net: introduce DST_NOCACHE flag

While doing stress tests with IP route cache disabled, and multi queue
devices, I noticed a very high contention on one rwlock used in
neighbour code.

When many cpus are trying to send frames (possibly using a high
performance multiqueue device) to the same neighbour, they fight for the
neigh->lock rwlock in order to call neigh_hh_init(), and fight on
hh->hh_refcnt (a pair of atomic_inc/atomic_dec_and_test())

But we dont need to call neigh_hh_init() for dst that are used only
once. It costs four atomic operations at least, on two contended cache
lines, plus the high contention on neigh->lock rwlock.

Introduce a new dst flag, DST_NOCACHE, that is set when dst was not
inserted in route cache.

With the stress test bench, sending 160000000 frames on one neighbour,
results are :

Before patch:

real	2m28.406s
user	0m11.781s
sys	36m17.964s

After patch:

real	1m26.532s
user	0m12.185s
sys	20m3.903s

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

diff --git a/include/net/dst.h b/include/net/dst.h
index aa53fbc..a217c83 100644 (file)
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -43,10 +43,11 @@ struct dst_entry {
        short                   error;
        short                   obsolete;
        int                     flags;
-#define DST_HOST               1
-#define DST_NOXFRM             2
-#define DST_NOPOLICY           4
-#define DST_NOHASH             8
+#define DST_HOST               0x0001
+#define DST_NOXFRM             0x0002
+#define DST_NOPOLICY           0x0004
+#define DST_NOHASH             0x0008
+#define DST_NOCACHE            0x0010
        unsigned long           expires;
 
        unsigned short          header_len;     /* more space at head required */

diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 96b1a74..b142a0d 100644 (file)
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -1210,7 +1210,9 @@ int neigh_resolve_output(struct sk_buff *skb)
        if (!neigh_event_send(neigh, skb)) {
                int err;
                struct net_device *dev = neigh->dev;
-               if (dev->header_ops->cache && !dst->hh) {
+               if (dev->header_ops->cache &&
+                   !dst->hh &&
+                   !(dst->flags & DST_NOCACHE)) {
                        write_lock_bh(&neigh->lock);
                        if (!dst->hh)
                                neigh_hh_init(neigh, dst, dst->ops->protocol);

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index a61acea..c3cb8bd 100644 (file)
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1107,6 +1107,7 @@ restart:
                 * on the route gc list.
                 */
 
+               rt->dst.flags |= DST_NOCACHE;
                if (rt->rt_type == RTN_UNICAST || rt->fl.iif == 0) {
                        int err = arp_bind_neighbour(&rt->dst);
                        if (err) {