Re: PIT clocksource makes invalid assumptions

devzero
2008-01-05T10:10:42Z

>I also didn't do a whole lot of digging to see what the problems this
>bug can cause in practice, but after the PIT clocksource was installed,
>I tried 'sleep 1' and this did not wake up.

i`m using clock=pit on many of our virtual servers for accurate time syncronization. (as recommended by vmware and even by microsoft - see: http://support.microsoft.com/?scid=kb%3Ben-us%3B918461&x=11&y=18 )

so this means i need "clock=pit nohz=off" now on those systems, if this won`t get fixed?

On 01/04/2008 12:18 PM, john stultz wrote:
> On Thu, 2008-01-03 at 15:52 -0800, Dan Hecht wrote:
>> Looking at pit_read() in arch/x86/kernel/i8253.c, it seems that the PIT
>> clocksource code assumes that the PIT CH0 is in periodic mode. With
>> clockevents, this assumption is no longer valid. There are at least two
>> places that make this assumption:
>>
>> 1) The calculation at the end of pit_read() assumes that the PIT is in
>> periodic mode. This isn't true unless the PIT is the current clockevent
>> and nohz is inactive. (Though #2 can end up forcing the PIT to be
>> reprogrammed).
>>
>> 2) The PIT clockevent is shutdown by using PIT mode 0 (interrupt on
>> terminal count) -- doesn't the PIT counter continue to count (even
>> though it won't be raising an interrupt)? If so, the test in pit_read()
>> under the VIA686a comment can succeed after the PIT clockevent has been
>> shutdown, and the PIT hardware may be reprogrammed to start firing
>> interrupts again. This doesn't seem intentional, and can defeat nohz
>> since now the PIT is firing periodically.
>>
>> Seems these problems can happen when the PIT is used as the clocksource
>> or even just the clocksource watchdog. It looks like there is some code
>> in clocksource.c that checks for CLOCK_SOURCE_IS_CONTINUOUS, which is
>> not set for the PIT clocksource, but it doesn't seem to be strong enough
>> to prevent these problematic scenarios (and it's not clear if that is
>> the intent of IS_CONTINUOUS anyway).
>
> The clocksource in use must have IS_CONTINUOUS set before we go into
> HRT/no_hz mode, so I think the situations above should not be possible
> (although I've not had a chance to check the current code).
>

Yes, I think that is correct. But, I don't think the code (always)
prevents nohz mode when the clocksource *watchdog* is !IS_CONTINUOUS.

Anyway, the bug doesn't require that nohz mode is enabled, it just
requires that the PIT clockevent is shutdown (or otherwise not
programmed in periodic mode).

>> To verify this really can happen, when I boot a kernel, I can see this
>> sequence:
>>
>> init_pit_timer (with mode==CLOCK_EVT_MODE_PERIODIC)
>> init_pit_timer (with mode==CLOCK_EVT_MODE_UNUSED)
>> init_pit_timer (with mode==CLOCK_EVT_MODE_SHUTDOWN)
>> pit_read() and count > LATCH (I believe the PIT is the watchdog at
>> this point), which causes the PIT to raise periodic interrupts.
>>
>> (Shortly after, the acpi pm clocksource is registered and replaces the
>> PIT as the watchdog. Later, the PIT clockevent is used as the broadcast
>> clockevent and reprogrammed into one-shot mode, stopping the PIT
>> interrupts.)
>>
>> Also, the user could force the PIT clocksource to be current_clocksource
>> even though the PIT is in one-shot mode (and therefore the calculation
>> in pit_read is bogus).
>
> Does this actually happen and cause problems? I thought there was some
> code to make sure we disable HRT/no_hz if we install a clocksource that
> does not have IS_CONTINUOUS set.
>

I didn't check if nohz was disabled when the PIT clocksource is switched
to, but I did check that the PIT was not the active clockevent, which is
enough for this bug.

I also didn't do a whole lot of digging to see what the problems this
bug can cause in practice, but after the PIT clocksource was installed,
I tried 'sleep 1' and this did not wake up.

Thanks,
Dan
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