[PATCH 06/10] timer: Report ignored local enqueue in nohz mode

[Frederic Weisbecker]

Enqueuing a local timer after the tick has been stopped will result in
the timer being ignored until the next random interrupt.

Perform sanity checks to report these situations.

Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/sched/core.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ca2bb629595f..24552911f92b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -674,6 +674,22 @@ int get_nohz_timer_target(void)
 	return cpu;
 }
 
+/* Make sure the timer won't be ignored in dynticks-idle case */
+static void wake_idle_assert_possible(void)
+{
+#ifdef CONFIG_SCHED_DEBUG
+	/*
+	 * Timers are re-evaluated after idle IRQs. In case of softirq,
+	 * we assume IRQ tail. Ksoftirqd shouldn't reach here as the
+	 * timer base wouldn't be idle. And inline softirq processing
+	 * after a call to local_bh_enable() within idle loop sound too
+	 * fun to be considered here.
+	 */
+	WARN_ONCE(in_task(),
+		  "Late timer enqueue may be ignored\n");
+#endif
+}
+
 /*
  * When add_timer_on() enqueues a timer into the timer wheel of an
  * idle CPU then this timer might expire before the next timer event
@@ -688,8 +704,10 @@ static void wake_up_idle_cpu(int cpu)
 {
 	struct rq *rq = cpu_rq(cpu);
 
-	if (cpu == smp_processor_id())
+	if (cpu == smp_processor_id()) {
+		wake_idle_assert_possible();
 		return;
+	}
 
 	if (set_nr_and_not_polling(rq->idle))
 		smp_send_reschedule(cpu);
-- 
2.25.1