Re: The HPET issue on Linux

[Beth Kon <eak@us.ibm.com>]

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?

>
>>
>> -- 
>>             Gleb.
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-- 
Regards,

Beth Kon