I think it's about time to enable invariant TSC timecounter on SMP by
default. Please see the attached patch. It is also available from
here:
http://people.freebsd.org/~jkim/tsc_smp_test4.diff
avg convinced me enough that it should be an opt-out feature going
forward. :-)
Comments?
Cheers,
Jung-uk Kim
Index: sys/x86/x86/tsc.c
===================================================================
--- sys/x86/x86/tsc.c (revision 222262)
+++ sys/x86/x86/tsc.c (working copy)
@@ -79,7 +79,8 @@ static void tsc_freq_changed(void *arg, const stru
int status);
static void tsc_freq_changing(void *arg, const struct cf_level *level,
int *status);
-static unsigned tsc_get_timecount(struct timecounter *tc);
+static unsigned tsc_get_timecount(struct timecounter *tc);
+static unsigned tsc_get_timecount_lowres(struct timecounter *tc);
static void tsc_levels_changed(void *arg, int unit);
static struct timecounter tsc_timecounter = {
@@ -385,7 +386,7 @@ test_smp_tsc(void)
if (bootverbose)
printf("SMP: %sed TSC synchronization test\n",
smp_tsc ? "pass" : "fail");
- return (smp_tsc ? 800 : -100);
+ return (smp_tsc ? (tsc_is_invariant ? 1000 : 800) : -100);
}
#undef N
@@ -395,11 +396,19 @@ test_smp_tsc(void)
static void
init_TSC_tc(void)
{
+ uint64_t max_freq;
+ int shift;
if ((cpu_feature & CPUID_TSC) == 0 || tsc_disabled)
return;
/*
+ * Limit timecounter frequency to fit in an int and prevent it from
+ * overflowing too fast.
+ */
+ max_freq = UINT_MAX;
+
+ /*
* We can not use the TSC if we support APM. Precise timekeeping
* on an APM'ed machine is at best a fools pursuit, since
* any and all of the time spent in various SMM code can't
@@ -421,13 +430,30 @@ init_TSC_tc(void)
* We can not use the TSC in SMP mode unless the TSCs on all CPUs are
* synchronized. If the user is sure that the system has synchronized
* TSCs, set kern.timecounter.smp_tsc tunable to a non-zero value.
+ * We also limit the frequency even lower to avoid "temporal anomalies"
+ * as much as possible.
*/
- if (smp_cpus > 1)
+ if (smp_cpus > 1) {
tsc_timecounter.tc_quality = test_smp_tsc();
+ max_freq >>= 8;
+ } else
#endif
+ if (tsc_is_invariant)
+ tsc_timecounter.tc_quality = 1000;
+
init:
+ for (shift = 0; shift < 32 && (tsc_freq >> shift) > max_freq; shift++)
+ ;
+ if (shift > 0) {
+ tsc_timecounter.tc_get_timecount = tsc_get_timecount_lowres;
+ tsc_timecounter.tc_name = "TSC-low";
+ if (bootverbose)
+ printf("TSC timecounter discards lower %d bit(s).\n",
+ shift);
+ }
if (tsc_freq != 0) {
- tsc_timecounter.tc_frequency = tsc_freq;
+ tsc_timecounter.tc_frequency = tsc_freq >> shift;
+ tsc_timecounter.tc_priv = (void *)(intptr_t)shift;
tc_init(&tsc_timecounter);
}
}
@@ -499,7 +525,8 @@ tsc_freq_changed(void *arg, const struct cf_level
/* Total setting for this level gives the new frequency in MHz. */
freq = (uint64_t)level->total_set.freq * 1000000;
atomic_store_rel_64(&tsc_freq, freq);
- atomic_store_rel_64(&tsc_timecounter.tc_frequency, freq);
+ tsc_timecounter.tc_frequency =
+ freq >> (int)(intptr_t)tsc_timecounter.tc_priv;
}
static int
@@ -514,7 +541,8 @@ sysctl_machdep_tsc_freq(SYSCTL_HANDLER_ARGS)
error = sysctl_handle_64(oidp, &freq, 0, req);
if (error == 0 && req->newptr != NULL) {
atomic_store_rel_64(&tsc_freq, freq);
- atomic_store_rel_64(&tsc_timecounter.tc_frequency, freq);
+ tsc_timecounter.tc_frequency =
+ freq >> (int)(intptr_t)tsc_timecounter.tc_priv;
}
return (error);
}
@@ -523,8 +551,15 @@ SYSCTL_PROC(_machdep, OID_AUTO, tsc_freq, CTLTYPE_
0, 0, sysctl_machdep_tsc_freq, "QU", "Time Stamp Counter frequency");
static u_int
-tsc_get_timecount(struct timecounter *tc)
+tsc_get_timecount(struct timecounter *tc __unused)
{
return (rdtsc32());
}
+
+static u_int
+tsc_get_timecount_lowres(struct timecounter *tc)
+{
+
+ return (rdtsc() >> (int)(intptr_t)tc->tc_priv);
+}
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