netdev - [PATCH v2 net-next] net/sched: acp_api: no longer acquire RTNL in tc_action_net

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Message-ID: <20250702071230.1892674-1-edumazet@google.com>
Date: Wed,  2 Jul 2025 07:12:30 +0000
From: Eric Dumazet <edumazet@...gle.com>
To: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>
Cc: Simon Horman <horms@...nel.org>, Jamal Hadi Salim <jhs@...atatu.com>, 
	Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>, 
	Kuniyuki Iwashima <kuniyu@...gle.com>, netdev@...r.kernel.org, eric.dumazet@...il.com, 
	Eric Dumazet <edumazet@...gle.com>, Vlad Buslov <vladbu@...dia.com>, 
	Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Subject: [PATCH v2 net-next] net/sched: acp_api: no longer acquire RTNL in tc_action_net_exit()

tc_action_net_exit() got an rtnl exclusion in commit
a159d3c4b829 ("net_sched: acquire RTNL in tc_action_net_exit()")

Since then, commit 16af6067392c ("net: sched: implement reference
counted action release") made this RTNL exclusion obsolete for
most cases.

Only tcf_action_offload_del() might still require it.

Move the rtnl locking into tcf_idrinfo_destroy() when
an offload action is found.

Most netns do not have actions, yet deleting them is adding a lot
of pressure on RTNL, which is for many the most contended mutex
in the kernel.

We are moving to a per-netns 'rtnl', so tc_action_net_exit()
will not be able to grab 'rtnl' a single time for a batch of netns.

Before the patch:

perf probe -a rtnl_lock

perf record -e probe:rtnl_lock -a /bin/bash -c 'unshare -n "/bin/true"; sleep 1'
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.305 MB perf.data (25 samples) ]

After the patch:

perf record -e probe:rtnl_lock -a /bin/bash -c 'unshare -n "/bin/true"; sleep 1'
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.304 MB perf.data (9 samples) ]

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Vlad Buslov <vladbu@...dia.com>
Cc: Jiri Pirko <jiri@...nulli.us>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
---
v2: Added conditional rtnl acquisition in tcf_idrinfo_destroy()
    after Cong feedback about tcf_action_offload_del() safety.
    Added some performance numbers before/after the patch.
v1: https://lore.kernel.org/netdev/20250701133006.812702-1-edumazet@google.com/

 include/net/act_api.h | 2 --
 net/sched/act_api.c   | 9 ++++++++-
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/include/net/act_api.h b/include/net/act_api.h
index 404df8557f6a13420b18d9c52b9710fe86d084aa..04781c92b43d6ab9cc6c81a88d5c6fe8c282c590 100644
--- a/include/net/act_api.h
+++ b/include/net/act_api.h
@@ -170,14 +170,12 @@ static inline void tc_action_net_exit(struct list_head *net_list,
 {
 	struct net *net;
 
-	rtnl_lock();
 	list_for_each_entry(net, net_list, exit_list) {
 		struct tc_action_net *tn = net_generic(net, id);
 
 		tcf_idrinfo_destroy(tn->ops, tn->idrinfo);
 		kfree(tn->idrinfo);
 	}
-	rtnl_unlock();
 }
 
 int tcf_generic_walker(struct tc_action_net *tn, struct sk_buff *skb,
diff --git a/net/sched/act_api.c b/net/sched/act_api.c
index 057e20cef3754f33357c4c1e30034f6b9b872d91..9e468e46346710c85c3a85b905d27dfe3972916a 100644
--- a/net/sched/act_api.c
+++ b/net/sched/act_api.c
@@ -933,18 +933,25 @@ void tcf_idrinfo_destroy(const struct tc_action_ops *ops,
 			 struct tcf_idrinfo *idrinfo)
 {
 	struct idr *idr = &idrinfo->action_idr;
+	bool mutex_taken = false;
 	struct tc_action *p;
-	int ret;
 	unsigned long id = 1;
 	unsigned long tmp;
+	int ret;
 
 	idr_for_each_entry_ul(idr, p, tmp, id) {
+		if (tc_act_in_hw(p) && !mutex_taken) {
+			rtnl_lock();
+			mutex_taken = true;
+		}
 		ret = __tcf_idr_release(p, false, true);
 		if (ret == ACT_P_DELETED)
 			module_put(ops->owner);
 		else if (ret < 0)
 			return;
 	}
+	if (mutex_taken)
+		rtnl_unlock();
 	idr_destroy(&idrinfo->action_idr);
 }
 EXPORT_SYMBOL(tcf_idrinfo_destroy);
-- 
2.50.0.727.gbf7dc18ff4-goog