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kern_tc.c/cputick2usec() (which is used to calculate cputime from

Browse source 
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
This commit is contained in:
firk 2022-03-21 09:33:11 -04:00 committed by George V. Neville-Neil
parent cab496e16c
commit bb53dd56c3
3 changed files with 10 additions and 23 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

9
lib/libkvm/kvm_proc.c
View file

@ -94,15 +94,10 @@ static uint64_t cpu_tick_frequency;
static uint64_t
cputick2usec(uint64_t tick)
{
if (cpu_tick_frequency == 0)
return (0);
if (tick > 18446744073709551) /* floor(2^64 / 1000) */
return (tick / (cpu_tick_frequency / 1000000));
else if (tick > 18446744073709) /* floor(2^64 / 1000000) */
return ((tick * 1000) / (cpu_tick_frequency / 1000));
else
return ((tick * 1000000) / cpu_tick_frequency);
return ((tick / cpu_tick_frequency) * 1000000ULL) +
((tick % cpu_tick_frequency) * 1000000ULL) / cpu_tick_frequency;
}
/*

12
sys/kern/kern_tc.c
View file

@ -2155,20 +2155,14 @@ 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;

12
sys/kern/kern_time.c
View file

@ -245,9 +245,10 @@ sys_clock_gettime(struct thread *td, struct clock_gettime_args *uap)
static inline void
cputick2timespec(uint64_t runtime, struct timespec *ats)
{
runtime = cputick2usec(runtime);
ats->tv_sec = runtime / 1000000;
ats->tv_nsec = runtime % 1000000 * 1000;
uint64_t tr;
tr = cpu_tickrate();
ats->tv_sec = runtime / tr;
ats->tv_nsec = ((runtime % tr) * 1000000000ULL) / tr;
}
void
@ -477,10 +478,7 @@ kern_clock_getres(struct thread *td, clockid_t clock_id, struct timespec *ts)
case CLOCK_THREAD_CPUTIME_ID:
case CLOCK_PROCESS_CPUTIME_ID:
cputime:
/* sync with cputick2usec */
ts->tv_nsec = 1000000 / cpu_tickrate();
if (ts->tv_nsec == 0)
ts->tv_nsec = 1000;
ts->tv_nsec = 1000000000 / cpu_tickrate() + 1;
break;
default:
if ((int)clock_id < 0)