Re: [PATCH] i386: Selectable Frequency of the Timer Interrupt

Jesper Juhl
2005-07-17T02:13:40Z

On 7/15/05, Jesper Juhl <jesper.juhl@gmail.com> wrote:
> On 7/15/05, Linus Torvalds <torvalds@osdl.org> wrote:
> >
> > On Fri, 15 Jul 2005, Jesper Juhl wrote:
> > >
> > > It's buggy, that I know. setting kernel_hz (the new boot parameter) to
> > > 250 causes my system clock to run at something like 4-5 times normal
> > > speed
> >
> > 4 times normal. You don't actually make the timer interrupt happen at
> > 250Hz, so the timer will be programmed to run at the full 1kHz.
> >
> Right, that's the basic problem. I increase jiffies at a higher rate
> but didn't slow the timer interrupt down at the same time.
>
> > You also need to actually change the LATCH define (in
> > include/linux/jiffies.h) to take this into account (there might be
> > something else too).
> >
> [...]
> > and you might be getting closer.
> >
> > Of course, you need to make sure that LATCH is used only after
> > jiffies_increment is set up. See "setup_pit_timer(void)" in
> > arch/i386/kernel/timers/timer_pit.c for more details.
> >
>
> Thank you for all the pointers and hints. This is a new area of code
> for me, so I'll need some time to poke around - the above helps a lot.
> Unfortunately I won't have any time to work on this today, but I'll
> see if I can get a working implementation together tomorrow.
>

Ok, I'm afraid I'm going to need another hint or two.

I've been looking at the timer code and getting thoroughly confused.
I've tried to find out where we actually program the interrupt
controller to say "this is the frequency I want you to interrupt me
at", but I can't seem to find it.
I'm aware that there are multiple possible time sources, and I've been
looking at the 8259 code, the ioapic code, the hpet code and various
other bits in arch/i386/kernel/ and arch/i386/kernel/timers/ , but I
seem to end up getting confused about all the different defines like
CLOCK_TICK_RATE, ACTHZ, TICK_NSEC, TICK_USEC, etc...

Where do we actually program the tick rate we want?

--
Jesper Juhl <jesper.juhl@gmail.com>
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