Re: PIT clocksource makes invalid assumptions

[john stultz <johnstul@us.ibm.com>]

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 clockevents, this assumption is no longer valid.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).

> 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.

thanks
-john