net: make sure struct dst_entry refcount is aligned on 64 bytes

As found in the past (commit f1dd9c3
[NET]: Fix tbench regression in 2.6.25-rc1), it is really
important that struct dst_entry refcount is aligned on a cache line.

We cannot use __atribute((aligned)), so manually pad the structure
for 32 and 64 bit arches.

for 32bit : offsetof(truct dst_entry, __refcnt) is 0x80
for 64bit : offsetof(truct dst_entry, __refcnt) is 0xc0

As it is not possible to guess at compile time cache line size,
we use a generic value of 64 bytes, that satisfies many current arches.
(Using 128 bytes alignment on 64bit arches would waste 64 bytes)

Add a BUILD_BUG_ON to catch future updates to "struct dst_entry" dont
break this alignment.

"tbench 8" is 4.4 % faster on a dual quad core (HP BL460c G1), Intel E5450 @3.00GHz
(2350 MB/s instead of 2250 MB/s)

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/dst.h

@@ -61,6 +61,8 @@ 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*);
@@ -71,8 +73,20 @@ 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[2];
+#else
+	long			__pad_to_align_refcnt[1];
+#endif
 	/*
 	 * __refcnt wants to be on a different cache line from
 	 * input/output/ops or performance tanks badly
@@ -157,6 +171,11 @@ dst_metric_locked(struct dst_entry *dst, int metric)
 
 static inline void dst_hold(struct dst_entry * dst)
 {
+	/*
+	 * If your kernel compilation stops here, please check
+	 * __pad_to_align_refcnt declaration in struct dst_entry
+	 */
+	BUILD_BUG_ON(offsetof(struct dst_entry, __refcnt) & 63);
 	atomic_inc(&dst->__refcnt);
 }