Make cpu_est_clockrate() more accurate by disabling interrupts for the

millisecond it is calibrating.  Suggested by jhb@ and bde@.  Don't clobber
the tsc_freq with the new value since it isn't accurate enough for
timecounters and the timecounter system as a whole needs support for
changing rates before we do this.  Subtract 0.5% from our measurement
to account for overhead in DELAY.  Note that this interface is for
estimating the clockrate and needs to work well at runtime so doing a full
calibration including disabling interrupts for a second is not feasible.

1 files changed, 10 insertions, 2 deletions

diff --git a/sys/amd64/amd64/machdep.c b/sys/amd64/amd64/machdep.c
index 527c61bd1dbb..422c0fb949d2 100644
--- a/sys/amd64/amd64/machdep.c
+++ b/sys/amd64/amd64/machdep.c
@@ -456,6 +456,7 @@ cpu_boot(int howto)
 int
 cpu_est_clockrate(int cpu_id, uint64_t *rate)
 {
+	register_t reg;
 	uint64_t tsc1, tsc2;
 
 	if (pcpu_find(cpu_id) == NULL || rate == NULL)
@@ -475,9 +476,11 @@ cpu_est_clockrate(int cpu_id, uint64_t *rate)
 #endif
 
 	/* Calibrate by measuring a short delay. */
+	reg = intr_disable();
 	tsc1 = rdtsc();
 	DELAY(1000);
 	tsc2 = rdtsc();
+	intr_restore(reg);
 
 #ifdef SMP
 	mtx_lock_spin(&sched_lock);
@@ -485,8 +488,13 @@ cpu_est_clockrate(int cpu_id, uint64_t *rate)
 	mtx_unlock_spin(&sched_lock);
 #endif
 
-	tsc_freq = (tsc2 - tsc1) * 1000;
-	*rate = tsc_freq;
+	/*
+	 * Calculate the difference in readings, convert to Mhz, and
+	 * subtract 0.5% of the total.  Empirical testing has shown that
+	 * overhead in DELAY() works out to approximately this value.
+	 */
+	tsc2 -= tsc1;
+	*rate = tsc2 * 1000 - tsc2 * 5;
 	return (0);
 }