Commit 5635c10d authored Nov 16, 2008 by Eric Dumazet Committed by David S. Miller Nov 16, 2008

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



As found in the past (commit f1dd9c37
[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>

parent 536533e6

include/net/dst.h

+19 −0

Original line number	Diff line number	Diff line
		@@ -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);
		}

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