To align btrace(8)'s behavior with bpftrace we need to schedule the
interval/profile clock interrupts relative to the start of recording,
not relative to the start of the uptime clock.

The problem is obvious when you compare behavior.

bpftrace:

alpine:~# uname -a
Linux alpine 6.6.16-0-lts #1-Alpine SMP PREEMPT_DYNAMIC Wed, 07 Feb 2024 18:00:38 +0000 x86_64 Linux
alpine:~# bpftrace --version
bpftrace v0.19.1
alpine:~# cat interval-start-latency.bt
BEGIN {
	@t0 = nsecs;
}

interval:hz:1 {
	$dt = nsecs - @t0;
	printf("elapsed %d.%09d\n", $dt / 1000000000, $dt % 1000000000);
	exit();
}
alpine:~# bpftrace -q interval-start-latency.bt | head -n 1
elapsed 1.014731440
alpine:~# cat profile-start-latency.bt
BEGIN {
	@t0 = nsecs;
}

profile:hz:1 {
	$dt = nsecs - @t0;
	printf("elapsed %d.%09d\n", $dt / 1000000000, $dt % 1000000000);
	exit();
}
alpine:~# bpftrace -q profile-start-latency.bt | head -n 1
elapsed 1.023011922

btrace(8):

$ doas btrace interval-start-latency.bt
elapsed 0.586779311
$ doas btrace profile-start-latency.bt
elapsed 0.252215375

Fix attached.  ok?

Index: dt_dev.c
===================================================================
RCS file: /cvs/src/sys/dev/dt/dt_dev.c,v
diff -u -p -r1.31 dt_dev.c
--- dt_dev.c	18 Feb 2024 00:54:03 -0000	1.31
+++ dt_dev.c	24 Feb 2024 15:59:17 -0000
@@ -477,7 +477,9 @@ dt_ioctl_get_stats(struct dt_softc *sc, 
 int
 dt_ioctl_record_start(struct dt_softc *sc)
 {
+	uint64_t now;
 	struct dt_pcb *dp;
+	int need_time = 1;
 
 	if (sc->ds_recording)
 		return EBUSY;
@@ -495,9 +497,14 @@ dt_ioctl_record_start(struct dt_softc *s
 		dtp->dtp_prov->dtpv_recording++;
 
 		if (dp->dp_nsecs != 0) {
+			if (need_time) {
+				need_time = 0;
+				now = nsecuptime();
+			}
 			clockintr_bind(&dp->dp_clockintr, dp->dp_cpu, dt_clock,
 			    dp);
-			clockintr_advance(&dp->dp_clockintr, dp->dp_nsecs);
+			clockintr_schedule(&dp->dp_clockintr,
+			    now + dp->dp_nsecs);
 		}
 	}
 	rw_exit_write(&dt_lock);