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SubjectRe: [PATCH 4/4 V2] net,rcu: don't assume the size of struct rcu_head
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Le mardi 01 mars 2011 à 16:53 +0800, Lai Jiangshan a écrit :
> 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__

Nope...

"lastuse" and "next" must be in this place, or this introduce false
sharing we wanted to avoid in the past.

I suggest you leave this code as is, we will address the problem when
rcu_head changes (assuming we can test a CONFIG_RCU_HEAD_DEBUG or
something)

First part of "struct dst_entry" is mostly read, while part beginning
after refcnt is often written.



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