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SubjectRe: [PATCH 4/4] net,rcu: don't assume the size of struct rcu_head
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Le mardi 01 mars 2011 à 16:03 +0800, Lai Jiangshan a écrit :
> 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>
> ---

> diff --git a/include/net/dst.h b/include/net/dst.h
> index 93b0310..4ef6c4a 100644
> --- a/include/net/dst.h
> +++ b/include/net/dst.h
> @@ -62,8 +62,6 @@ struct dst_entry {
> struct hh_cache *hh;
> #ifdef CONFIG_XFRM
> struct xfrm_state *xfrm;
> -#else
> - void *__pad1;
> #endif
> int (*input)(struct sk_buff*);
> int (*output)(struct sk_buff*);
> @@ -74,23 +72,18 @@ struct dst_entry {
>
> #ifdef CONFIG_NET_CLS_ROUTE
> __u32 tclassid;
> -#else
> - __u32 __pad2;
> #endif
>
>
> /*
> * Align __refcnt to a 64 bytes alignment
> * (L1_CACHE_SIZE would be too much)
> - */
> -#ifdef CONFIG_64BIT
> - long __pad_to_align_refcnt[1];
> -#endif
> - /*
> + *
> * __refcnt wants to be on a different cache line from
> * input/output/ops or performance tanks badly
> */
> - atomic_t __refcnt; /* client references */
> + atomic_t __refcnt /* client references */
> + __attribute__((aligned (64)));
> int __use;
> unsigned long lastuse;
> union {

If struct rcu_head is bigger, this is for debugging purposes, so we dont
care about performance, and can avoid wasting ~64 bytes.

Some machines still have about 2.000.000 active dst entries : the
convoluted checks we added in include/net/dst.h are here to make sure we
dont have huge holes in the dst structure.

(This might change when/if IP route cache is gone)



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