Commit c7d4426a authored Oct 03, 2010 by Eric Dumazet Committed by David S. Miller Oct 03, 2010

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>

parent 9a7241c2

include/net/dst.h

+5 −4

Original line number	Diff line number	Diff line
		@@ -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 */

net/core/neighbour.c

+3 −1

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

net/ipv4/route.c

+1 −0

Original line number	Diff line number	Diff line
		@@ -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) {

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