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 <[e-mail]>
Cc: Vlad Buslov <[e-mail]>
Cc: Jiri Pirko <[e-mail]>
Cc: Marcelo Ricardo Leitner <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

include/net/act_api.h

···
 {
 	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,

net/sched/act_api.c

···
 			 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);