linux-kernel - Re: [PATCH] i386 add idle notifier

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Date:	Thu, 30 Nov 2006 00:16:37 -0800
From:	Stephane Eranian <eranian@....hp.com>
To:	Andi Kleen <ak@...e.de>
Cc:	Andrew Morton <akpm@...l.org>,
	Christoph Hellwig <hch@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] i386 add idle notifier

Hi,

On Thu, Nov 30, 2006 at 12:21:41AM +0100, Andi Kleen wrote:
> 
> > An alternative approach might be to change perfmon so that it works out
> > whether it is being called by an idle thread
> > 
> > 	if ((current->flags & PF_IDLE) && (other stuff to do with irqs?))
> > 		return;
> 
> The problem is that the performance counters just keep running in the CPU.
> Perfmon needs to do something actively to disable them.
> 
Exactly.

> Actually on x86 they usually stop in true idle state in hardware, but 
> they don't do in polling mode and it sometimes seems to depend on
> the firmware.  So it mostly would be for idle=poll
> 
Most likely. But we also have the deal with other architectures such
as MIPS,IA64, and PPC.

> But if you do walk clock time profiling exactly because they stop 
> a profiler should account for this somehow. Otherwise the profiling time
> doesn't add up to 100%
> 
Yes. Note that perfmon does maintain the duration when monitornig was active.
So it is possible to determine active time and use it to scale counts.

-- 
-Stephane
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