[RFC PATCH 4/5] cpuidle/ppc: CPU goes tickless if there are no arch-specific constraints

[Preeti U Murthy <preeti@linux.vnet.ibm.com>]

In the current design of timer offload framework, the broadcast cpu should
*not* go into tickless idle so as to avoid missed wakeups on CPUs in deep idle states.

Since we prevent the CPUs entering deep idle states from programming the lapic of the
broadcast cpu for their respective next local events for reasons mentioned in
PATCH[3/5], the broadcast CPU checks if there are any CPUs to be woken up during
each of its timer interrupt programmed to its local events.

With tickless idle, the broadcast CPU might not get a timer interrupt till after
many ticks which can result in missed wakeups on CPUs in deep idle states. By
disabling tickless idle, worst case, the tick_sched hrtimer will trigger a
timer interrupt every period to check for broadcast.

However the current setup of tickless idle does not let us make the choice
of tickless on individual cpus. NOHZ_MODE_INACTIVE which disables tickless idle,
is a system wide setting. Hence resort to an arch specific call to check if a cpu
can go into tickless idle.

Signed-off-by: Preeti U Murthy <preeti@linux.vnet.ibm.com>
---

 arch/powerpc/kernel/time.c |    5 +++++
 kernel/time/tick-sched.c   |    7 +++++++
 2 files changed, 12 insertions(+)

diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index 8ed0fb3..68a636f 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -862,6 +862,11 @@ static void decrementer_timer_broadcast(const struct cpumask *mask)
 	arch_send_tick_broadcast(mask);
 }
 
+int arch_can_stop_idle_tick(int cpu)
+{
+	return cpu != bc_cpu;
+}
+
 static void register_decrementer_clockevent(int cpu)
 {
 	struct clock_event_device *dec = &per_cpu(decrementers, cpu);
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 6960172..e9ffa84 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -700,8 +700,15 @@ static void tick_nohz_full_stop_tick(struct tick_sched *ts)
 #endif
 }
 
+int __weak arch_can_stop_idle_tick(int cpu)
+{
+	return 1;
+}
+
 static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
 {
+	if (!arch_can_stop_idle_tick(cpu))
+		return false;
 	/*
 	 * If this cpu is offline and it is the one which updates
 	 * jiffies, then give up the assignment and let it be taken by