Re: [PATCH] ACPI: Read TSC upon resume

[Chen Gong]

ä 10/16/2010 11:03 AM, Koornstra, Reinoud åé:
> > -----Original Message-----
> > From: linux-acpi-owner@xxxxxxxxxxxxxxx [mailto:linux-acpi-
> > owner@xxxxxxxxxxxxxxx] On Behalf Of Rafael J. Wysocki
> > Sent: Thursday, October 07, 2010 2:44 PM
> > To: Greg KH
> > Cc: Sameer Nanda; lenb@xxxxxxxxxx; stefan.bader@xxxxxxxxxxxxx;
> > brad.figg@xxxxxxxxxxxxx; apw@xxxxxxxxxxxxx; linux-acpi@xxxxxxxxxxxxxxx;
> > linux-kernel@xxxxxxxxxxxxxxx
> > 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@xxxxxxx>  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@xxxxxxx>  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.
If I'm wrong, please feel free to fix me.

I have 2 questions to your answer:

1.  CPU has a flag named constant_tsc to keep TSC always working in a 
constant way, so it is irrelvant to the CPU freq. whether in turbo mode 
or any P-state CPU currently belongs to, TSC should be not affected. 
IIRC, this flag should exist long before, at least before Core 2 duo. If 
so, TSC shoule be stable in this kind of environment.

2. though during idle-time TSC will not be incremented, here I want to
remind it is right before Westmere (TSC not always running), and you 
mentioned "upon resume the cpu is spinning up and the first tsc's will 
be slower", I don't know if this commit cd7240c0b can fix it up.
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