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

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
...@@ -156,7 +156,8 @@ EXPORT_SYMBOL(jiffies_64); ...@@ -156,7 +156,8 @@ EXPORT_SYMBOL(jiffies_64);
/* /*
* The time start value for each level to select the bucket at enqueue * 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)) #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) ...@@ -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, static inline unsigned calc_index(unsigned long expires, unsigned lvl,
unsigned long *bucket_expiry) 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); expires = (expires + LVL_GRAN(lvl)) >> LVL_SHIFT(lvl);
*bucket_expiry = expires << LVL_SHIFT(lvl); *bucket_expiry = expires << LVL_SHIFT(lvl);
return LVL_OFFS(lvl) + (expires & LVL_MASK); return LVL_OFFS(lvl) + (expires & LVL_MASK);
......
Markdown is supported
0%
or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment