[PATCH tip/core/rcu 13/17] rcutorture: Print time since GP end upon forward-progress failure

["Paul E. McKenney" <paulmck@linux.ibm.com>]

If rcutorture's forward-progress tests fail while a grace period is not
in progress, it is useful to print the time since the last grace period
ended as a way to detect failure to launch a new grace period.  This
commit therefore makes this change.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
---
 kernel/rcu/tree.c | 5 ++++-
 kernel/rcu/tree.h | 2 ++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 95a3825b1b19..4d8b50a7750a 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1997,7 +1997,8 @@ static void rcu_gp_cleanup(void)
 
 	WRITE_ONCE(rcu_state.gp_activity, jiffies);
 	raw_spin_lock_irq_rcu_node(rnp);
-	gp_duration = jiffies - rcu_state.gp_start;
+	rcu_state.gp_end = jiffies;
+	gp_duration = rcu_state.gp_end - rcu_state.gp_start;
 	if (gp_duration > rcu_state.gp_max)
 		rcu_state.gp_max = gp_duration;
 
@@ -2683,6 +2684,8 @@ void rcu_fwd_progress_check(unsigned long j)
 			__func__, jiffies - rcu_state.gp_start);
 		show_rcu_gp_kthreads();
 	} else {
+		pr_info("%s: Last GP end %lu jiffies ago\n",
+			__func__, jiffies - rcu_state.gp_end);
 		preempt_disable();
 		rdp = this_cpu_ptr(&rcu_data);
 		rcu_check_gp_start_stall(rdp->mynode, rdp, j);
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 14f6758f0989..3397089490ec 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -328,6 +328,8 @@ struct rcu_state {
 						/*  force_quiescent_state(). */
 	unsigned long gp_start;			/* Time at which GP started, */
 						/*  but in jiffies. */
+	unsigned long gp_end;			/* Time last GP ended, again */
+						/*  in jiffies. */
 	unsigned long gp_activity;		/* Time of last GP kthread */
 						/*  activity in jiffies. */
 	unsigned long gp_req_activity;		/* Time of last GP request */
-- 
2.17.1