[PATCH 01/10] kern_tc.c/cputick2usec()
Sebastian Huber
sebastian.huber at embedded-brains.de
Tue Mar 7 06:32:11 UTC 2023
From: firk <firk at cantconnect.ru>
(which is used to calculate cputime from cpu ticks) has some imprecision and,
worse, huge timestep (about 20 minutes on 4GHz CPU) near 53.4 days of elapsed
time.
kern_time.c/cputick2timespec() (it is used for clock_gettime() for
querying process or thread consumed cpu time) Uses cputick2usec()
and then needlessly converting usec to nsec, obviously losing
precision even with fixed cputick2usec().
kern_time.c/kern_clock_getres() uses some weird (anyway wrong)
formula for getting cputick resolution.
PR: 262215
Reviewed by: gnn
Differential Revision: https://reviews.freebsd.org/D34558
---
cpukit/score/src/kern_tc.c | 12 +++---------
1 file changed, 3 insertions(+), 9 deletions(-)
diff --git a/cpukit/score/src/kern_tc.c b/cpukit/score/src/kern_tc.c
index 643026a1c8..8e43169934 100644
--- a/cpukit/score/src/kern_tc.c
+++ b/cpukit/score/src/kern_tc.c
@@ -2617,20 +2617,14 @@ cpu_tickrate(void)
* years) and in 64 bits at 4 GHz (146 years), but if we do a multiply
* before divide conversion (to retain precision) we find that the
* margin shrinks to 1.5 hours (one millionth of 146y).
- * With a three prong approach we never lose significant bits, no
- * matter what the cputick rate and length of timeinterval is.
*/
uint64_t
cputick2usec(uint64_t tick)
{
-
- if (tick > 18446744073709551LL) /* floor(2^64 / 1000) */
- return (tick / (cpu_tickrate() / 1000000LL));
- else if (tick > 18446744073709LL) /* floor(2^64 / 1000000) */
- return ((tick * 1000LL) / (cpu_tickrate() / 1000LL));
- else
- return ((tick * 1000000LL) / cpu_tickrate());
+ uint64_t tr;
+ tr = cpu_tickrate();
+ return ((tick / tr) * 1000000ULL) + ((tick % tr) * 1000000ULL) / tr;
}
cpu_tick_f *cpu_ticks = tc_cpu_ticks;
--
2.35.3
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