netfilter: conntrack: resched gc again if eviction rate is high

If we evicted a large fraction of the scanned conntrack entries re-schedule
the next gc cycle for immediate execution.

This triggers during tests where load is high, then drops to zero and
many connections will be in TW/CLOSE state with < 30 second timeouts.

Without this change it will take several minutes until conntrack count
comes back to normal.

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_conntrack_core.c

@@ -945,6 +945,7 @@ static void gc_worker(struct work_struct *work)
 {
 	unsigned int i, goal, buckets = 0, expired_count = 0;
 	unsigned long next_run = GC_INTERVAL;
+	unsigned int ratio, scanned = 0;
 	struct conntrack_gc_work *gc_work;
 
 	gc_work = container_of(work, struct conntrack_gc_work, dwork.work);
@@ -969,6 +970,7 @@ static void gc_worker(struct work_struct *work)
 		hlist_nulls_for_each_entry_rcu(h, n, &ct_hash[i], hnnode) {
 			tmp = nf_ct_tuplehash_to_ctrack(h);
 
+			scanned++;
 			if (nf_ct_is_expired(tmp)) {
 				nf_ct_gc_expired(tmp);
 				expired_count++;
@@ -988,6 +990,10 @@ static void gc_worker(struct work_struct *work)
 	if (gc_work->exiting)
 		return;
 
+	ratio = scanned ? expired_count * 100 / scanned : 0;
+	if (ratio >= 90)
+		next_run = 0;
+
 	gc_work->last_bucket = i;
 	schedule_delayed_work(&gc_work->dwork, next_run);
 }