timer: Report ignored local enqueue in nohz mode?

["Paul E. McKenney" <paulmck@kernel.org>]

Hello, Frederic!

I don't see the following commit in mainline, but figured I should
check with you guys to see if the problem got solved in some other way.
Unless I hear otherwise, I will continue to carry this patch in -rcu
and will send it along for the v5.13 merge window.

							Thanx, Paul

------------------------------------------------------------------------

commit 650c433b46ca9601378c9d170d5dc0e24dd42822
Author: Frederic Weisbecker <frederic@kernel.org>
Date:   Fri Jan 8 13:50:12 2021 +0100

    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.
    
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Ingo Molnar<mingo@kernel.org>
    Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
    Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
    Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ca2bb62..4822371 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -674,6 +674,26 @@ 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
@@ -688,8 +708,10 @@ 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);