neigh: Keep neighbour cache entries if number of them is small enough.

Since we have removed NCE (Neighbour Cache Entry) reference from
routing entries, the only refcnt holders of an NCE are its timer
(if running) and its owner table, in usual cases.  As a result,
neigh_periodic_work() purges NCEs over and over again even for
gateways.

It does not make sense to purge entries, if number of them is
very small, so keep them.  The minimum number of entries to keep
is specified by gc_thresh1.
Signed-off-by: NYOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -26,6 +26,11 @@ route/max_size - INTEGER
 	Maximum number of routes allowed in the kernel.  Increase
 	this when using large numbers of interfaces and/or routes.
 
+neigh/default/gc_thresh1 - INTEGER
+	Minimum number of entries to keep.  Garbage collector will not
+	purge entries if there are fewer than this number.
+	Default: 256
+
 neigh/default/gc_thresh3 - INTEGER
 	Maximum number of neighbor entries allowed.  Increase this
 	when using large numbers of interfaces and when communicating

net/core/neighbour.c

@@ -778,6 +778,9 @@ static void neigh_periodic_work(struct work_struct *work)
 	nht = rcu_dereference_protected(tbl->nht,
 					lockdep_is_held(&tbl->lock));
 
+	if (atomic_read(&tbl->entries) < tbl->gc_thresh1)
+		goto out;
+
 	/*
 	 *	periodically recompute ReachableTime from random function
 	 */
@@ -832,6 +835,7 @@ static void neigh_periodic_work(struct work_struct *work)
 		nht = rcu_dereference_protected(tbl->nht,
 						lockdep_is_held(&tbl->lock));
 	}
+out:
 	/* Cycle through all hash buckets every base_reachable_time/2 ticks.
 	 * ARP entry timeouts range from 1/2 base_reachable_time to 3/2
 	 * base_reachable_time.