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

[Tony Lindgren <tony@atomide.com>]

* Nishanth Aravamudan <nacc@us.ibm.com> [050907 18:07]:
> On 07.09.2005 [10:37:43 +0300], Tony Lindgren wrote:
> > * Nishanth Aravamudan <nacc@us.ibm.com> [050905 20:02]:
> > > On 05.09.2005 [10:27:05 +0300], Tony Lindgren wrote:
> > > > * Srivatsa Vaddagiri <vatsa@in.ibm.com> [050905 10:03]:
> > > > > On Sun, Sep 04, 2005 at 01:10:54PM -0700, Nishanth Aravamudan wrote:
> > > > > > 
> > > > > > Also, I am a bit confused by the use of "dynamic-tick" to describe these
> > > > > > changes. To me, these are all NO_IDLE_HZ implementations, as they are
> > > > > > only invoked from cpu_idle() (or their equivalent) routines. I know this
> > > > > > is true of s390 and the x86 code, and I believe it is true of the ARM
> > > > > > code? If it were dynamic-tick, I would think we would be adjusting the
> > > > > > timer interrupt frequency continuously (e.g., at the end of
> > > > > > __run_timers() and at every call to {add,mod,del}_timer()). I was
> > > > > > working on a patch which did some renaming to no_idle_hz_timer, etc.,
> > > > > > but it's mostly code churn :)
> > > > > 
> > > > > Yes, the name 'dynamic-tick' is misleading!
> > > > 
> > > > Huh? For most people dynamic-tick is much more descriptive name than
> > > > NO_IDLE_HZ or VST!
> > > 
> > > I understand this. My point is that the structures are *not*
> > > dynamic-tick specific. They are interrupt source specific, generally
> > > (also known as hardware timers) -- dynamic tick or NO_IDLE_HZ are the
> > > users of the interrupt source reprogramming functions, but not the
> > > reprogrammers themselves, in my mind. Also, it still would be confusing
> > > to use dynamic-tick, when the .config option is NO_IDLE_HZ! :)
> > 
> > I see what you mean, it's a confusing naming issue currently :) Would
> > the following solution work for you:
> > 
> > - Dynamic tick is the structure you register with, and then you use it
> >   for any kind of non-continuous timer tinkering 
> > 
> > - This structure has at least two possible users, NO_IDLE_HZ and
> >   sub-jiffie timers
> > 
> > So we could have following config options:
> > 
> > CONFIG_DYNTICK
> > CONFIG_NO_IDLE_HZ	depends on dyntick
> > CONFIG_SUBJIFFIE_TIMER	depends on dyntick
> 
> Hrm, yes, first you are right with the dependency ordering. I take it
> CONFIG_DYNTICK is simply there as NO_IDLE_HZ and SUBJIFFIE_TIMER are
> independent users of the same underlying infrastructure.

Cool, I'm glad we got the dependencies figured out now rather than later :)
 
> > > > If you wanted, you could reprogram the next timer to happen from
> > > > {add,mod,del}_timer() just by calling the timer_dyn_reprogram() there.
> > > 
> > > I messed with this with my soft-timer rework (which has since has fallen
> > > by the wayside). It is a bit of overhead, especially del_timer(), but
> > > it's possible. This is what I would consider "dynamic-tick." And I would
> > > setup a *different* .config option to enable it. Perhaps depending on
> > > CONFIG_NO_IDLE_HZ.
> > 
> > Yes, I agree it should be a different .config option. Maybe the example
> > above would work for that?
> 
> Yes, I'm thinking it might.
> 
> > > > And you would want to do that if you wanted sub-jiffie timer
> > > > interrupts.
> > > 
> > > Yes, true, it does enable that. Well, to be honest, it completely
> > > redefines (in some sense) the jiffy, as it is potentially continuously
> > > changing, not just at idle times.
> > 
> > Yeah. But should still work as we already accept interrupts at any point
> > inbetween jiffies to update time, and update the system time from a
> > second continuously running timer :)
> 
> The problem with subjiffie timers is that the precision of soft-timers
> is jiffies currently. It requires some serious effort to modify the
> soft-timer subsystem to be aware of the extra bits it needs,
> efficiently -- take a look at what HRT has had to do.

Yes, we should coordinate that with HRT. BTW, we can reduce the overhead
of del_timer() by _not_ calling next_timer_interrupt(), and programming
the next timer interrupt to happen where next jiffie would be. Then once
we get to the idle, we call next_timer_interrupt()...
 
> > > > So I'd rather not limit the name to the currently implemented
> > > > functionality only :)
> > > 
> > > I'm not trying to limit the name, but make sure we are tying the
> > > strcutures and functions to the right abstraction (interrupt source, in
> > > my opinion).
> > 
> > But other devices are interrupt sources too... And really the only use
> > for this stuct is non-continuous timer stuff, right?
> 
> Would "tick_source" be better? I guess you are right, that there is only
> this one consumer... Although if that is the case, then maybe a separate
> .h/.c file is the right way to go, to isolate the code, reduce
> #ifdeffery in timer.h/.c.

Hmmm, seems like dyntick.[ch] is still the best name for it...

Tony
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