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Date:   Sat,  9 Jan 2021 03:05:36 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, stable@...r.kernel.org
Subject: [RFC PATCH 8/8] timer: Report ignored local enqueue in nohz mode

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.

Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar<mingo@...nel.org>
Cc: Paul E. McKenney <paulmck@...nel.org>
Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
---
 kernel/sched/core.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 6056f0374674..6c8b04272a9a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -715,6 +715,26 @@ int get_nohz_timer_target(void)
 	return cpu;
 }
 
+static void wake_idle_assert_possible(void)
+{
+#ifdef CONFIG_SCHED_DEBUG
+	/* Timers are re-evaluated after idle IRQs */
+	if (in_hardirq())
+		return;
+	/*
+	 * Same as hardirqs, assuming they are executing
+	 * on 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.
+	 */
+	if (in_serving_softirq())
+		return;
+
+	WARN_ON_ONCE("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
@@ -729,8 +749,10 @@ 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

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