		*destructor_arg;
+
+	/*
+	 * Warning: all fields from here until dataref are cleared in
+	 * __alloc_skb()
+	 *
+	 */
 	unsigned char	nr_frags;
 	__u8		tx_flags;
 	unsigned short	gso_size;
@@ -276,14 +285,10 @@ struct skb_shared_info {
 	__be32          ip6_frag_id;
 
 	/*
-	 * Warning : all fields before dataref are cleared in __alloc_skb()
+	 * Warning: all fields before dataref are cleared in __alloc_skb()
 	 */
 	atomic_t	dataref;
 
-	/* Intermediate layers must ensure that destructor_arg
-	 * remains valid until skb destructor */
-	void *		destructor_arg;
-
 	/* must be last field, see pskb_expand_head() */
 	skb_frag_t	frags[MAX_SKB_FRAGS];
 };
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index d4e139e..b8a41d6 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -214,7 +214,10 @@ struct sk_buff *__alloc_skb(unsigned int size, gfp_t gfp_mask,
 
 	/* make sure we initialize shinfo sequentially */
 	shinfo = skb_shinfo(skb);
-	memset(shinfo, 0, offsetof(struct skb_shared_info, dataref));
+
+	memset(&shinfo->nr_frags, 0,
+	       offsetof(struct skb_shared_info, dataref)
+	       - offsetof(struct skb_shared_info, nr_frags));
 	atomic_set(&shinfo->dataref, 1);
 	kmemcheck_annotate_variable(shinfo->destructor_arg);
 
-- 
1.7.2.5

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