Re: [PATCH 16/20] sched/idle: Use explicit broadcast oneshot control function

[Rafael J. Wysocki]

On Tuesday, April 28, 2015 02:58:37 PM Sudeep Holla wrote:
> 
> On 28/04/15 15:14, Rafael J. Wysocki wrote:
> > On Tuesday, April 28, 2015 03:37:44 PM Rafael J. Wysocki wrote:
> >> On Tuesday, April 28, 2015 03:31:54 PM Rafael J. Wysocki wrote:
> >>> On Tuesday, April 28, 2015 02:37:10 PM Linus Walleij wrote:
> >>>> On Tue, Apr 28, 2015 at 2:19 PM, Rafael J. Wysocki <rafael@xxxxxxxxxx> wrote:
> >>>>> Sudeep:
> >>>>>> At-least I observed issue only when I am using hardware broadcast timer.
> >>>>>> It doesn't hang when I am using hrtimer as broadcast timer in which case
> >>>>>> one of the cpu will be not enter deeper idle states that lose timer.
> >>>>>> I will rerun on v4.1-rc1 and post the complete log.
> >>>>>
> >>>>> So the bug here is that cpuidle_enter() enables interrupts, so the
> >>>>> assumption about them being not enabled made by
> >>>>> tick_broadcast_oneshot_control() is actually not valid.
> >>>>>
> >>>>> It looks like we need to acquire the clockevents_lock at least in this
> >>>>> particular case.  Let me see where to put it and I'll send a patch for
> >>>>> testing.
> >>>>
> >>>> Aha that looks very much like it. Put me on the patch and I'll
> >>>> take it for a spin.
> >>>
> >>> OK, so something like the below for starters (the _irqsave variant is used to
> >>> avoid adding one more WARN_ON(irqs_disabled()) in there).
> >>>
> >>> I haven't tested it, but then I can't reproduce the original issue in the
> >>> first place.
> >>
> >> Of course, the whole "broadcast" thing could be done from cpuidle_enter()
> >> in the first place, but then we could not avoid the problem with the cpuidle
> >> *callback* enabling interrupts possibly in there anyway (not to mention the
> >> "coupled" stuff).
> >
> > That said, if the given state is marked with CPUIDLE_FLAG_TIMER_STOP, I really
> > wouldn't expect it to re-enable interrupts on exit and the "coupled" thing
> > seems to be fundamentally at odds with that flag either.
> >
> > So it should be possible to move the "broadcast" logic into the cpuidle layer,
> > which I'm going to try to do.
> >
> 
> Makes sense.
> 
> > Please test the patch I've sent, though, as it should bring the code back to
> > where it was before the clockevents_notify() removal and it'd be good to verify
> > that.
> >
> 
> I tested your patch and it works now. Anyways I am continuing to run
> stress tests on my board. I will report if I find any issues.

Great, thanks!

Below is the patch I came up with in the meantime.

This moves the "switch to broadcast" timer logic into
cpuidle_enter_state() which allows tick_broadcast_exit() to be
called directly with interrupts disabled (as required), but
it also adds a fallback branch reflecting the 4.0 and earlier
behavior for idle states that enable interrupts on exit
from their ->enter callbacks.

I'm not aware of any valid cases when CPUIDLE_FLAG_TIMER_STOP can be
set for such states, but people may try to add stuff like that in the
future, so it's better to catch that (hence the WARN_ON_ONCE) and do
our best to handle it gracefully anyway, IMO.

The "if (entered_state == -EBUSY)" check is conservative.  It may
be better to do "if (entered_state < 0)" and fall back to the default
on all errors, but that's not what we do today (I guess the concern
would be "what if the state ->enter returns an error after entering
and exiting the idle state, in which case we may miss a wakeup event
if we fall back to the default").

---
 drivers/cpuidle/cpuidle.c  |   16 ++++++++++++++++
 include/linux/clockchips.h |    2 ++
 kernel/sched/idle.c        |   16 ++--------------
 kernel/time/clockevents.c  |   13 +++++++++++++
 4 files changed, 33 insertions(+), 14 deletions(-)

Index: linux-pm/include/linux/clockchips.h
===================================================================
--- linux-pm.orig/include/linux/clockchips.h
+++ linux-pm/include/linux/clockchips.h
@@ -198,9 +198,11 @@ extern int tick_receive_broadcast(void);
 # if defined(CONFIG_GENERIC_CLOCKEVENTS_BROADCAST) && defined(CONFIG_TICK_ONESHOT)
 extern void tick_setup_hrtimer_broadcast(void);
 extern int tick_check_broadcast_expired(void);
+extern void tick_broadcast_exit_idle_fallback(void);
 # else
 static inline int tick_check_broadcast_expired(void) { return 0; }
 static inline void tick_setup_hrtimer_broadcast(void) { }
+static inline void tick_broadcast_exit_idle_fallback(void) { }
 # endif
 
 extern int clockevents_notify(unsigned long reason, void *arg);
Index: linux-pm/kernel/time/clockevents.c
===================================================================
--- linux-pm.orig/kernel/time/clockevents.c
+++ linux-pm/kernel/time/clockevents.c
@@ -735,6 +735,19 @@ static ssize_t sysfs_unbind_tick_dev(str
 static DEVICE_ATTR(unbind_device, 0200, NULL, sysfs_unbind_tick_dev);
 
 #ifdef CONFIG_GENERIC_CLOCKEVENTS_BROADCAST
+/**
+ * tick_broadcast_exit_idle_fallback - Fallback broadcast oneshot mode exit.
+ *
+ * Called from within the CPU idle subsystem when exiting the broadcast oneshot
+ * mode with interrupts enabled (fallback case only).
+ */
+void tick_broadcast_exit_idle_fallback(void)
+{
+	raw_spin_lock_irq(&clockevents_lock);
+	tick_broadcast_exit();
+	raw_spin_unlock_irq(&clockevents_lock);
+}
+
 static struct device tick_bc_dev = {
 	.init_name	= "broadcast",
 	.id		= 0,
Index: linux-pm/kernel/sched/idle.c
===================================================================
--- linux-pm.orig/kernel/sched/idle.c
+++ linux-pm/kernel/sched/idle.c
@@ -81,7 +81,6 @@ static void cpuidle_idle_call(void)
 	struct cpuidle_device *dev = __this_cpu_read(cpuidle_devices);
 	struct cpuidle_driver *drv = cpuidle_get_cpu_driver(dev);
 	int next_state, entered_state;
-	unsigned int broadcast;
 	bool reflect;
 
 	/*
@@ -150,17 +149,6 @@ static void cpuidle_idle_call(void)
 		goto exit_idle;
 	}
 
-	broadcast = drv->states[next_state].flags & CPUIDLE_FLAG_TIMER_STOP;
-
-	/*
-	 * Tell the time framework to switch to a broadcast timer
-	 * because our local timer will be shutdown. If a local timer
-	 * is used from another cpu as a broadcast timer, this call may
-	 * fail if it is not available
-	 */
-	if (broadcast && tick_broadcast_enter())
-		goto use_default;
-
 	/* Take note of the planned idle state. */
 	idle_set_state(this_rq(), &drv->states[next_state]);
 
@@ -174,8 +162,8 @@ static void cpuidle_idle_call(void)
 	/* The cpu is no longer idle or about to enter idle. */
 	idle_set_state(this_rq(), NULL);
 
-	if (broadcast)
-		tick_broadcast_exit();
+	if (entered_state == -EBUSY)
+		goto use_default;
 
 	/*
 	 * Give the governor an opportunity to reflect on the outcome
Index: linux-pm/drivers/cpuidle/cpuidle.c
===================================================================
--- linux-pm.orig/drivers/cpuidle/cpuidle.c
+++ linux-pm/drivers/cpuidle/cpuidle.c
@@ -158,9 +158,18 @@ int cpuidle_enter_state(struct cpuidle_d
 	int entered_state;
 
 	struct cpuidle_state *target_state = &drv->states[index];
+	bool broadcast = !!(target_state->flags & CPUIDLE_FLAG_TIMER_STOP);
 	ktime_t time_start, time_end;
 	s64 diff;
 
+	/*
+	 * Tell the time framework to switch to a broadcast timer because our
+	 * local timer will be shut down.  If a local timer is used from another
+	 * CPU as a broadcast timer, this call may fail if it is not available.
+	 */
+	if (broadcast && tick_broadcast_enter())
+		return -EBUSY;
+
 	trace_cpu_idle_rcuidle(index, dev->cpu);
 	time_start = ktime_get();
 
@@ -169,6 +178,13 @@ int cpuidle_enter_state(struct cpuidle_d
 	time_end = ktime_get();
 	trace_cpu_idle_rcuidle(PWR_EVENT_EXIT, dev->cpu);
 
+	if (broadcast) {
+		if (WARN_ON_ONCE(!irqs_disabled()))
+			tick_broadcast_exit_idle_fallback();
+		else
+			tick_broadcast_exit();
+	}
+
 	if (!cpuidle_state_is_coupled(dev, drv, entered_state))
 		local_irq_enable();
 

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