[PATCH 32/38] tick-sched: add comment about 'idle_active' in tick_nohz_idle_exit()

[Viresh Kumar]

The sequence of calls for dynticks CPUs was a bit confusing and so adding a
comment in tick_nohz_idle_exit() routine to mention it clearly. All information
required is in commit and this conversation with Frederic.

https://lkml.org/lkml/2014/4/10/355

Suggested-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Signed-off-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
---
 kernel/time/tick-sched.c | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index f1bc258..85a4e90 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -925,6 +925,22 @@ void tick_nohz_idle_exit(void)
 
 	ts->inidle = 0;
 
+	/*
+	 * Can idle_active be false here?
+	 * Ideally this would be the sequence of calls:
+	 * - tick_nohz_idle_enter(), i.e. idle_active = true;
+	 * - local_irq_disable()
+	 * - IDLE
+	 * - wake up due to IPI or other interrupt
+	 * - local_irq_enable()
+	 * - tick_nohz_irq_enter(), i.e. idle_active = false;
+	 * - tick_nohz_irq_exit(), i.e. idle_active = true; This is not called
+	 *   in case of IPI's as need_resched() will prevent that in
+	 *   tick_irq_exit(), as we don't need to account any more for idle time
+	 *   or try to enter dyntics mode (We are going to exit idle state).
+	 *
+	 * - tick_nohz_idle_exit()
+	 */
 	if (ts->idle_active || ts->tick_stopped)
 		now = ktime_get();
 
-- 
1.7.12.rc2.18.g61b472e

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