Re: timer: Report ignored local enqueue in nohz mode?

[Frederic Weisbecker <frederic@kernel.org>]

On Wed, Mar 03, 2021 at 11:49:45AM -0800, Paul E. McKenney wrote:
> 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.

I have it included in a nohz series I'm about to post but since RCU is the
motivation behind doing this, it's fine if you carry it.

I've just modified it a bit after a review from Peter:

---
From 7876725b8631147967bb9e65158ef1cb2bb94372 Mon Sep 17 00:00:00 2001
From: Frederic Weisbecker <frederic@kernel.org>
Date: Fri, 8 Jan 2021 13:50:12 +0100
Subject: [PATCH] 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@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar<mingo@kernel.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---
 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