Commit 44688972 authored by Frederic Weisbecker's avatar Frederic Weisbecker Committed by Thomas Gleixner
Browse files

timers: Add comments about calc_index() ceiling work 



calc_index() adds 1 unit of the level granularity to the expiry passed
in parameter to ensure that the timer doesn't expire too early. Add a
comment to explain that and the resulting layout in the wheel.

Suggested-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarFrederic Weisbecker <frederic@kernel.org>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Tested-by: default avatarJuri Lelli <juri.lelli@redhat.com>
Link: https://lkml.kernel.org/r/20200717140551.29076-6-frederic@kernel.org
parent 9a2b764b

kernel/time/timer.c

+11 −1
Original line number Diff line number Diff line
@@ -156,7 +156,8 @@ EXPORT_SYMBOL(jiffies_64); 


/*

 * The time start value for each level to select the bucket at enqueue

 * time.

 * time. We start from the last possible delta of the previous level

 * so that we can later add an extra LVL_GRAN(n) to n (see calc_index()).

 */

#define LVL_START(n)	((LVL_SIZE - 1) << (((n) - 1) * LVL_CLK_SHIFT))
@@ -490,6 +491,15 @@ static inline void timer_set_idx(struct timer_list *timer, unsigned int idx) 
static inline unsigned calc_index(unsigned long expires, unsigned lvl,

				  unsigned long *bucket_expiry)

{



	/*

	 * The timer wheel has to guarantee that a timer does not fire

	 * early. Early expiry can happen due to:

	 * - Timer is armed at the edge of a tick

	 * - Truncation of the expiry time in the outer wheel levels

	 *

	 * Round up with level granularity to prevent this.

	 */

	expires = (expires + LVL_GRAN(lvl)) >> LVL_SHIFT(lvl);

	*bucket_expiry = expires << LVL_SHIFT(lvl);

	return LVL_OFFS(lvl) + (expires & LVL_MASK);