netfilter: conntrack: fix the gc rescheduling delay

Commit 2cfadb76 ("netfilter: conntrack: revisit gc autotuning")
changed the eviction rescheduling to the use average expiry of scanned
entries (within 1-60s) by doing:

  for (...) {
      expires = clamp(nf_ct_expires(tmp), ...);
      next_run += expires;
      next_run /= 2;
  }

The issue is the above will make the average ('next_run' here) more
dependent on the last expiration values than the firsts (for sets > 2).
Depending on the expiration values used to compute the average, the
result can be quite different than what's expected. To fix this we can
do the following:

  for (...) {
      expires = clamp(nf_ct_expires(tmp), ...);
      next_run += (expires - next_run) / ++count;
  }

Fixes: 2cfadb76 ("netfilter: conntrack: revisit gc autotuning")
Cc: Florian Westphal <fw@strlen.de>
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: Florian Westphal <fw@strlen.de>

net/netfilter/nf_conntrack_core.c

@@ -67,6 +67,7 @@ struct conntrack_gc_work {
 	struct delayed_work	dwork;
 	u32			next_bucket;
 	u32			avg_timeout;
+	u32			count;
 	u32			start_time;
 	bool			exiting;
 	bool			early_drop;
@@ -1466,6 +1467,7 @@ static void gc_worker(struct work_struct *work)
 	unsigned int expired_count = 0;
 	unsigned long next_run;
 	s32 delta_time;
+	long count;
 
 	gc_work = container_of(work, struct conntrack_gc_work, dwork.work);
 
@@ -1475,10 +1477,12 @@ static void gc_worker(struct work_struct *work)
 
 	if (i == 0) {
 		gc_work->avg_timeout = GC_SCAN_INTERVAL_INIT;
+		gc_work->count = 1;
 		gc_work->start_time = start_time;
 	}
 
 	next_run = gc_work->avg_timeout;
+	count = gc_work->count;
 
 	end_time = start_time + GC_SCAN_MAX_DURATION;
 
@@ -1498,8 +1502,8 @@ static void gc_worker(struct work_struct *work)
 
 		hlist_nulls_for_each_entry_rcu(h, n, &ct_hash[i], hnnode) {
 			struct nf_conntrack_net *cnet;
-			unsigned long expires;
 			struct net *net;
+			long expires;
 
 			tmp = nf_ct_tuplehash_to_ctrack(h);
 
@@ -1513,6 +1517,7 @@ static void gc_worker(struct work_struct *work)
 
 				gc_work->next_bucket = i;
 				gc_work->avg_timeout = next_run;
+				gc_work->count = count;
 
 				delta_time = nfct_time_stamp - gc_work->start_time;
 
@@ -1528,8 +1533,8 @@ static void gc_worker(struct work_struct *work)
 			}
 
 			expires = clamp(nf_ct_expires(tmp), GC_SCAN_INTERVAL_MIN, GC_SCAN_INTERVAL_CLAMP);
+			expires = (expires - (long)next_run) / ++count;
 			next_run += expires;
-			next_run /= 2u;
 
 			if (nf_conntrack_max95 == 0 || gc_worker_skip_ct(tmp))
 				continue;
@@ -1570,6 +1575,7 @@ static void gc_worker(struct work_struct *work)
 		delta_time = nfct_time_stamp - end_time;
 		if (delta_time > 0 && i < hashsz) {
 			gc_work->avg_timeout = next_run;
+			gc_work->count = count;
 			gc_work->next_bucket = i;
 			next_run = 0;
 			goto early_exit;