[PATCH tip/core/rcu 07/17] rcutorture: Also use GP sequence to judge forward progress

[Paul E. McKenney]

Currently, rcutorture relies solely on the progress of
rcu_torture_writer() to judge grace-period forward progress.  In theory,
this is the gold standard of forward progress, but in practice rcutorture
separately detects and reports rcu_torture_writer() stalls.  This commit
therefore adds the grace-period sequence number (when provided) to the
judgment of grace-period forward progress, which makes it easier to
distinguish between failure of actual grace periods to progress on the
one hand and downstream forward-progress failures on the other.

For example, given this change, if rcu_torture_writer() stalls,
but rcu_torture_fwd_prog() does not complain, then the grace-period
computation is working, which is a hint that the failure lies in callback
processing, wakeup of the rcu_torture_writer() kthread, or similar.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
 kernel/rcu/rcutorture.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index fd3ce6cc8eea..dee7b45b2186 100644
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -1673,7 +1673,8 @@ static int __init rcu_torture_stall_init(void)
 /* Carry out grace-period forward-progress testing. */
 static int rcu_torture_fwd_prog(void *args)
 {
-	unsigned long cvar;
+	unsigned long cver;
+	unsigned long gps;
 	int idx;
 	unsigned long stopat;
 	bool tested = false;
@@ -1681,7 +1682,8 @@ static int rcu_torture_fwd_prog(void *args)
 	VERBOSE_TOROUT_STRING("rcu_torture_fwd_progress task started");
 	do {
 		schedule_timeout_interruptible(fwd_progress_holdoff * HZ);
-		cvar = READ_ONCE(rcu_torture_current_version);
+		cver = READ_ONCE(rcu_torture_current_version);
+		gps = cur_ops->get_gp_seq();
 		stopat = jiffies + cur_ops->stall_dur() / fwd_progress_div;
 		while (time_before(jiffies, stopat) && !torture_must_stop()) {
 			idx = cur_ops->readlock();
@@ -1692,8 +1694,9 @@ static int rcu_torture_fwd_prog(void *args)
 		}
 		if (!time_before(jiffies, stopat) && !torture_must_stop()) {
 			tested = true;
-			WARN_ON_ONCE(cvar ==
-				     READ_ONCE(rcu_torture_current_version));
+			cver = cver == READ_ONCE(rcu_torture_current_version);
+			gps = rcutorture_seq_diff(cur_ops->get_gp_seq(), gps);
+			WARN_ON_ONCE(cver && gps < 2);
 		}
 		/* Avoid slow periods, better to test when busy. */
 		stutter_wait("rcu_torture_fwd_prog");
-- 
2.17.1