linux-kernel - [PATCH tip/core/rcu 02/12] timer: Report ignored local enqueue in nohz mode

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Message-Id: <20210304002311.23655-2-paulmck@kernel.org>
Date:   Wed,  3 Mar 2021 16:23:01 -0800
From:   paulmck@...nel.org
To:     rcu@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, kernel-team@...com, mingo@...nel.org,
        jiangshanlai@...il.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
        dhowells@...hat.com, edumazet@...gle.com, fweisbec@...il.com,
        oleg@...hat.com, joel@...lfernandes.org,
        Frederic Weisbecker <frederic@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        "Paul E . McKenney" <paulmck@...nel.org>
Subject: [PATCH tip/core/rcu 02/12] timer: Report ignored local enqueue in nohz mode

From: Frederic Weisbecker <frederic@...nel.org>

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@...radead.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar<mingo@...nel.org>
Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
---
 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 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);
-- 
2.9.5