kern/59691: tstohz(9) fails to round up on some inputs

[campbell+netbsd%mumble.net@localhost]

>Number:         59691
>Category:       kern
>Synopsis:       tstohz(9) fails to round up on some inputs
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Oct 05 12:25:01 +0000 2025
>Originator:     Taylor R Campbell
>Release:        current, 11, 10, 9, ...
>Organization:
The NetTSD Roundation
>Environment:
>Description:

	tstohz(9) is supposed to return the number of ticks needed to
	sleep before a certain duration, like tvtohz(9) but for struct
	timespec rather than struct timeval.

	However, it works by first rounding the struct timespec
	(nanosecond precision) _down_ to a struct timeval (microsecond
	precision), and then calling tvtohz(9).  So, for example, while
	tvtohz(0.000001 sec) gives 2 ticks (one full tick, plus
	whatever time is left between now and the next hardclock tick,
	which may be time epsilon away), tstohz(0.000000001 sec) gives
	0 ticks (tvtohz returns 0 ticks for 0 time).

	This is a likely cause of one source of infinite loops in
	timers leading to heartbeat panics:

	PR kern/59339: heartbeat watchdog fires since 10.99.14
	https://gnats.NetBSD.org/59339

	PR kern/59465: Recurring kernel panic with -current (10.99.14):
	"heart stopped beating"
	https://gnats.NetBSD.org/59465

	PR kern/59679: Multiple "heart stopped beating" / "softints
	stuck for 16 seconds" panics
	https://gnats.NetBSD.org/59679


>How-To-Repeat:

	tvtohz(&(struct timespec){.tv_sec = 0, .tv_nsec = 1})


>Fix:

	in progress