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From:   paulmck@kernel.org
To:     rcu@vger.kernel.org
Cc:     linux-kernel@vger.kernel.org, kernel-team@fb.com, mingo@kernel.org,
        jiangshanlai@gmail.com, akpm@linux-foundation.org,
        mathieu.desnoyers@efficios.com, josh@joshtriplett.org,
        tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org,
        dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com,
        oleg@redhat.com, joel@joelfernandes.org,
        Frederic Weisbecker <frederic@kernel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
        "Paul E . McKenney" <paulmck@kernel.org>
Subject: [PATCH tip/core/rcu 02/12] timer: Report ignored local enqueue in nohz mode
Date:   Wed,  3 Mar 2021 16:23:01 -0800
Message-Id: <20210304002311.23655-2-paulmck@kernel.org>
In-Reply-To: <20210304002225.GA23492@paulmck-ThinkPad-P72>
References: <20210304002225.GA23492@paulmck-ThinkPad-P72>
Precedence: bulk

From: Frederic Weisbecker <frederic@kernel.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@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>
---
 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