net: fib_rules: Only hold RTNL for the first IPv4 RTM_NEWRULE.

Now, RTM_DELRULE no longer needs RTNL, and the only RTNL dependant
in RTM_NEWRULE is fib_unmerge(), which is called for the first
IPv4 rule.

Let's add fib_rules_ops.need_rtnl() and hold RTNL only for the
first IPv4 rule.

Tested:
The script below creates 1K rules in parallel in 4K netns, and
it got 20x/30x faster for IPv4/IPv6.

  #!/bin/bash
  N=4096
  F=rules.txt

  for i in $(seq $N); do ip netns add ns-$i; done
  printf 'rule add from all table %d\n' {1..1024} > $F

  for v in 4 6; do
  	echo "=== IPv${v} ==="
  	time { for i in $(seq $N); do nsenter \
  	--net=/var/run/netns/ns-$i ip -$v -batch $F & done; wait; }
  done

  for i in $(seq $N); do ip netns del ns-$i; done
  rm -f $F

Without this series:

  # ./test.sh
  === IPv4 ===

  real	0m22.752s
  user	0m7.834s
  sys	92m46.721s
  === IPv6 ===

  real	0m35.181s
  user	0m8.635s
  sys	142m30.479s

With this series:

  # ./test.sh
  === IPv4 ===

  real	0m0.918s
  user	0m5.675s
  sys	2m7.024s
  === IPv6 ===

  real	0m1.214s
  user	0m7.917s
  sys	4m19.489s

Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Link: https://patch.msgid.link/20260629181226.1929658-10-kuniyu@google.com
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/net/ipv4/fib_rules.c b/net/ipv4/fib_rules.c
index 16d202246a36..4edb0dca7be8 100644
--- a/net/ipv4/fib_rules.c
+++ b/net/ipv4/fib_rules.c
@@ -460,6 +460,11 @@ static void fib4_rule_flush_cache(struct fib_rules_ops *ops)
 	rt_cache_flush(ops->fro_net);
 }
 
+static bool fib4_rule_need_rtnl(struct net *net)
+{
+	return !net->ipv4.fib_has_custom_rules;
+}
+
 static const struct fib_rules_ops __net_initconst fib4_rules_ops_template = {
 	.family		= AF_INET,
 	.rule_size	= sizeof(struct fib4_rule),
@@ -473,6 +478,7 @@ static const struct fib_rules_ops __net_initconst fib4_rules_ops_template = {
 	.fill		= fib4_rule_fill,
 	.nlmsg_payload	= fib4_rule_nlmsg_payload,
 	.flush_cache	= fib4_rule_flush_cache,
+	.need_rtnl	= fib4_rule_need_rtnl,
 	.nlgroup	= RTNLGRP_IPV4_RULE,
 	.owner		= THIS_MODULE,
 };