Re: The HPET issue on Linux

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

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?
Anthony just explained that on x86, even edge-triggered interrupts are 
queued in the apic and an eoi will occur, so this is not different than 
the PIT.
>
>>
>>>
>>> -- 
>>>             Gleb.
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>>
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>
>


-- 
Regards,

Beth Kon