Re: [PATCH 3/5] tick/broadcast: Prefer per-cpu oneshot wakeup timers to broadcast

Some SoCs have two per-cpu timer implementations where the timer with
the higher rating stops in deep idle (i.e. suffers from
CLOCK_EVT_FEAT_C3STOP) but is otherwise preferable to the timer with the
lower rating. In such a design, we rely on a global broadcast timer and
IPIs to wake up from deep idle states.

To avoid the reliance on a global broadcast timer and also to reduce the
overhead associated with the IPI wakeups, extend
tick_install_broadcast_device() to manage per-cpu wakeup timers
separately from the broadcast device.

For now, these timers remain unused.

Cc: Frederic Weisbecker <redacted>
Cc: Thomas Gleixner <redacted>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Lorenzo Colitti <redacted>
Cc: John Stultz <redacted>
Cc: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>
---
  kernel/time/tick-broadcast.c | 57 +++++++++++++++++++++++++++++++++++-
  kernel/time/tick-common.c    |  2 +-
  kernel/time/tick-internal.h  |  4 +--
  3 files changed, 59 insertions(+), 4 deletions(-)

diff --git a/kernel/time/tick-broadcast.c b/kernel/time/tick-broadcast.c
index f3f2f4ba4321..8bd8cd69c8c9 100644
--- a/kernel/time/tick-broadcast.c
+++ b/kernel/time/tick-broadcast.c

@@ -33,6 +33,8 @@ static int tick_broadcast_forced;
  static __cacheline_aligned_in_smp DEFINE_RAW_SPINLOCK(tick_broadcast_lock);
  
  #ifdef CONFIG_TICK_ONESHOT
+static DEFINE_PER_CPU(struct clock_event_device *, tick_oneshot_wakeup_device);
+
  static void tick_broadcast_setup_oneshot(struct clock_event_device *bc);
  static void tick_broadcast_clear_oneshot(int cpu);
  static void tick_resume_broadcast_oneshot(struct clock_event_device *bc);

@@ -88,13 +90,59 @@ static bool tick_check_broadcast_device(struct clock_event_device *curdev,
  	return !curdev || newdev->rating > curdev->rating;
  }
  
+#ifdef CONFIG_TICK_ONESHOT
+static struct clock_event_device *tick_get_oneshot_wakeup_device(int cpu)
+{
+	return per_cpu(tick_oneshot_wakeup_device, cpu);
+}
+
+static bool tick_set_oneshot_wakeup_device(struct clock_event_device *newdev,
+					   int cpu)
+{
+	struct clock_event_device *curdev;
+
+	if (!newdev)
+		goto set_device;
+
+	if ((newdev->features & CLOCK_EVT_FEAT_DUMMY) ||
+	    (newdev->features & CLOCK_EVT_FEAT_C3STOP) ||
+	    !(newdev->features & CLOCK_EVT_FEAT_ONESHOT))
+		return false;
+
+	if (!cpumask_equal(newdev->cpumask, cpumask_of(cpu)))
+		return false;
+
+	curdev = tick_get_oneshot_wakeup_device(cpu);
+	if (curdev && newdev->rating <= curdev->rating)
+		return false;
+
+set_device:
+	per_cpu(tick_oneshot_wakeup_device, cpu) = newdev;
+	return true;
+}
+#else
+static struct clock_event_device *tick_get_oneshot_wakeup_device(int cpu)
+{
+	return NULL;
+}
+
+static bool tick_set_oneshot_wakeup_device(struct clock_event_device *newdev,
+					   int cpu)
+{
+	return false;
+}
+#endif
+
  /*
   * Conditionally install/replace broadcast device
   */
-void tick_install_broadcast_device(struct clock_event_device *dev)
+void tick_install_broadcast_device(struct clock_event_device *dev, int cpu)
  {
  	struct clock_event_device *cur = tick_broadcast_device.evtdev;
  
+	if (tick_set_oneshot_wakeup_device(dev, cpu))
+		return;
+
  	if (!tick_check_broadcast_device(cur, dev))
  		return;
  

@@ -996,6 +1044,13 @@ void hotplug_cpu__broadcast_tick_pull(int deadcpu)
   */
  static void tick_broadcast_oneshot_offline(unsigned int cpu)
  {
+	struct clock_event_device *dev = tick_get_oneshot_wakeup_device(cpu);
+
+	if (dev) {
+		clockevents_switch_state(dev, CLOCK_EVT_STATE_DETACHED);
+		tick_set_oneshot_wakeup_device(NULL, cpu);
+	}
+
  	/*
  	 * Clear the broadcast masks for the dead cpu, but do not stop
  	 * the broadcast device!

diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index e15bc0ef1912..d663249652ef 100644
--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c

@@ -373,7 +373,7 @@ void tick_check_new_device(struct clock_event_device *newdev)
  	/*
  	 * Can the new device be used as a broadcast device ?
  	 */
-	tick_install_broadcast_device(newdev);
+	tick_install_broadcast_device(newdev, cpu);
  }
  
  /**

diff --git a/kernel/time/tick-internal.h b/kernel/time/tick-internal.h
index 7a981c9e87a4..30c89639e305 100644
--- a/kernel/time/tick-internal.h
+++ b/kernel/time/tick-internal.h

@@ -61,7 +61,7 @@ extern ssize_t sysfs_get_uname(const char *buf, char *dst, size_t cnt);
  /* Broadcasting support */
  # ifdef CONFIG_GENERIC_CLOCKEVENTS_BROADCAST
  extern int tick_device_uses_broadcast(struct clock_event_device *dev, int cpu);
-extern void tick_install_broadcast_device(struct clock_event_device *dev);
+extern void tick_install_broadcast_device(struct clock_event_device *dev, int cpu);
  extern int tick_is_broadcast_device(struct clock_event_device *dev);
  extern void tick_suspend_broadcast(void);
  extern void tick_resume_broadcast(void);

@@ -72,7 +72,7 @@ extern int tick_broadcast_update_freq(struct clock_event_device *dev, u32 freq);
  extern struct tick_device *tick_get_broadcast_device(void);
  extern struct cpumask *tick_get_broadcast_mask(void);
  # else /* !CONFIG_GENERIC_CLOCKEVENTS_BROADCAST: */
-static inline void tick_install_broadcast_device(struct clock_event_device *dev) { }
+static inline void tick_install_broadcast_device(struct clock_event_device *dev, int cpu) { }
  static inline int tick_is_broadcast_device(struct clock_event_device *dev) { return 0; }
  static inline int tick_device_uses_broadcast(struct clock_event_device *dev, int cpu) { return 0; }
  static inline void tick_do_periodic_broadcast(struct clock_event_device *d) { }