[PATCH V2] Add kernel logs when sched clock unstable and NO_HZ_FULL is not possible

[Ani Sinha]

Commit 4f49b90abb4aca ("sched-clock: Migrate to use new tick
dependency mask model") had also removed the kernel warning
message informing the user that it was not possible to turn
on NO_HZ_FULL. Adding back that log message here. It is
unhelpful when the kernel turns off NO_HZ_FULL silently
without informing anyone.
Also added a kernel log when sched clock is marked as unstable.

Signed-off-by: Ani Sinha <ani@xxxxxxxxxxx>
---
 kernel/sched/clock.c     |  5 ++++-
 kernel/time/tick-sched.c | 14 +++++++++++---
 2 files changed, 15 insertions(+), 4 deletions(-)

changelog:
v1: original patch
v2: updated log message to make it non specific to sched clock. Also
    added another log when sched clock is marked as unstable.
 
diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index c2b2859ddd82..49f8c68faf56 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -192,8 +192,11 @@ void clear_sched_clock_stable(void)
 
 	smp_mb(); /* matches sched_clock_init_late() */
 
-	if (static_key_count(&sched_clock_running.key) == 2)
+	if (static_key_count(&sched_clock_running.key) == 2) {
 		__clear_sched_clock_stable();
+		WARN_ONCE(sched_clock_stable(),
+			  "sched clock is now marked unstable.");
+	}
 }
 
 static void __sched_clock_gtod_offset(void)
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 828b091501ca..6e583aa2d160 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -937,10 +937,18 @@ static void tick_nohz_full_update_tick(struct tick_sched *ts)
 	if (!ts->tick_stopped && ts->nohz_mode == NOHZ_MODE_INACTIVE)
 		return;
 
-	if (can_stop_full_tick(cpu, ts))
+	if (can_stop_full_tick(cpu, ts)) {
 		tick_nohz_stop_sched_tick(ts, cpu);
-	else if (ts->tick_stopped)
-		tick_nohz_restart_sched_tick(ts, ktime_get());
+	} else {
+		/*
+		 * Don't allow the user to think they can get
+		 * full NO_HZ with this machine.
+		 */
+		WARN_ONCE(tick_nohz_full_running,
+			  "NO_HZ_FULL will not work for the current system.");
+		if (ts->tick_stopped)
+			tick_nohz_restart_sched_tick(ts, ktime_get());
+	}
 #endif
 }
 
-- 
2.25.1