linux-kernel - RE: [PATCH] ACPI: Read TSC upon resume

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Date:	Sat, 16 Oct 2010 03:03:54 +0000
From:	"Koornstra, Reinoud" <koornstra@...com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>, Greg KH <gregkh@...e.de>
CC:	Sameer Nanda <snanda@...omium.org>,
	"lenb@...nel.org" <lenb@...nel.org>,
	"stefan.bader@...onical.com" <stefan.bader@...onical.com>,
	"brad.figg@...onical.com" <brad.figg@...onical.com>,
	"apw@...onical.com" <apw@...onical.com>,
	"linux-acpi@...r.kernel.org" <linux-acpi@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] ACPI: Read TSC upon resume

> -----Original Message-----
> From: linux-acpi-owner@...r.kernel.org [mailto:linux-acpi-
> owner@...r.kernel.org] On Behalf Of Rafael J. Wysocki
> Sent: Thursday, October 07, 2010 2:44 PM
> To: Greg KH
> Cc: Sameer Nanda; lenb@...nel.org; stefan.bader@...onical.com;
> brad.figg@...onical.com; apw@...onical.com; linux-acpi@...r.kernel.org;
> linux-kernel@...r.kernel.org
> Subject: Re: [PATCH] ACPI: Read TSC upon resume
> 
> On Thursday, October 07, 2010, Greg KH wrote:
> > On Thu, Oct 07, 2010 at 11:05:21AM -0700, Sameer Nanda wrote:
> > > On Thu, Oct 7, 2010 at 10:46 AM, Greg KH <gregkh@...e.de> wrote:
> > > > On Thu, Oct 07, 2010 at 10:43:34AM -0700, Sameer Nanda wrote:
> > > >> On Wed, Oct 6, 2010 at 7:19 PM, Greg KH <gregkh@...e.de> wrote:
> > > >> > And are you always going to be printing this out?  Why do we
> want to
> > > >> > know this every time?
> > > >> >
> > > >>
> > > >> Yes, every time.  This helps track variance in BIOS resume times
> within a
> > > >> single boot.
> > > >
> > > > Is that really something that users can do something about?
> > >
> > > Aside from complaining to the BIOS vendors, no :)
> >
> > Then I would not recommend adding this patch, as it is irrelevant for
> > 99.9999% of all Linux users.
> 
> It may be somewhat useful, but the rdtscll() call seems to be x86-
> specific, in
> which case it shouldn't be used at this place.

Also, in the case of an intel core 2 duo cpu, the tsc is not stable, hence upon resume the cpu is spinning up and the first tsc's will be slower.
During idle-time the tsc will not be incremented. The tsc is only stably incremented upon 100% cpu usage. It also doesn't increment faster in turbo mode in case of some core 2 duo and certainly the Nehalem cpu's. Calculating in time in terms of tsc might not be so reliable.

> 
> Thanks,
> Rafael
> --
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