rcu: Modulate grace-period slow init to normalize delay

Currently, the larger the gp_init_delay boot parameter, the slower
rcutorture will sequence through grace periods.  This commit avoids this
issue by decreasing the probability of slowing initialization of a given
grace period as the degree of slowness increases.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -169,7 +169,17 @@ module_param(gp_init_delay, int, 0644);
 #else /* #ifdef CONFIG_RCU_TORTURE_TEST_SLOW_INIT */
 static const int gp_init_delay;
 #endif /* #else #ifdef CONFIG_RCU_TORTURE_TEST_SLOW_INIT */
-#define PER_RCU_NODE_PERIOD 10	/* Number of grace periods between delays. */
+
+/*
+ * Number of grace periods between delays, normalized by the duration of
+ * the delay.  The longer the the delay, the more the grace periods between
+ * each delay.  The reason for this normalization is that it means that,
+ * for non-zero delays, the overall slowdown of grace periods is constant
+ * regardless of the duration of the delay.  This arrangement balances
+ * the need for long delays to increase some race probabilities with the
+ * need for fast grace periods to increase other race probabilities.
+ */
+#define PER_RCU_NODE_PERIOD 3	/* Number of grace periods between delays. */
 
 /*
  * Track the rcutorture test sequence number and the update version
@@ -1848,7 +1858,8 @@ static int rcu_gp_init(struct rcu_state *rsp)
 		cond_resched_rcu_qs();
 		WRITE_ONCE(rsp->gp_activity, jiffies);
 		if (gp_init_delay > 0 &&
-		    !(rsp->gpnum % (rcu_num_nodes * PER_RCU_NODE_PERIOD)))
+		    !(rsp->gpnum %
+		      (rcu_num_nodes * PER_RCU_NODE_PERIOD * gp_init_delay)))
 			schedule_timeout_uninterruptible(gp_init_delay);
 	}