timer: Report ignored local enqueue in nohz mode?

timer: Report ignored local enqueue in nohz mode?

[Paul E. McKenney]

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);

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

[Frederic Weisbecker]

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

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

[Paul E. McKenney]

On Fri, Mar 05, 2021 at 02:41:11PM +0100, Frederic Weisbecker wrote:
> 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.

Actually, please feel free to run this up the normal nohz path.
I will remove my version once yours hits mainline, as I did with
the others.

I was just curious.  ;-)

							Thanx, Paul

> 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
>