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
--- a/include/net/dst.h
+++ b/include/net/dst.h
@@ -43,10 +43,11 @@
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 96b1a749a..b142a0d 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -1210,7 +1210,9 @@
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
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1107,6 +1107,7 @@
* 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) {
