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nohz: Clarify magic in tick_nohz_stop_sched_tick()

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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>
This commit is contained in:
Peter Zijlstra 2015-11-19 17:21:06 +01:00 committed by Thomas Gleixner
parent 09a9982016
commit 82bbe34b3d
1 changed files with 18 additions and 2 deletions
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20
kernel/time/tick-sched.c
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@ -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;
}