rcutorture: Add ftrace-compatible timestamp to GP# failure/close-call output

This commit adds an ftrace-compatible microsecond-scale timestamp
to the failure/close-call output, but only in kernels built with
CONFIG_RCU_TORTURE_TEST_LOG_GP=y.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>

Author: paulmckrcu

kernel/rcu/rcutorture.c

@@ -275,6 +275,7 @@ struct rt_read_seg {
 	int rt_end_cpu;
 	unsigned long long rt_gp_seq;
 	unsigned long long rt_gp_seq_end;
+	u64 rt_ts;
 };
 static int err_segs_recorded;
 static struct rt_read_seg err_segs[RCUTORTURE_RDR_MAX_SEGS];
@@ -1989,6 +1990,7 @@ static void rcutorture_one_extend(int *readstate, int newstate, bool insoftirq,
 	// Sample grace-period sequence number, as good a place as any.
 	if (IS_ENABLED(CONFIG_RCU_TORTURE_TEST_LOG_GP) && cur_ops->gather_gp_seqs) {
 		rtrsp->rt_gp_seq = cur_ops->gather_gp_seqs();
+		rtrsp->rt_ts = ktime_get_mono_fast_ns();
 		if (!first)
 			rtrsp[-1].rt_gp_seq_end = rtrsp->rt_gp_seq;
 	}
@@ -3663,7 +3665,11 @@ rcu_torture_cleanup(void)
 			pr_alert("\t: No segments recorded!!!\n");
 		firsttime = 1;
 		for (i = 0; i < rt_read_nsegs; i++) {
-			pr_alert("\t%d: %#4x", i, err_segs[i].rt_readstate);
+			if (IS_ENABLED(CONFIG_RCU_TORTURE_TEST_LOG_GP))
+				pr_alert("\t%lluus ", div64_u64(err_segs[i].rt_ts, 1000ULL));
+			else
+				pr_alert("\t");
+			pr_cont("%d: %#4x", i, err_segs[i].rt_readstate);
 			if (err_segs[i].rt_delay_jiffies != 0) {
 				pr_cont("%s%ldjiffies", firsttime ? "" : "+",
 					err_segs[i].rt_delay_jiffies);