Re: [PATCH 06/10] timer: Report ignored local enqueue in nohz mode
On Tue, Mar 16, 2021 at 04:27:56PM +0100, Peter Zijlstra wrote:
> On Thu, Mar 11, 2021 at 01:37:04PM +0100, Frederic Weisbecker wrote:
> > 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.
> >
> > Reviewed-by: Rafael J. Wysocki
> > Signed-off-by: Frederic Weisbecker
> > Cc: Peter Zijlstra
> > Cc: Thomas Gleixner
> > Cc: Ingo Molnar
> > Cc: Paul E. McKenney
> > ---
> > 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);
>
> I'm not entirely sure I understand this one. What's the callchain that
> leads to this?
That's while calling add_timer*() or mod_timer() on an idle target.
Now the issue is only relevant when these timer functions are called
after cpuidle_select(), which arguably makes a small vulnerable window
that could be spotted in the future if the timer functions are called
after instrumentation_end()?
Thanks.
Re: [PATCH 06/10] timer: Report ignored local enqueue in nohz mode
On Thu, Mar 11, 2021 at 01:37:04PM +0100, Frederic Weisbecker wrote:
> 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.
>
> Reviewed-by: Rafael J. Wysocki
> Signed-off-by: Frederic Weisbecker
> Cc: Peter Zijlstra
> Cc: Thomas Gleixner
> Cc: Ingo Molnar
> Cc: Paul E. McKenney
> ---
> 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);
I'm not entirely sure I understand this one. What's the callchain that
leads to this?
[PATCH 06/10] 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.
Reviewed-by: Rafael J. Wysocki
Signed-off-by: Frederic Weisbecker
Cc: Peter Zijlstra
Cc: Thomas Gleixner
Cc: Ingo Molnar
Cc: Paul E. McKenney
---
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
