[RFC] Event timers on MIPS

[Alexander Motin]

Jayachandran C. wrote:
> On Sun, Jul 18, 2010 at 1:04 AM, Alexander Motin <mav at freebsd.org> wrote:
>> Jayachandran C. wrote:
>>> On XLR we would like to use the count/compare which is faster but less
>>> accurate on all cpus - we can have upto 32 cpus now.  We also have a
>>> PIC which can provide a better timestamp and timer interrupts.  This
>>> PIC timestamp can be read from all CPUs but the timer interrupt can be
>>> delivered to just one CPU at a time.  I think this is how we ended up
>>> with the current implementation, but any suggestions on how to improve
>>> this is welcome.
> 
> As a first step, I have copied the count /compare code from mips with
> minor modifications into mips/rmi, this lets me boot up (checked in as
> r210528).
> 
> I would like to add the PIC based clock next.

Thanks.

>> I would prefer to not mix the things.
>>
>> I think:
>>  - PIC timestamp looks like the best candidate for system timecounter.
>>  - per-CPU counters could be registered as per-CPU timecounters with
>> set_cputicker() - the main criteria there is a speed.
>>  - if per-CPU counters are synchronized between CPUs - they could be
>> registered as alternative timecounter for people who wish fastest
>> timecounting; if they are not - they are useless in that role.
>>  - both PIC timer and per-CPU comparators should be independently
>> registered as eventtimers - it is better to have two of them to from
>> accounting correctness PoV, and it will allow user to experiment which
>> one he likes more.
>>  - if there is any other timer hardware - it also should be registered -
>> it will give additional flexibility.
> 
> The per-cpu count/compare counters are not synchronized on XLR.

Then tick_ticker() function looks broken. counter_lower_last and
counter_upper should be tracked per-CPU. Otherwise you will have huge
forward jumps due to false overflows.

> So your suggestion would be to add a PIC based clock which calls
> tc_init() and et_register(), and to leave the set_cputicker() to be
> the count/compare?

Yes. And I would leave count/compare also calling tc_init() and
et_register() as it is now. It won't hurt.

> Also, with just the count/compare, I get these print on early mutiuser bootup.
> ---
> calcru: runtime went backwards from 85936878 usec to 236488 usec for
> pid 1286 (rpcbind)
> calcru: runtime went backwards from 7158742 usec to 19700 usec for pid
> 1285 (nfsiod 0)
> calcru: runtime went backwards from 111005442 usec to 305474 usec for
> pid 1257 (syslogd)
> calcru: runtime went backwards from 10740196 usec to 29555 usec for
> pid 1048 (devd)
> --
> Did not get much time to investigate, any idea what the cause  can be?

I think it can easily be result of broken tick_ticker().

-- 
Alexander Motin