linux-kernel - Re: 2.6.21-rc6-mm1

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Message-Id: <200704111519.43738.ak@suse.de>
Date:	Wed, 11 Apr 2007 15:19:43 +0200
From:	Andi Kleen <ak@...e.de>
To:	eranian@....hp.com
Cc:	Venki Pallipadi <venkatesh.pallipadi@...el.com>,
	Adam Belay <abelay@...ell.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, randy.dunlap@...cle.com,
	lenb@...nel.org
Subject: Re: 2.6.21-rc6-mm1


> 
> So my point is that you cannot use the PMU to account for wall-clock time when you
> go in halted state. Yet, you could use the idle notifier to compensate for it by
> recording let's say TSC prior to entry and computing delta on exit. But then I am
> not sure what you would do with it given that other events may or may not have
> counted during that time, making scaling harder to do.

I think correcting it with the TSC is fine. It might not be perfect, but it's
good enough for most uses. Certainly much better than idle=poll

> Once solution, implemented by Itanium Montecito processor, is to have a PMU event
> that counts cycles spent in halted state. 

This solution is incompatible with aggressive power management.

-Andi
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