Re: [PATCH] use 64bit timer for hpet

[Linux <zhaoyan.liao@linux.alibaba.com>]

Gleixner,

> Seriously? The wrap-around time for 32bit HPET @24MHz is ~3 minutes.

In some cases, our system will be very busy, and the timeout of 3 minutes 
is not an exaggeration. Then, the system considers that the tsc clock is 
inaccurate and switches the tsc clock to the hpet clock, which brings 
greater performance overhead.

> Aside of that the reason why the kernel does not support 64bit HPET is
> that there are HPETs which advertise 64bit support, but the
> implementation is buggy.

Can you tell me what is the buggy with the 64-bit hpet clock? In my opinion, 
it is unreasonable to use a lower-bit width clock to calibrate a higher-bit width
 clock, and the hardware already supports the higher-bit width.


> 2021年7月7日 下午6:04，Thomas Gleixner <tglx@linutronix.de> 写道：
> 
> Liao,
> 
> On Fri, Jul 02 2021 at 16:13, zhaoyan liao wrote:
>> The kernel judges whether the tsc clock is accurate in the
>> clocksource_watchdog background thread function. The hpet clock source
>> is 32-bit, but tsc is 64-bit. Therefore, when the system is busy and the
>> clocksource_watchdog cannot be scheduled in time, the hpet clock may
>> overflow and cause the system to misjudge tsc as unreliable.
> 
> Seriously? The wrap-around time for 32bit HPET @24MHz is ~3 minutes.
> 
>> In this case, we recommend that the kernel adopts the 64-bit hpet clock
>> by default to keep the width of the two clock sources the same to reduce
>> misjudgment. Some CPU models may not support 64-bit hpet, but according
>> to the description of the CPU's register manual, it does not affect our
>> reading action.
> 
> So much for the theory.
> 
>> -#define HPET_MASK			CLOCKSOURCE_MASK(32)
>> +#define HPET_MASK			CLOCKSOURCE_MASK(64)
> 
> How is that valid for a 32bit HPET? This breaks the clocksource.
> 
>> +inline unsigned long hpet_readq(unsigned int a)
>> +{
>> +	return readq(hpet_virt_address + a);
> 
> Breaks 32bit build immediately.
> 
> Aside of that the reason why the kernel does not support 64bit HPET is
> that there are HPETs which advertise 64bit support, but the
> implementation is buggy.
> 
> IOW, while this works for your hardware this breaks quite some parts of
> the universe. Not really a good approach.
> 
> Thanks,
> 
>        tglx