Re: The HPET issue on Linux

[Sheng Yang <sheng@linux.intel.com>]

On Thursday 07 January 2010 02:36:26 Beth Kon wrote:
> Dor Laor wrote:
> > On 01/06/2010 12:09 PM, Gleb Natapov wrote:
> >> On Wed, Jan 06, 2010 at 05:48:52PM +0800, Sheng Yang wrote:
> >>> Hi Beth
> >>>
> >>> I still found the emulated HPET would result in some boot failure. For
> >>> example, on my 2.6.30, with HPET enabled, the kernel would fail
> >>> check_timer(),
> >>> especially in timer_irq_works().
> >>>
> >>> The testing of timer_irq_works() is let 10 ticks pass(using
> >>> mdelay()), and
> >>> want to confirm the clock source with at least 5 ticks advanced in
> >>> jiffies.
> >>> I've checked that, on my machine, it would mostly get only 4 ticks
> >>> when HPET
> >>> enabled, then fail the test. On the other hand, if I using PIT, it
> >>> would get
> >>> more than 10 ticks(maybe understandable if some complementary ticks
> >>> there). Of
> >>> course, extend the ticks count/mdelay() time can work.
> >>>
> >>> I think it's a major issue of HPET. And it maybe just due to a too long
> >>> userspace path for interrupt injection... If it's true, I think it's
> >>> not easy
> >>> to deal with it.
> >>
> >> PIT tick are reinjected automatically, HPET should probably do the same
> >> although it may just create another set of problems.
> >
> > Older Linux do automatic adjustment for lost ticks so automatic
> > reinjection causes time to run too fast. This is why we added the
> > -no-kvm-pit-reinject flag...
> >
> > It took lots of time to pit/rtc to stabilize, in order of seriously
> > consider the hpet emulation, lots of testing should be done.
> 
> I will try to look into this. Since HPET is edge-triggered, looks like
> this problem is of a different nature than PIT.  Is this a solid failure
> or intermittent?
> 
At least for v2.6.30 in my box, it always fails... Of course, I believe the 
chance of successful injecting enough interrupt depends on the many factors. 
So I think out target can be: not far behind what PIT can do...

-- 
regards
Yang, Sheng