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SubjectRe: [BUG] 2.6.21-rc1,2,3 regressions on my system that I found so far
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Len,

On Fri, 2007-03-16 at 21:32 -0400, Len Brown wrote:
> > > [ 36.433917] APIC timer disabled due to verification failure.
> > >
> > > And NO_HZ is disabled due to that (I get 1000/s timer's interrupts)
> > > I haven't investigated that yet.
> > > It looks like another new test that my hardware fails to perform...
> >
> > Yes, this is probably caused by SMM code trying to emulate a PS/2
> > keyboard from a (maybe connected or not) USB keyboard. Unfortunately we
> > have no way to disable this BIOS misfeature in the early boot process.
> > Arjan, Len ?????
>
> Nope. By definition, SMM is invisible to the OS -- we don't even
> get a bit that said it occurred (though we'd like one -- it would
> be really helpful to diagnose issues like this one)

I know that it is invisible. Nevertheless I know that the BIOSes emulate
PS/2 keyboards from USB via SMM during the boot process until we call
the usb_handoff function.

> So go into BIOS SETUP and see if there is a USB Legacy Emulation
> feature that you can disable. Sometimes there is not, but disabling
> onboard USB altogether may help at least prove the issue is in that area.

I have more than one box (even original Intel mainboards), where either
plugging a PS/2 keyboard or switching off USB makes this problem go
away.

> > I built in this test to rule out bogus LAPIC timer calibration values
> > which are sometimes off by factor 2-10.
> >
> > But I also built in a calibration against the PM-Timer, which turned out
> > to be quite reliable and I think the additional verification step is
> > only necessary for sytems without PM-Timer.
> >
> > That was a bit over cautious from my side. I send a patch to avoid this
> > when PM-Timer is available in a separate mail.
>
> PM-Timer was invented to work-around the issue that the TSC became unreliable
> in the face of power management on laptops. In particular, to be able
> to time duration of OS idle where TSC stopped.
>
> While it is not fine grain, and it is not low-latency, is should
> be very reliable. My understanding is that it is implemented as
> a simple divider right off the system 14MHz clock -- the signal
> which most motherboard clocks are PLL multiplied up from --
> including the 100MHz front-side bus which drives the LAPIC timer.
>
> But that said, I don't understand why calibrating the LAPIC timer
> using the PM-timer is going to be more reliable -- exactly how
> and why did the previous calibration scheme fail?
> Maybe I could follow the new logic in apic.c if I saw the "apic=debug"
> output for this box.

calibrating APIC timer ...
... lapic delta = 2426884
... PM timer delta = 833908
APIC calibration PIT not consistent with PM Timer: 232ms instead of 100ms
APIC delta adjusted to PM-Timer: 1041737 (2426884)
..... delta 1041737
..... mult: 44749065
..... calibration result: 166677
..... CPU clock speed is 4659.0624 MHz.
..... host bus clock speed is 166.0677 MHz.

This box is off by factor 2.3 and using the PM-Timer instead of the
PIT/jiffies values gives me a correct result.

Another one:
APIC calibration not consistent with PM Timer: 2020ms instead of 100ms
APIC delta adjusted to PM-Timer: 1254436 (25341111)

Off by factor 20 !!

The original APIC timer calibration did:

local_irq_disable();
wait_until_pit_underflows();
t1 = read_apic_counter();
for (i = 0; i < HZ/10; i++)
wait_until_pit_underflows();
t2 = read_apic_counter();

and calculated the APIC timer frequency from the delta of t1 and t2 vs.
the 100ms time.

This had 2 problems:
1. It gave results, which are off by factor 2-10 on a couple of boxen.
2. Some systems stop there dead as the PIT readout is broken.

I changed it to do:

local_irq_disable();
original_pit_handler = pit->handler;
pit->handler = lapic_calibration_handler;
loops = 0;
local_irq_enable();
wait_until_handler_has_done_HZ/10_loops();

The handler does:

if (!loops++) {
t1_apic = read_apic_counter();
t1_jiffies = jiffies;
t1_pm = read_pm_timer();
}

if (loops == HZ/10) {
t2_apic = read_apic_counter();
t2_jiffies = jiffies;
t2_pm = read_pm_timer();
done = 1;
}

If the pmtimer is available, then calculate the APIC timer frequency
from the t1_pm/t2_pm delta, otherwise use jiffies.

When pm_timer is there, we can trust the calculated value, if not we do
a verify run of the periodic apic timer and the pit timer. If this fails
- and it fails often due to the SMM crap - then I use the PIT and IPIs.

In the first version I did a verification run even when pm_timer was
there, but this produced false positives as well, because the lapic
timer interrupt is in the same way delayed as the PIT interrupt. I
removed this to avoid unnecessary switching to IPIs after I verified,
that it always produced false positives when the calibration was done
against the PM-Timer.

tglx


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