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

[Eric Dumazet <eric.dumazet@gmail.com>]

Le mardi 01 mars 2011 A  16:53 +0800, Lai Jiangshan a A(C)crit :
> 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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