From: Christoph Paasch <cpaasch@openai.com>

When we have a (very) large number of nexthops, they do not fit within a
single message. rtm_dump_walk_nexthops() thus will be called repeatedly
and ctx->idx is used to avoid dumping the same nexthops again.

The approach in which we avoid dumping the same nexthops is by basically
walking the entire nexthop rb-tree from the left-most node until we find
a node whose id is >= s_idx. That does not scale well.

Instead of this inefficient approach, rather go directly through the
tree to the nexthop that should be dumped (the one whose nh_id >=
s_idx). This allows us to find the relevant node in O(log(n)).

We have quite a nice improvement with this:

Before:
=======

--> ~1M nexthops:
$ time ~/libnl/src/nl-nh-list | wc -l
1050624

real	0m21.080s
user	0m0.666s
sys	0m20.384s

--> ~2M nexthops:
$ time ~/libnl/src/nl-nh-list | wc -l
2101248

real	1m51.649s
user	0m1.540s
sys	1m49.908s

After:
======

--> ~1M nexthops:
$ time ~/libnl/src/nl-nh-list | wc -l
1050624

real	0m1.157s
user	0m0.926s
sys	0m0.259s

--> ~2M nexthops:
$ time ~/libnl/src/nl-nh-list | wc -l
2101248

real	0m2.763s
user	0m2.042s
sys	0m0.776s

Signed-off-by: Christoph Paasch <cpaasch@openai.com>
---
 net/ipv4/nexthop.c | 36 +++++++++++++++++++++++++++++++++---
 1 file changed, 33 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/nexthop.c b/net/ipv4/nexthop.c
index 29118c43ebf5f1e91292fe227d4afde313e564bb..509004bfd08ec43de44c7ce4a540c983d0e70201 100644
--- a/net/ipv4/nexthop.c
+++ b/net/ipv4/nexthop.c
@@ -3511,12 +3511,42 @@ static int rtm_dump_walk_nexthops(struct sk_buff *skb,
 	int err;
 
 	s_idx = ctx->idx;
-	for (node = rb_first(root); node; node = rb_next(node)) {
+
+	/* If this is not the first invocation, ctx->idx will contain the id of
+	 * the last nexthop we processed. Instead of starting from the very
+	 * first element of the red/black tree again and linearly skipping the
+	 * (potentially large) set of nodes with an id smaller than s_idx, walk
+	 * the tree and find the left-most node whose id is >= s_idx.  This
+	 * provides an efficient O(log n) starting point for the dump
+	 * continuation.
+	 */
+	if (s_idx != 0) {
+		struct rb_node *tmp = root->rb_node;
+
+		node = NULL;
+		while (tmp) {
+			struct nexthop *nh;
+
+			nh = rb_entry(tmp, struct nexthop, rb_node);
+			if (nh->id < s_idx) {
+				tmp = tmp->rb_right;
+			} else {
+				/* Track current candidate and keep looking on
+				 * the left side to find the left-most
+				 * (smallest id) that is still >= s_idx.
+				 */
+				node = tmp;
+				tmp = tmp->rb_left;
+			}
+		}
+	} else {
+		node = rb_first(root);
+	}
+
+	for (; node; node = rb_next(node)) {
 		struct nexthop *nh;
 
 		nh = rb_entry(node, struct nexthop, rb_node);
-		if (nh->id < s_idx)
-			continue;
 
 		ctx->idx = nh->id;
 		err = nh_cb(skb, cb, nh, data);

-- 
2.50.1