net: Make nexthop-dumps scale linearly with the number of nexthops

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>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20250816-nexthop_dump-v2-1-491da3462118@openai.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ipv4/nexthop.c b/net/ipv4/nexthop.c
index 29118c43ebf5f1e91292fe227d4afde313e564bb..509004bfd08ec43de44c7ce4a540c983d0e70201 100644 (file)
--- 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);