On Thu, 25 Mar 2021 19:41:35 +0100 (CET)
Mark Kettenis <mark.kette...@xs4all.nl> wrote:
>> From: Scott Cheloha <scottchel...@gmail.com>
>> Date: Thu, 25 Mar 2021 13:18:04 -0500
>> > On Wed, Mar 24, 2021 at 05:40:21PM +0900, YASUOKA Masahiko wrote:
>> Which diff did you apply? Yasuoka provided two diffs.
>>
>> In any case, ignore this diff:
>>
>> > diff --git a/sys/arch/amd64/amd64/tsc.c b/sys/arch/amd64/amd64/tsc.c
>> > index 238a5a068e1..3b951a8b5a3 100644
>> > --- a/sys/arch/amd64/amd64/tsc.c
>> > +++ b/sys/arch/amd64/amd64/tsc.c
>> > @@ -212,7 +212,8 @@ cpu_recalibrate_tsc(struct timecounter *tc)
>> > u_int
>> > tsc_get_timecount(struct timecounter *tc)
>> > {
>> > - return rdtsc_lfence() + curcpu()->ci_tsc_skew;
>> > + //return rdtsc_lfence() + curcpu()->ci_tsc_skew;
>> > + return rdtsc_lfence();
>> > }
>> >
>> > void
>>
>>
>> We don't want to discard the skews, that's wrong.
I'm sorry for the confusion.
>> The reason it "fixes" Yasuoka's problem is because the real skews
>> on the ESXi VMs in question are probably close to zero but our
>> synchronization algorithm is picking huge (wrong) skews due to
>> some other variable interfering with our measurement.
>
> Right. If a VM exit happens while we're doing our measurement, you'll
> see a significant delay. And a guest OS can't prevent those from
> happening. But even on real hardware SMM mode may interfere with our
> measurement.
For machines like the ESXi VMs, the measurement seems to have to
exclude such delayed values as outliers. I think taking a lot of
samples and choice the minimum is a good enough way for the purpose.
I updated the diff.
- delete lines for debug
- make tsc quality lower if skew is not good enough
- reduce difference from NetBSD
comment? ok?
Index: sys/arch/amd64//amd64/tsc.c
===================================================================
RCS file: /disk/cvs/openbsd/src/sys/arch/amd64/amd64/tsc.c,v
retrieving revision 1.23
diff -u -p -r1.23 tsc.c
--- sys/arch/amd64//amd64/tsc.c 23 Feb 2021 04:44:30 -0000 1.23
+++ sys/arch/amd64//amd64/tsc.c 29 Mar 2021 04:18:31 -0000
@@ -38,6 +38,7 @@ int tsc_is_invariant;
#define TSC_DRIFT_MAX 250
#define TSC_SKEW_MAX 100
+#define TSC_SYNC_ROUNDS 1000
int64_t tsc_drift_observed;
volatile int64_t tsc_sync_val;
@@ -235,6 +236,7 @@ tsc_timecounter_init(struct cpu_info *ci
printf("%s: disabling user TSC (skew=%lld)\n",
ci->ci_dev->dv_xname, (long long)ci->ci_tsc_skew);
tsc_timecounter.tc_user = 0;
+ tsc_timecounter.tc_quality = -1000;
}
if (!(ci->ci_flags & CPUF_PRIMARY) ||
@@ -314,13 +316,19 @@ tsc_read_bp(struct cpu_info *ci, uint64_
void
tsc_sync_bp(struct cpu_info *ci)
{
+ int i, val, diff;
uint64_t bptsc, aptsc;
- tsc_read_bp(ci, &bptsc, &aptsc); /* discarded - cache effects */
- tsc_read_bp(ci, &bptsc, &aptsc);
+ val = INT_MAX;
+ for (i = 0; i < TSC_SYNC_ROUNDS; i++) {
+ tsc_read_bp(ci, &bptsc, &aptsc);
+ diff = bptsc - aptsc;
+ if (abs(diff) < abs(val))
+ val = diff;
+ }
/* Compute final value to adjust for skew. */
- ci->ci_tsc_skew = bptsc - aptsc;
+ ci->ci_tsc_skew = val;
}
/*
@@ -351,8 +359,10 @@ tsc_post_ap(struct cpu_info *ci)
void
tsc_sync_ap(struct cpu_info *ci)
{
- tsc_post_ap(ci);
- tsc_post_ap(ci);
+ int i;
+
+ for (i = 0; i < TSC_SYNC_ROUNDS; i++)
+ tsc_post_ap(ci);
}
void
