Time keeping Issues with the low-resolution TSC timecounter

[Jung-uk Kim]

On Wednesday 15 June 2011 06:19 pm, Jung-uk Kim wrote:
> On Wednesday 15 June 2011 04:39 pm, Jung-uk Kim wrote:
> > On Wednesday 15 June 2011 03:27 pm, Ian FREISLICH wrote:
> > > > > The problem I noticed first is that it takes unusually long
> > > > > until a key press is repeated. With the default eventtimer
> > > > > (HPET) it seems to take about 4s, which can be slightly
> > > > > improved by switching to i8254.
> > > > >
> > > > > The "error beep" seems to take longer than usual, too,
> > > > > and the system "feels sluggish" in general.
> > > > >
> > > > > An effect that is easier to measure is that the system is
> > > > > unable to properly keep the time. Again the problem is less
> > > > > severe when using i8254 instead of HPET:
> > > >
> > > > [SNIP]
> > > >
> > > > First of all, please do not mix timecounter issues with
> > > > eventtimer. They are not directly related.
> > > >
> > > > Can you please show me verbose boot messages *without* your
> > > > patch? Does "sysctl kern.timecounter.hardware=HPET" help
> > > > *without* touching eventtimers?
> > >
> > > I have the same issue with my system (Atom N270).  The effect
> > > that I see is about 29 wall clock seconds are recorded as 1
> > > system second.
> >
> > Can please you send me output from the following?
> >
> > sh -c 'count=10; while [ $count -gt 0 ]; do count=$((count -
> > 1));\ sysctl kern.timecounter; sleep 1; done'
>
> The OP sent me some data.  The following is cooked data from what I
> got:
>
> TSC		Delta		Elapsed Time
> ---------------+---------------+------------
> 2245026970
> 2249708604	4681634		0.3003693189
> 2255874998	6166394		0.3956301509
> 2260656402	4781404		0.3067704701
> 2261993048	1336646		0.0857579744
> 2264175164	2182116		0.1400025497
> 2266445706	2270542		0.1456758803
> 2266987003	541297		0.0347291162
> 2267645095	658092		0.0422225757
> 2268274965	629870		0.0404118782
> 2268979787	704822		0.0452207294
> 2270482069	1502282		0.0963850274
> 2271140877	658808		0.0422685136
>
> HPET		Delta		Elapsed Time
> ---------------+---------------+------------
> 1322734365
> 1337156221	14421856	1.0072408644
> 1351548130	14391909	1.0051493276
> 1365949265	14401135	1.0057936833
> 1380376349	14427084	1.0076059946
> 1394823985	14447636	1.0090413726
> 1409273964	14449979	1.0092050107
> 1423719753	14445789	1.0089123757
> 1438167064	14447311	1.0090186742
> 1452613630	14446566	1.0089666424
> 1467062977	14449347	1.0091608710
> 1481522037	14459060	1.0098392393
> 1495969404	14447367	1.0090225853
>
> As you can see, HPET increases normally (within errors from
> sleep(3) accuracy, syscall overhead, etc.) but TSC-low is totally
> erratic (and too low).  I don't know how this can happen, though.
> :-(
>
> I need some time to figure it out.

Can you please test the attached patch?  Theoretically, it should not 
make a difference but I'd like to see if it does, just in case.

Thanks,

Jung-uk Kim
-------------- next part --------------
Index: sys/x86/x86/tsc.c
===================================================================
--- sys/x86/x86/tsc.c	(revision 223102)
+++ sys/x86/x86/tsc.c	(working copy)
@@ -579,6 +579,9 @@ tsc_get_timecount(struct timecounter *tc __unused)
 static u_int
 tsc_get_timecount_low(struct timecounter *tc)
 {
+	uint32_t low, high;
 
-	return (rdtsc() >> (int)(intptr_t)tc->tc_priv);
+	__asm __volatile("rdtsc; shrd %2,%1,%0"
+	: "=a" (low), "=d" (high) : "Ic" ((uint8_t)(intptr_t)tc->tc_priv));
+	return (low);
 }