nohz: Clarify magic in tick_nohz_stop_sched_tick()
While going through the nohz code I got stumped by some of it.
This patch adds a few comments clarifying the code; based on discussion
with Thomas.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/r/20151119162106.GO3816@twins.programming.kicks-ass.net
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 7c7ec451..7ea28ed 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -603,15 +603,31 @@
/*
* If the tick is due in the next period, keep it ticking or
- * restart it proper.
+ * force prod the timer.
*/
delta = next_tick - basemono;
if (delta <= (u64)TICK_NSEC) {
tick.tv64 = 0;
+ /*
+ * We've not stopped the tick yet, and there's a timer in the
+ * next period, so no point in stopping it either, bail.
+ */
if (!ts->tick_stopped)
goto out;
+
+ /*
+ * If, OTOH, we did stop it, but there's a pending (expired)
+ * timer reprogram the timer hardware to fire now.
+ *
+ * We will not restart the tick proper, just prod the timer
+ * hardware into firing an interrupt to process the pending
+ * timers. Just like tick_irq_exit() will not restart the tick
+ * for 'normal' interrupts.
+ *
+ * Only once we exit the idle loop will we re-enable the tick,
+ * see tick_nohz_idle_exit().
+ */
if (delta == 0) {
- /* Tick is stopped, but required now. Enforce it */
tick_nohz_restart(ts, now);
goto out;
}