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POSIX timer: Optimize overrun check by replacing loop with division.

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Converted the timer overrun check from a loop-based approach to a division-based method. This change ensures a deterministic worst-case execution time (WCET), even if it might not outperform the loop in average scenarios.

Signed-off-by: ouyangxiangzhen <ouyangxiangzhen@xiaomi.com>
This commit is contained in:
ouyangxiangzhen 2024-09-13 17:57:25 +08:00 committed by Xiang Xiao
parent 578ecef975
commit 5a01f0f7f6
1 changed files with 17 additions and 11 deletions
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28
sched/timer/timer_settime.c
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@ -98,6 +98,7 @@ static inline void timer_restart(FAR struct posix_timer_s *timer,
{
clock_t ticks;
sclock_t delay;
sclock_t frame;
/* If this is a repetitive timer, then restart the watchdog */
@ -106,19 +107,24 @@ static inline void timer_restart(FAR struct posix_timer_s *timer,
/* Check whether next expected time is reached */
ticks = clock_systime_ticks();
timer->pt_overrun = 0;
delay = ticks - timer->pt_expected;
for (; ; )
{
timer->pt_expected += timer->pt_delay;
delay = timer->pt_expected - ticks;
if (delay > 0)
{
break;
}
/* Calculate the number of timer overruns and the next expected tick.
* The next expired tick frame can be computed as align up:
* frame <- (elapsed_ticks + pt_delay) / pt_delay
* For instance:
* | pt_delay | pt_delay | pt_delay | ... |
* ^ pt_expected ^ ticks ^ next pt_expected
* In this case, frame equals 3.
* Then, pt_overrun <- frame - 1 and
* the next pt_expected <- pt_expected + frame * pt_delay.
* Assumption of correctness:
* (delay + timer->pt_delay) should not overflow.
*/
timer->pt_overrun++;
}
frame = (delay + timer->pt_delay) / timer->pt_delay;
timer->pt_overrun = frame - 1;
timer->pt_expected += frame * timer->pt_delay;
wd_start_abstick(&timer->pt_wdog, timer->pt_expected,
timer_timeout, itimer);