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kern: clarify boot time

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In FreeBSD, the current time is computed from uptime + boottime. Uptime
is a continuous, smooth function that's monotonically increasing. To
effect changes to the current time, boottime is adjusted.  boottime is
mutable and shouldn't be cached against future need. Document the
current implementation, with the caveat that we may stop stepping
boottime on resume in the future and will step uptime instead (noted in
the commit message, but not in the code).

Sponsored by:		Netflix
Reviewed by:		phk, rpokala
Differential Revision:	https://reviews.freebsd.org/D30116
This commit is contained in:
Warner Losh
2021-05-05 12:32:13 -06:00
committed by Sebastian Huber
parent 8d5fc57e65
commit 87d7a6e54b
1 changed files with 7 additions and 1 deletions
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8
cpukit/score/src/kern_tc.c
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@@ -252,11 +252,17 @@ volatile int32_t time_uptime = 1;
#endif /* __rtems__ */
#ifndef __rtems__
/*
* The system time is always computed by summing the estimated boot time and the
* system uptime. The timehands track boot time, but it changes when the system
* time is set by the user, stepped by ntpd or adjusted when resuming. It
* is set to new_time - uptime.
*/
static int sysctl_kern_boottime(SYSCTL_HANDLER_ARGS);
SYSCTL_PROC(_kern, KERN_BOOTTIME, boottime,
CTLTYPE_STRUCT | CTLFLAG_RD | CTLFLAG_MPSAFE, NULL, 0,
sysctl_kern_boottime, "S,timeval",
"System boottime");
"Estimated system boottime");
SYSCTL_NODE(_kern, OID_AUTO, timecounter, CTLFLAG_RW | CTLFLAG_MPSAFE, 0,
"");