[RFC PATCH 8/8] timer: Report ignored local enqueue in nohz mode

Frederic Weisbecker
2021-01-09T02:07:39Z

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 | 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