net: dst: Switch to rcuref_t reference counting

Under high contention dst_entry::__refcnt becomes a significant bottleneck.

atomic_inc_not_zero() is implemented with a cmpxchg() loop, which goes into
high retry rates on contention.

Switch the reference count to rcuref_t which results in a significant
performance gain. Rename the reference count member to __rcuref to reflect
the change.

The gain depends on the micro-architecture and the number of concurrent
operations and has been measured in the range of +25% to +130% with a
localhost memtier/memcached benchmark which amplifies the problem
massively.

Running the memtier/memcached benchmark over a real (1Gb) network
connection the conversion on top of the false sharing fix for struct
dst_entry::__refcnt results in a total gain in the 2%-5% range over the
upstream baseline.

Reported-by: Wangyang Guo <wangyang.guo@intel.com>
Reported-by: Arjan Van De Ven <arjan.van.de.ven@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230307125538.989175656@linutronix.de
Link: https://lore.kernel.org/r/20230323102800.215027837@linutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

5 files changed, 12 insertions, 28 deletions

diff --git a/net/bridge/br_nf_core.c b/net/bridge/br_nf_core.c
index 8c69f0c95a8e..98aea5485aae 100644
--- a/net/bridge/br_nf_core.c
+++ b/net/bridge/br_nf_core.c
@@ -73,7 +73,7 @@ void br_netfilter_rtable_init(struct net_bridge *br)
 {
 	struct rtable *rt = &br->fake_rtable;
 
-	atomic_set(&rt->dst.__refcnt, 1);
+	rcuref_init(&rt->dst.__rcuref, 1);
 	rt->dst.dev = br->dev;
 	dst_init_metrics(&rt->dst, br_dst_default_metrics, true);
 	rt->dst.flags	= DST_NOXFRM | DST_FAKE_RTABLE;
diff --git a/net/core/dst.c b/net/core/dst.c
index 31c08a3386d3..3247e84045ca 100644
--- a/net/core/dst.c
+++ b/net/core/dst.c
@@ -66,7 +66,7 @@ void dst_init(struct dst_entry *dst, struct dst_ops *ops,
 	dst->tclassid = 0;
 #endif
 	dst->lwtstate = NULL;
-	atomic_set(&dst->__refcnt, initial_ref);
+	rcuref_init(&dst->__rcuref, initial_ref);
 	dst->__use = 0;
 	dst->lastuse = jiffies;
 	dst->flags = flags;
@@ -162,31 +162,15 @@ EXPORT_SYMBOL(dst_dev_put);
 
 void dst_release(struct dst_entry *dst)
 {
-	if (dst) {
-		int newrefcnt;
-
-		newrefcnt = atomic_dec_return(&dst->__refcnt);
-		if (WARN_ONCE(newrefcnt < 0, "dst_release underflow"))
-			net_warn_ratelimited("%s: dst:%p refcnt:%d\n",
-					     __func__, dst, newrefcnt);
-		if (!newrefcnt)
-			call_rcu_hurry(&dst->rcu_head, dst_destroy_rcu);
-	}
+	if (dst && rcuref_put(&dst->__rcuref))
+		call_rcu_hurry(&dst->rcu_head, dst_destroy_rcu);
 }
 EXPORT_SYMBOL(dst_release);
 
 void dst_release_immediate(struct dst_entry *dst)
 {
-	if (dst) {
-		int newrefcnt;
-
-		newrefcnt = atomic_dec_return(&dst->__refcnt);
-		if (WARN_ONCE(newrefcnt < 0, "dst_release_immediate underflow"))
-			net_warn_ratelimited("%s: dst:%p refcnt:%d\n",
-					     __func__, dst, newrefcnt);
-		if (!newrefcnt)
-			dst_destroy(dst);
-	}
+	if (dst && rcuref_put(&dst->__rcuref))
+		dst_destroy(dst);
 }
 EXPORT_SYMBOL(dst_release_immediate);
 
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index b7b1661d0d56..906aebdc566b 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -843,7 +843,7 @@ int rtnl_put_cacheinfo(struct sk_buff *skb, struct dst_entry *dst, u32 id,
 	if (dst) {
 		ci.rta_lastuse = jiffies_delta_to_clock_t(jiffies - dst->lastuse);
 		ci.rta_used = dst->__use;
-		ci.rta_clntref = atomic_read(&dst->__refcnt);
+		ci.rta_clntref = rcuref_read(&dst->__rcuref);
 	}
 	if (expires) {
 		unsigned long clock;
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 12e163dec34e..35085fc0cf15 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -293,7 +293,7 @@ static const struct fib6_info fib6_null_entry_template = {
 
 static const struct rt6_info ip6_null_entry_template = {
 	.dst = {
-		.__refcnt	= ATOMIC_INIT(1),
+		.__rcuref	= RCUREF_INIT(1),
 		.__use		= 1,
 		.obsolete	= DST_OBSOLETE_FORCE_CHK,
 		.error		= -ENETUNREACH,
@@ -307,7 +307,7 @@ static const struct rt6_info ip6_null_entry_template = {
 
 static const struct rt6_info ip6_prohibit_entry_template = {
 	.dst = {
-		.__refcnt	= ATOMIC_INIT(1),
+		.__rcuref	= RCUREF_INIT(1),
 		.__use		= 1,
 		.obsolete	= DST_OBSOLETE_FORCE_CHK,
 		.error		= -EACCES,
@@ -319,7 +319,7 @@ static const struct rt6_info ip6_prohibit_entry_template = {
 
 static const struct rt6_info ip6_blk_hole_entry_template = {
 	.dst = {
-		.__refcnt	= ATOMIC_INIT(1),
+		.__rcuref	= RCUREF_INIT(1),
 		.__use		= 1,
 		.obsolete	= DST_OBSOLETE_FORCE_CHK,
 		.error		= -EINVAL,
diff --git a/net/netfilter/ipvs/ip_vs_xmit.c b/net/netfilter/ipvs/ip_vs_xmit.c
index 80448885c3d7..99c349c0d968 100644
--- a/net/netfilter/ipvs/ip_vs_xmit.c
+++ b/net/netfilter/ipvs/ip_vs_xmit.c
@@ -339,7 +339,7 @@ __ip_vs_get_out_rt(struct netns_ipvs *ipvs, int skb_af, struct sk_buff *skb,
 			spin_unlock_bh(&dest->dst_lock);
 			IP_VS_DBG(10, "new dst %pI4, src %pI4, refcnt=%d\n",
 				  &dest->addr.ip, &dest_dst->dst_saddr.ip,
-				  atomic_read(&rt->dst.__refcnt));
+				  rcuref_read(&rt->dst.__rcuref));
 		}
 		if (ret_saddr)
 			*ret_saddr = dest_dst->dst_saddr.ip;
@@ -507,7 +507,7 @@ __ip_vs_get_out_rt_v6(struct netns_ipvs *ipvs, int skb_af, struct sk_buff *skb,
 			spin_unlock_bh(&dest->dst_lock);
 			IP_VS_DBG(10, "new dst %pI6, src %pI6, refcnt=%d\n",
 				  &dest->addr.in6, &dest_dst->dst_saddr.in6,
-				  atomic_read(&rt->dst.__refcnt));
+				  rcuref_read(&rt->dst.__rcuref));
 		}
 		if (ret_saddr)
 			*ret_saddr = dest_dst->dst_saddr.in6;