net: dst: Prevent false sharing vs. dst_entry:: __refcnt

dst_entry::__refcnt is highly contended in scenarios where many connections
happen from and to the same IP. The reference count is an atomic_t, so the
reference count operations have to take the cache-line exclusive.

Aside of the unavoidable reference count contention there is another
significant problem which is caused by that: False sharing.

perf top identified two affected read accesses. dst_entry::lwtstate and
rtable::rt_genid.

dst_entry:__refcnt is located at offset 64 of dst_entry, which puts it into
a seperate cacheline vs. the read mostly members located at the beginning
of the struct.

That prevents false sharing vs. the struct members in the first 64
bytes of the structure, but there is also

  dst_entry::lwtstate

which is located after the reference count and in the same cache line. This
member is read after a reference count has been acquired.

struct rtable embeds a struct dst_entry at offset 0. struct dst_entry has a
size of 112 bytes, which means that the struct members of rtable which
follow the dst member share the same cache line as dst_entry::__refcnt.
Especially

  rtable::rt_genid

is also read by the contexts which have a reference count acquired
already.

When dst_entry:__refcnt is incremented or decremented via an atomic
operation these read accesses stall. This was found when analysing the
memtier benchmark in 1:100 mode, which amplifies the problem extremly.

Move the rt[6i]_uncached[_list] members out of struct rtable and struct
rt6_info into struct dst_entry to provide padding and move the lwtstate
member after that so it ends up in the same cache line.

The resulting improvement depends on the micro-architecture and the number
of CPUs. It ranges from +20% to +120% with a localhost memtier/memcached
benchmark.

[ tglx: Rearrange struct ]

Signed-off-by: Wangyang Guo <wangyang.guo@intel.com>
Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20230323102800.042297517@linutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 13 insertions, 13 deletions

diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 244df77fac87..12e163dec34e 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -139,20 +139,20 @@ void rt6_uncached_list_add(struct rt6_info *rt)
 {
 	struct uncached_list *ul = raw_cpu_ptr(&rt6_uncached_list);
 
-	rt->rt6i_uncached_list = ul;
+	rt->dst.rt_uncached_list = ul;
 
 	spin_lock_bh(&ul->lock);
-	list_add_tail(&rt->rt6i_uncached, &ul->head);
+	list_add_tail(&rt->dst.rt_uncached, &ul->head);
 	spin_unlock_bh(&ul->lock);
 }
 
 void rt6_uncached_list_del(struct rt6_info *rt)
 {
-	if (!list_empty(&rt->rt6i_uncached)) {
-		struct uncached_list *ul = rt->rt6i_uncached_list;
+	if (!list_empty(&rt->dst.rt_uncached)) {
+		struct uncached_list *ul = rt->dst.rt_uncached_list;
 
 		spin_lock_bh(&ul->lock);
-		list_del_init(&rt->rt6i_uncached);
+		list_del_init(&rt->dst.rt_uncached);
 		spin_unlock_bh(&ul->lock);
 	}
 }
@@ -169,7 +169,7 @@ static void rt6_uncached_list_flush_dev(struct net_device *dev)
 			continue;
 
 		spin_lock_bh(&ul->lock);
-		list_for_each_entry_safe(rt, safe, &ul->head, rt6i_uncached) {
+		list_for_each_entry_safe(rt, safe, &ul->head, dst.rt_uncached) {
 			struct inet6_dev *rt_idev = rt->rt6i_idev;
 			struct net_device *rt_dev = rt->dst.dev;
 			bool handled = false;
@@ -188,7 +188,7 @@ static void rt6_uncached_list_flush_dev(struct net_device *dev)
 				handled = true;
 			}
 			if (handled)
-				list_move(&rt->rt6i_uncached,
+				list_move(&rt->dst.rt_uncached,
 					  &ul->quarantine);
 		}
 		spin_unlock_bh(&ul->lock);
@@ -334,7 +334,7 @@ static const struct rt6_info ip6_blk_hole_entry_template = {
 static void rt6_info_init(struct rt6_info *rt)
 {
 	memset_after(rt, 0, dst);
-	INIT_LIST_HEAD(&rt->rt6i_uncached);
+	INIT_LIST_HEAD(&rt->dst.rt_uncached);
 }
 
 /* allocate dst with ip6_dst_ops */
@@ -2638,7 +2638,7 @@ struct dst_entry *ip6_route_output_flags(struct net *net,
 	dst = ip6_route_output_flags_noref(net, sk, fl6, flags);
 	rt6 = (struct rt6_info *)dst;
 	/* For dst cached in uncached_list, refcnt is already taken. */
-	if (list_empty(&rt6->rt6i_uncached) && !dst_hold_safe(dst)) {
+	if (list_empty(&rt6->dst.rt_uncached) && !dst_hold_safe(dst)) {
 		dst = &net->ipv6.ip6_null_entry->dst;
 		dst_hold(dst);
 	}
@@ -2748,7 +2748,7 @@ INDIRECT_CALLABLE_SCOPE struct dst_entry *ip6_dst_check(struct dst_entry *dst,
 	from = rcu_dereference(rt->from);
 
 	if (from && (rt->rt6i_flags & RTF_PCPU ||
-	    unlikely(!list_empty(&rt->rt6i_uncached))))
+	    unlikely(!list_empty(&rt->dst.rt_uncached))))
 		dst_ret = rt6_dst_from_check(rt, from, cookie);
 	else
 		dst_ret = rt6_check(rt, from, cookie);
@@ -6477,7 +6477,7 @@ static int __net_init ip6_route_net_init(struct net *net)
 	net->ipv6.ip6_null_entry->dst.ops = &net->ipv6.ip6_dst_ops;
 	dst_init_metrics(&net->ipv6.ip6_null_entry->dst,
 			 ip6_template_metrics, true);
-	INIT_LIST_HEAD(&net->ipv6.ip6_null_entry->rt6i_uncached);
+	INIT_LIST_HEAD(&net->ipv6.ip6_null_entry->dst.rt_uncached);
 
 #ifdef CONFIG_IPV6_MULTIPLE_TABLES
 	net->ipv6.fib6_has_custom_rules = false;
@@ -6489,7 +6489,7 @@ static int __net_init ip6_route_net_init(struct net *net)
 	net->ipv6.ip6_prohibit_entry->dst.ops = &net->ipv6.ip6_dst_ops;
 	dst_init_metrics(&net->ipv6.ip6_prohibit_entry->dst,
 			 ip6_template_metrics, true);
-	INIT_LIST_HEAD(&net->ipv6.ip6_prohibit_entry->rt6i_uncached);
+	INIT_LIST_HEAD(&net->ipv6.ip6_prohibit_entry->dst.rt_uncached);
 
 	net->ipv6.ip6_blk_hole_entry = kmemdup(&ip6_blk_hole_entry_template,
 					       sizeof(*net->ipv6.ip6_blk_hole_entry),
@@ -6499,7 +6499,7 @@ static int __net_init ip6_route_net_init(struct net *net)
 	net->ipv6.ip6_blk_hole_entry->dst.ops = &net->ipv6.ip6_dst_ops;
 	dst_init_metrics(&net->ipv6.ip6_blk_hole_entry->dst,
 			 ip6_template_metrics, true);
-	INIT_LIST_HEAD(&net->ipv6.ip6_blk_hole_entry->rt6i_uncached);
+	INIT_LIST_HEAD(&net->ipv6.ip6_blk_hole_entry->dst.rt_uncached);
 #ifdef CONFIG_IPV6_SUBTREES
 	net->ipv6.fib6_routes_require_src = 0;
 #endif