[RFC] Event timers on MIPS

[Jayachandran C.]

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.

> 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.

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?

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?

Thanks,
JC.