Subject: Re: [RFC][PATCH] Runtime switching of the idle function [take 2]
From: Steven Rostedt <rostedt@goodmis.org>
To: john stultz <johnstul@us.ibm.com>
Cc: Andrew Morton <akpm@osdl.org>, mingo@elte.hu,
       acpi-devel@lists.sourceforge.net, len.brown@intel.com,
       nando@ccrma.Stanford.EDU, rlrevell@joe-job.com,
       linux-kernel@vger.kernel.org, paulmck@us.ibm.com, kr@cybsft.com,
       tglx@linutronix.de, pluto@agmk.net, john.cooper@timesys.com,
       bene@linutronix.de, dwalker@mvista.com, trini@kernel.crashing.org,
       george@mvista.com
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Date: Tue, 29 Nov 2005 09:22:37 -0500
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On Mon, 2005-11-28 at 20:22 -0800, john stultz wrote:
> On Mon, 2005-11-28 at 22:42 -0500, Steven Rostedt wrote:
> > On Mon, 2005-11-28 at 19:02 -0800, Andrew Morton wrote:
> > > Steven Rostedt <rostedt@goodmis.org> wrote:
> > > >
> > > > This patch creates a directory in /sys/kernel called idle.
> > > >
> > > 
> > > At no point do you appear to explain _why_ the kernel needs this feature?
> > 
> > Sorry about that.  This originally came up when we had problems with the
> > AMD64 x2 in the -rt patch.  It was noted that the TSCs would get very
> > far out of sync and cause problems.  The way to solve this was to set
> > idle=poll.  The original patch I sent was to allow the user to change to
> > idle=poll dynamically.  This way they could switch to the poll_idle and
> > run there tests (requiring tsc not to drift) and then switch back to the
> > default idle to save on electricity.
> 
> The problem with this is that this must be a one way transition. That
> is, once the TSCs have become unsynchronized, there is no use going back
> to using the polling idle unless you add some code to re-sync the TSCs
> which would be ugly to do after the system has booted.
> 

I've thought about that too.  But this patch does allow you to start
with  idle=poll and then switch back.  Also, if you do lock to a cpu,
you don't need to worry about the tsc from slipping if you switch to
idle=poll.

-- Steve

> Using idle=poll (for anything other then debugging) is really a worst
> case workaround for systems that do not have alternative clocksources
> like ACPI PM or HPET.
> 
> Its an interesting bit of code, but I'm not really sure I understand its
> usefulness.
> 
> thanks
> -john
> 
> 
> 

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