netfilter: nf_conncount: reduce unnecessary GC

Currently nf_conncount can trigger garbage collection (GC)
at multiple places. Each GC process takes a spin_lock_bh
to traverse the nf_conncount_list. We found that when testing
port scanning use two parallel nmap, because the number of
connection increase fast, the nf_conncount_count and its
subsequent call to __nf_conncount_add take too much time,
causing several CPU lockup. This happens when user set the
conntrack limit to +20,000, because the larger the limit,
the longer the list that GC has to traverse.

The patch mitigate the performance issue by avoiding unnecessary
GC with a timestamp. Whenever nf_conncount has done a GC,
a timestamp is updated, and beforce the next time GC is
triggered, we make sure it's more than a jiffies.
By doin this we can greatly reduce the CPU cycles and
avoid the softirq lockup.

To reproduce it in OVS,
$ ovs-appctl dpctl/ct-set-limits zone=1,limit=20000
$ ovs-appctl dpctl/ct-get-limits

At another machine, runs two nmap
$ nmap -p1- <IP>
$ nmap -p1- <IP>

Signed-off-by: William Tu <[e-mail]>
Co-authored-by: Yifeng Sun <[e-mail]>
Reported-by: Greg Rose <[e-mail]>
Suggested-by: Florian Westphal <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

include/net/netfilter/nf_conntrack_count.h

···
 
 struct nf_conncount_list {
 	spinlock_t list_lock;
+	u32 last_gc;		/* jiffies at most recent gc */
 	struct list_head head;	/* connections with the same filtering key */
 	unsigned int count;	/* length of list */
 };

net/netfilter/nf_conncount.c

···
 	struct nf_conn *found_ct;
 	unsigned int collect = 0;
 
+	if (time_is_after_eq_jiffies((unsigned long)list->last_gc))
+		goto add_new_node;
+
 	/* check the saved connections */
 	list_for_each_entry_safe(conn, conn_n, &list->head, node) {
 		if (collect > CONNCOUNT_GC_MAX_NODES)
···
 		nf_ct_put(found_ct);
 	}
 
+add_new_node:
 	if (WARN_ON_ONCE(list->count > INT_MAX))
 		return -EOVERFLOW;
 
···
 	conn->jiffies32 = (u32)jiffies;
 	list_add_tail(&conn->node, &list->head);
 	list->count++;
+	list->last_gc = (u32)jiffies;
 	return 0;
 }
 
···
 	spin_lock_init(&list->list_lock);
 	INIT_LIST_HEAD(&list->head);
 	list->count = 0;
+	list->last_gc = (u32)jiffies;
 }
 EXPORT_SYMBOL_GPL(nf_conncount_list_init);
 
···
 	struct nf_conn *found_ct;
 	unsigned int collected = 0;
 	bool ret = false;
+
+	/* don't bother if we just did GC */
+	if (time_is_after_eq_jiffies((unsigned long)READ_ONCE(list->last_gc)))
+		return false;
 
 	/* don't bother if other cpu is already doing GC */
 	if (!spin_trylock(&list->list_lock))
···
 
 	if (!list->count)
 		ret = true;
+	list->last_gc = (u32)jiffies;
 	spin_unlock(&list->list_lock);
 
 	return ret;