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 7c7ec45159834a1b25576fbed037c9951f3c076f..7ea28ed3109d83e118b65c52e4aaa2e99d0c3638 100644 (file)
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -603,15 +603,31 @@ static ktime_t tick_nohz_stop_sched_tick(struct tick_sched *ts,
 
        /*
         * 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;
                }