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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@cn.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@cn.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@cn.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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