Re: Bogomips and loops_per_jiffy

On Thu, 9 May 2002, Troy Benjegerdes wrote:

quoted

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while ((next_dec = tb_ticks_per_jiffy - tb_delta(&jiffy_stamp)) < 0)
{

Now that next_dec is unsigned, this condition is always false.

Yes I wrote this in the most possible compact way.  But I don't understand
(yet) how this can cause problems. You have a security margin of about 2
billion timbase ticks.

The problem occurs when calibrating the delay loop, and the timebase is
very large.

The fact that the TB is large or not should be irrelevanet, after all
the TB wraps around every 2 to 5 minutes or so on most processors. Do the
math, it's rather simple modulo 2^32 arithmetic. If I lost decrementer
interrupts every time the TB wraps around, I would see strange things
under top for example.

Now if you do a set_tb after the time_init at some point, you have to
update the timekeeping variables (jiffy_stamp or whatever) in sync.
Otherwise you are going to have problems.

Note that there is a very simple possible patch: truncate the new value
you load into the decrementer to tb_ticks_per_jiffy. And for debugging,
keep the last few values of the timebase at the decrementer interrupt
around and print them when you have to truncate. I'm afraid that this is a
symptom of lost timekeeping after reading the RTC and that you simply
cure the symptom.

http://lists.linuxppc.org/linuxppc-embedded/200205/msg00040.html

We could probably just avoid all this and do 'set_tb(0,0)' in time_init
(since we go and try to sync the timebase on SMP systems anyway)

Better set the TB to a large value to force the problem and see if you can
reproduce it.

	Regards,
	Gabriel.


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