netdev - [PATCH 4/4 V2] net,rcu: don't assume the size of struct rcu

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Date:	Tue, 01 Mar 2011 16:53:40 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	David Miller <davem@...emloft.net>
CC:	mingo@...e.hu, paulmck@...ux.vnet.ibm.com, cl@...ux-foundation.org,
	penberg@...nel.org, eric.dumazet@...il.com, mpm@...enic.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org
Subject: [PATCH 4/4 V2] net,rcu: don't assume the size of struct rcu_head

On 03/01/2011 04:16 PM, David Miller wrote:
> From: Lai Jiangshan <laijs@...fujitsu.com>
> Date: Tue, 01 Mar 2011 16:03:44 +0800
> 
>>
>> struct dst_entry assumes the size of struct rcu_head as 2 * sizeof(long)
>> and manually adds pads for aligning for "__refcnt".
>>
>> When the size of struct rcu_head is changed, these manual padding
>> is wrong. Use __attribute__((aligned (64))) instead.
>>
>> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> 
> We don't want to use the align if it's going to waste lots of space.
> 
> Instead we want to rearrange the structure so that the alignment comes
> more cheaply.

Subject: [PATCH 4/4 V2] net,rcu: don't assume the size of struct rcu_head

struct dst_entry assumes the size of struct rcu_head as 2 * sizeof(long)
and manually adds pads for aligning for "__refcnt".

When the size of struct rcu_head is changed, these manual padding
are hardly suit for the changes. So we rearrange the structure,
and move the seldom access rcu_head to the end of the structure.

Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
---

diff --git a/include/net/dst.h b/include/net/dst.h
index 93b0310..d8c5296 100644
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -37,7 +37,6 @@
 struct sk_buff;
 
 struct dst_entry {
-	struct rcu_head		rcu_head;
 	struct dst_entry	*child;
 	struct net_device       *dev;
 	short			error;
@@ -78,6 +77,13 @@ struct dst_entry {
 	__u32			__pad2;
 #endif
 
+	unsigned long		lastuse;
+	union {
+		struct dst_entry	*next;
+		struct rtable __rcu	*rt_next;
+		struct rt6_info		*rt6_next;
+		struct dn_route __rcu	*dn_next;
+	};
 
 	/*
 	 * Align __refcnt to a 64 bytes alignment
@@ -92,13 +98,7 @@ struct dst_entry {
 	 */
 	atomic_t		__refcnt;	/* client references	*/
 	int			__use;
-	unsigned long		lastuse;
-	union {
-		struct dst_entry	*next;
-		struct rtable __rcu	*rt_next;
-		struct rt6_info		*rt6_next;
-		struct dn_route __rcu	*dn_next;
-	};
+	struct rcu_head		rcu_head;
 };
 
 #ifdef __KERNEL__
--
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