linux-kernel - Re: [RFC] maximum latency tracking infrastructure

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Message-Id: <1156457795.6951.18.camel@dwalker1.mvista.com>
Date:	Thu, 24 Aug 2006 15:16:35 -0700
From:	Daniel Walker <dwalker@...sta.com>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	linux-kernel@...r.kernel.org, len.brown@...el.com
Subject: Re: [RFC] maximum latency tracking infrastructure

On Thu, 2006-08-24 at 23:57 +0200, Arjan van de Ven wrote:
> > I was just thinking that it might be cleaner to register a structure
> > instead of tracking identifiers to usecs. You might get a speed up on
> > some of the operations, like unregister.
> 
> it makes things a lot more complex for both the user and the
> infrastructure though, and I doubt it's going to be a performance gain;
> you need to walk all registered items anyway to decide the new minimum
> value if you unregister one for example.

Might be time for a priority list (lib/plist.c), but that might be like
swatting a gnat with a sledge hammer.

> > Another thing I was thinking about is that this seems somewhat contrary
> > to the idea of using dynamic tick (assuming it was in mainline) to
> > heuristically pick a power state. Do you have any thoughts on how you
> > would combine the two?
> 
> Actually it's designed in part FOR this case!
> So how that will work (thought experiment, I don't have the code yet)
> 
> In idle, determine the time the next scheduled event is.
> Then given that time go over the C-states and pick the deepest C-state
> that
> 1) satisfies the requested latency
> 2) has a latency that is a small enough fraction of the total time
> 
> (2 is needed to not pick a 1 msec-latency C state for a 1ms idle, that
> won't save you power most likely, so you need to have enough time in
> "real" idle)
> 
> so when you know your latency requirements, you now can pick a DEEPER
> sleepstate than you could before (or at least the right one)... dynticks
> needs this more than anything :)

Sounds pretty good. Since dynamic tick tracks timer events one could
also add a method to track interrupts in general if they are regular
enough to do so. That's just thinking while typing, so it might not be
sane.

Daniel

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