Re: [PATCH 1/3] dynticks - implement no idle hz for x86

[Srivatsa Vaddagiri <vatsa@in.ibm.com>]

On Mon, Sep 05, 2005 at 09:57:30AM -0700, Nishanth Aravamudan wrote:
> I think it's ok where it is. Currently, with x86, at least, you can have
> an independent interrupt source and time source (not true for all archs,
> of course, ppc64 being a good example, I think?) Perhaps "handler"

By "independent" do you mean driven by separate clocks? PPC64 does
use decrementer as its interrupt source and Time-base-register as 
its timesource AFAIK. Both are driven by the same clock I think.

> What may be useful is something similar to what John Stultz does in his
> rework, attaching priorities to the various interrupt sources. For
> example, on x86, if we have an HPET, then we should use it, if not, then
> use APIC and PIT, but if the APIC doesn't exist in h/w, or is buggy
> (perhaps determined via a calibration loop), then only use the PIT.

This logic is what the arch-code should follow in picking its interrupt
source and is independent of dyn-tick. dyn-tick just works with whatever 
arch-code has chosen as its interrupt source.

> I agree. I guess max_skip, to me, is what the kernel thinks the
> interrupt source should maximally skip by, not what the interrupt source
> thinks it can do. So, I think it fits in fine with what you are saying
> and with the code you have in the current patch.

Great!

> I was just wondering; I guess it makes sense, but did you check to see
> if it ever *doesn't* get called? Like I said, __run_timers() [from how I

Haven't tested that, but I feel can happen in practice, since we dont
control device interrupts.

> base->timer_jiffies) [the condition in run_timer_softirq()] is not. How
> much does it cost to raise the softirq, if it is going to return
> immediately from the callback?

Don't know. It just felt nice to avoid any unnecessary invocations.

-- 


Thanks and Regards,
Srivatsa Vaddagiri,
Linux Technology Center,
IBM Software Labs,
Bangalore, INDIA - 560017