svn commit: r184293 - in head/sys: amd64/amd64 i386/i386

[Maxim Sobolev]

Bruce Evans wrote:
> On Mon, 27 Oct 2008, John Baldwin wrote:
> 
>> On Sunday 26 October 2008 02:58:04 pm Maxim Sobolev wrote:
>>> Author: sobomax
>>> Date: Sun Oct 26 18:58:04 2008
>>> New Revision: 184293
>>> URL: http://svn.freebsd.org/changeset/base/184293
>>>
>>> Log:
>>>   Fix division by zero panic if kern.hz less than 32.
>>>
>>>   MFC after:    1 day
>>
>> This is wrong.  In the case you are worried about here, lapic_timer_hz 
>> is less
>> than 128.  There is no way you are going to fire stathz 128 times per 
>> second
>> from a timer running at < 128 hz.  You are effectively running stathz at
>> lapic_timer_hz, so I would just set stathz = lapic_timer_hz in this case.
> 
> stathz needs to be about 128 to work as intended, at least for SCHED_4BSD.
> 
>> Also, I would drop the extra {}'s to match style(9) as well as the 
>> existing
>> style of the file.
> 
> I noticed this bug in the main commit too.
> 
> Also, hz = 10 cannot work on i386 without lapic_timer, since the i8254 
> timer
> has a maximum interrupt period of 55 ms and thus a minimum frequency of
> 18.2 Hz.  Attempts to set it to 10 Hz cause similar bugs to the ones here
> -- the best approximation of 18.2 is (supposed to be) used, but the system
> is not informed about the enormous error in this approximation and still
> thinks that 10 Hz is used.

Does it suggest that we simply need to place lower limit of 128 on 
lapic_timer_hz and lower limit of 18 on i8254 timer? As far as I 
understand timer frequency could be higher than actual HZ (at least 
lapic code suggest that it can). Will it help or cause any other issues?

-Maxim