inet: frags: delay fqdir_free_fn()

fqdir_free_fn() is using very expensive rcu_barrier()

When one netns is dismantled, we often call fqdir_exit()
multiple times, typically lauching fqdir_free_fn() twice.

Delaying by one second fqdir_free_fn() helps to reduce
the number of rcu_barrier() calls, and lock contention
on rcu_state.barrier_mutex.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/inet_fragment.c

@@ -175,7 +175,7 @@ static void fqdir_free_fn(struct work_struct *work)
 	}
 }
 
-static DECLARE_WORK(fqdir_free_work, fqdir_free_fn);
+static DECLARE_DELAYED_WORK(fqdir_free_work, fqdir_free_fn);
 
 static void fqdir_work_fn(struct work_struct *work)
 {
@@ -184,7 +184,7 @@ static void fqdir_work_fn(struct work_struct *work)
 	rhashtable_free_and_destroy(&fqdir->rhashtable, inet_frags_free_cb, NULL);
 
 	if (llist_add(&fqdir->free_list, &fqdir_free_list))
-		queue_work(system_wq, &fqdir_free_work);
+		queue_delayed_work(system_wq, &fqdir_free_work, HZ);
 }
 
 int fqdir_init(struct fqdir **fqdirp, struct inet_frags *f, struct net *net)