Kernel/linux-next
1
0
Fork 0
mirror of https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git synced 2025-01-19 14:56:21 +00:00
Code Issues Actions Packages Projects Releases Wiki Activity

timers: Add comments about calc_index() ceiling work

Browse Source 
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: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lkml.kernel.org/r/20200717140551.29076-6-frederic@kernel.org
This commit is contained in:
Frederic Weisbecker 2020-07-17 16:05:44 +02:00 committed by Thomas Gleixner
parent 9a2b764b06
commit 4468897211
1 changed files with 11 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

12
kernel/time/timer.c
View File

@ -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);