[PATCH 06/13] nohz: Introduce full dynticks' default timekeeping target

[Frederic Weisbecker]

When a full dynticks CPU wakes up while the whole rest of the system
is idle, we need to wake up the CPU in charge of the timekeeping duty
handling.

As of today, the CPU that maintains this duty is CPU 0 when
CONFIG_NO_HZ_FULL=y. So referring to tick_do_timer_cpu like we
currently do is correct. But this behaviour is subject to change
in the future because we want to balance the timekeeping duty over all
the CPUs outside the full dynticks range.

As such we now need to define a default timekeeping CPU which receives
the timekeeping wakeup IPIs and which can't be offlined so that it's
guaranteed to always be present for full dynticks CPUs housekeeping.

So lets stick to CPU 0 for this purpose. It's convenient because
rejecting any other CPU's offlining request may result in suspend
failure.

We can optimize this solution later by adaptively sending the IPI
to a potential timekeeping CPU that is already running a non idle task.

Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: John Stultz <john.stultz@xxxxxxxxxx>
Cc: Alex Shi <alex.shi@xxxxxxxxxx>
Cc: Kevin Hilman <khilman@xxxxxxxxxx>
---
 include/linux/tick.h     | 16 ++++++++++++++++
 kernel/rcu/tree_plugin.h |  4 ++--
 kernel/time/tick-sched.c |  2 +-
 3 files changed, 19 insertions(+), 3 deletions(-)

diff --git a/include/linux/tick.h b/include/linux/tick.h
index cf2fd34..af98d2c 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -180,6 +180,22 @@ static inline bool tick_nohz_full_cpu(int cpu)
 }
 
 /**
+ * tick_timekeeping_default_cpu - seek timekeeping default CPU
+
+ * @return the default target which we send an IPI to
+ * when a full dynticks CPU wakes up and exits from full
+ * system idle state.
+ *
+ * This target is always CPU 0 in full dynticks environment.
+ * If we were to pick up any other CPU, that would result in suspend
+ * failures due to rejected offlining request.
+ */
+static inline int tick_timekeeping_default_cpu(void)
+{
+	return 0;
+}
+
+/**
  * tick_timeeping_cpu - check if a CPU is elligble to handle timekeeping duty
  * @cpu:	the cpu to check
  *
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 84d90c8..1795265 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -2488,7 +2488,7 @@ void rcu_sysidle_force_exit(void)
 				      oldstate, RCU_SYSIDLE_NOT);
 		if (oldstate == newoldstate &&
 		    oldstate == RCU_SYSIDLE_FULL_NOTED) {
-			smp_send_reschedule(tick_do_timer_cpu);
+			smp_send_reschedule(tick_timekeeping_default_cpu());
 			return; /* We cleared it, done! */
 		}
 		oldstate = newoldstate;
@@ -2597,7 +2597,7 @@ static bool is_sysidle_rcu_state(struct rcu_state *rsp)
  */
 static void rcu_bind_gp_kthread(void)
 {
-	int cpu = ACCESS_ONCE(tick_do_timer_cpu);
+	int cpu = tick_timekeeping_default_cpu();
 
 	if (cpu < 0 || cpu >= nr_cpu_ids)
 		return;
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index ea0d411..9a91c31 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -305,7 +305,7 @@ static int tick_nohz_cpu_down_callback(struct notifier_block *nfb,
 		 * If we handle the timekeeping duty for full dynticks CPUs,
 		 * we can't safely shutdown that CPU.
 		 */
-		if (tick_nohz_full_running && tick_do_timer_cpu == cpu)
+		if (tick_nohz_full_running && tick_timekeeping_default_cpu() == cpu)
 			return NOTIFY_BAD;
 		break;
 	}
-- 
1.8.3.1

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