time: tick-sched: Reorganize idle tick management code

Prepare the scheduler tick code for reworking the idle loop to
avoid stopping the tick in some cases.

The idea is to split the nohz idle entry call to decouple the idle
time stats accounting and preparatory work from the actual tick stop
code, in order to later be able to delay the tick stop once we reach
more power-knowledgeable callers.

Move away the tick_nohz_start_idle() invocation from
__tick_nohz_idle_enter(), rename the latter to
__tick_nohz_idle_stop_tick() and define tick_nohz_idle_stop_tick()
as a wrapper around it for calling it from the outside.

Make tick_nohz_idle_enter() only call tick_nohz_start_idle() instead
of calling the entire __tick_nohz_idle_enter(), add another wrapper
disabling and enabling interrupts around tick_nohz_idle_stop_tick()
and make the current callers of tick_nohz_idle_enter() call it too
to retain their current functionality.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

2 files changed, 26 insertions, 21 deletions

diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index 2975f195e1c4..c0bc111878e6 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -216,6 +216,7 @@ static void do_idle(void)
 
 	__current_set_polling();
 	tick_nohz_idle_enter();
+	tick_nohz_idle_stop_tick_protected();
 
 	while (!need_resched()) {
 		check_pgt_cache();
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 5d4a0342f934..678349aec483 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -528,14 +528,11 @@ static void tick_nohz_stop_idle(struct tick_sched *ts, ktime_t now)
 	sched_clock_idle_wakeup_event();
 }
 
-static ktime_t tick_nohz_start_idle(struct tick_sched *ts)
+static void tick_nohz_start_idle(struct tick_sched *ts)
 {
-	ktime_t now = ktime_get();
-
-	ts->idle_entrytime = now;
+	ts->idle_entrytime = ktime_get();
 	ts->idle_active = 1;
 	sched_clock_idle_sleep_event();
-	return now;
 }
 
 /**
@@ -894,19 +891,21 @@ static bool can_stop_idle_tick(int cpu, struct tick_sched *ts)
 	return true;
 }
 
-static void __tick_nohz_idle_enter(struct tick_sched *ts)
+static void __tick_nohz_idle_stop_tick(struct tick_sched *ts)
 {
-	ktime_t now, expires;
+	ktime_t expires;
 	int cpu = smp_processor_id();
 
-	now = tick_nohz_start_idle(ts);
-
 	if (can_stop_idle_tick(cpu, ts)) {
 		int was_stopped = ts->tick_stopped;
 
 		ts->idle_calls++;
 
-		expires = tick_nohz_stop_sched_tick(ts, now, cpu);
+		/*
+		 * The idle entry time should be a sufficient approximation of
+		 * the current time at this point.
+		 */
+		expires = tick_nohz_stop_sched_tick(ts, ts->idle_entrytime, cpu);
 		if (expires > 0LL) {
 			ts->idle_sleeps++;
 			ts->idle_expires = expires;
@@ -920,16 +919,19 @@ static void __tick_nohz_idle_enter(struct tick_sched *ts)
 }
 
 /**
- * tick_nohz_idle_enter - stop the idle tick from the idle task
+ * tick_nohz_idle_stop_tick - stop the idle tick from the idle task
  *
  * When the next event is more than a tick into the future, stop the idle tick
- * Called when we start the idle loop.
- *
- * The arch is responsible of calling:
+ */
+void tick_nohz_idle_stop_tick(void)
+{
+	__tick_nohz_idle_stop_tick(this_cpu_ptr(&tick_cpu_sched));
+}
+
+/**
+ * tick_nohz_idle_enter - prepare for entering idle on the current CPU
  *
- * - rcu_idle_enter() after its last use of RCU before the CPU is put
- *  to sleep.
- * - rcu_idle_exit() before the first use of RCU after the CPU is woken up.
+ * Called when we start the idle loop.
  */
 void tick_nohz_idle_enter(void)
 {
@@ -941,7 +943,7 @@ void tick_nohz_idle_enter(void)
 
 	ts = this_cpu_ptr(&tick_cpu_sched);
 	ts->inidle = 1;
-	__tick_nohz_idle_enter(ts);
+	tick_nohz_start_idle(ts);
 
 	local_irq_enable();
 }
@@ -958,10 +960,12 @@ void tick_nohz_irq_exit(void)
 {
 	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
 
-	if (ts->inidle)
-		__tick_nohz_idle_enter(ts);
-	else
+	if (ts->inidle) {
+		tick_nohz_start_idle(ts);
+		__tick_nohz_idle_stop_tick(ts);
+	} else {
 		tick_nohz_full_update_tick(ts);
+	}
 }
 
 /**