[PATCH] Add missing kernel log when enabling 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.

Signed-off-by: Ani Sinha <ani@xxxxxxxxxxx>
---
 kernel/time/tick-sched.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 828b091501ca..a82480c036e2 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 with current sched clock");
+		if (ts->tick_stopped)
+			tick_nohz_restart_sched_tick(ts, ktime_get());
+	}
 #endif
 }
 
-- 
2.25.1