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

[Nishanth Aravamudan <nacc@us.ibm.com>]

On 05.09.2005 [22:55:01 +0530], Srivatsa Vaddagiri wrote:
> 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.

Well, independent as in not the same, I meant. Let me think about it and
look at the code a bit, before making myself or anyone else more
confused. John, do you have any input on what I'm getting at? I know we
have discussed this before...

> > 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.

Yes, true. I didn't mean for the h/w interrupt source selection to be
part of the arch-independent code, but that we might need to include a
priority field in the interrupt_source structure to allow the
arch-dependent code to do so.

> > 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.

Well, it would be interesting to see if there's any difference without
that function, or if it's even getting called.

> > 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.

Yes, but it also might add a function which doesn't need to be. I'll
take a closer look at this too.

Thanks,
Nish