net dst: use a percpu_counter to track entries

struct dst_ops tracks number of allocated dst in an atomic_t field,
subject to high cache line contention in stress workload.

Switch to a percpu_counter, to reduce number of time we need to dirty a
central location. Place it on a separate cache line to avoid dirtying
read only fields.

Stress test :

(Sending 160.000.000 UDP frames,
IP route cache disabled, dual E5540 @2.53GHz,
32bit kernel, FIB_TRIE, SLUB/NUMA)

Before:

real    0m51.179s
user    0m15.329s
sys     10m15.942s

After:

real	0m45.570s
user	0m15.525s
sys	9m56.669s

With a small reordering of struct neighbour fields, subject of a
following patch, (to separate refcnt from other read mostly fields)

real	0m41.841s
user	0m15.261s
sys	8m45.949s

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 36 insertions, 1 deletions

diff --git a/include/net/dst_ops.h b/include/net/dst_ops.h
index d1ff9b7..1fa5306 100644
--- a/include/net/dst_ops.h
+++ b/include/net/dst_ops.h
@@ -1,6 +1,7 @@
 #ifndef _NET_DST_OPS_H
 #define _NET_DST_OPS_H
 #include <linux/types.h>
+#include <linux/percpu_counter.h>
 
 struct dst_entry;
 struct kmem_cachep;
@@ -22,7 +23,41 @@ struct dst_ops {
 	void			(*update_pmtu)(struct dst_entry *dst, u32 mtu);
 	int			(*local_out)(struct sk_buff *skb);
 
-	atomic_t		entries;
 	struct kmem_cache	*kmem_cachep;
+
+	struct percpu_counter	pcpuc_entries ____cacheline_aligned_in_smp;
 };
+
+static inline int dst_entries_get_fast(struct dst_ops *dst)
+{
+	return percpu_counter_read_positive(&dst->pcpuc_entries);
+}
+
+static inline int dst_entries_get_slow(struct dst_ops *dst)
+{
+	int res;
+
+	local_bh_disable();
+	res = percpu_counter_sum_positive(&dst->pcpuc_entries);
+	local_bh_enable();
+	return res;
+}
+
+static inline void dst_entries_add(struct dst_ops *dst, int val)
+{
+	local_bh_disable();
+	percpu_counter_add(&dst->pcpuc_entries, val);
+	local_bh_enable();
+}
+
+static inline int dst_entries_init(struct dst_ops *dst)
+{
+	return percpu_counter_init(&dst->pcpuc_entries, 0);
+}
+
+static inline void dst_entries_destroy(struct dst_ops *dst)
+{
+	percpu_counter_destroy(&dst->pcpuc_entries);
+}
+
 #endif