Date: Thu, 8 Sep 2005 14:22:13 -0700
From: Nishanth Aravamudan <nacc@us.ibm.com>
To: Tony Lindgren <tony@atomide.com>
Cc: Srivatsa Vaddagiri <vatsa@in.ibm.com>, Con Kolivas <kernel@kolivas.org>,
       Russell King <rmk+lkml@arm.linux.org.uk>, linux-kernel@vger.kernel.org,
       akpm@osdl.org, ck list <ck@vds.kolivas.org>
Subject: Re: [PATCH 1/3] dynticks - implement no idle hz for x86
Message-ID: <20050908212213.GB2997@us.ibm.com>
References: <200509031801.09069.kernel@kolivas.org> <20050903090650.B26998@flint.arm.linux.org.uk> <200509031814.49666.kernel@kolivas.org> <20050904201054.GA4495@us.ibm.com> <20050905070053.GA7329@in.ibm.com> <20050905072704.GB5734@atomide.com> <20050905170202.GJ25856@us.ibm.com> <20050907073743.GB5804@atomide.com> <20050907150517.GC4590@us.ibm.com> <20050908100035.GD25847@atomide.com>
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On 08.09.2005 [13:00:36 +0300], Tony Lindgren wrote:
> * 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 :)

Yup, I think that makes the most sense. I appreciate your help with it!

> > > > > 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()...

Yes, I agree with the del_timer() changes.

> > > > > 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...

I guess you are right, I'll change my little summary and send it out so
it's archived.

Thanks,
Nish
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