Commit 82bbe34b authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Thomas Gleixner
Browse files

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: default avatarPeter 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: default avatarThomas Gleixner <tglx@linutronix.de>
parent 09a99820

kernel/time/tick-sched.c

+18 −2
Original line number Diff line number Diff line
@@ -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;

		}