linux-kernel - Re: [RFC PATCH] x86:Use cpu_khz for loops_per

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Message-ID: <20080627081202.GA26613@elf.ucw.cz>
Date:	Fri, 27 Jun 2008 10:12:02 +0200
From:	Pavel Machek <pavel@...e.cz>
To:	Alok Kataria <akataria@...are.com>
Cc:	kernel list <linux-kernel@...r.kernel.org>,
	Dan Hecht <dhecht@...are.com>
Subject: Re: [RFC PATCH] x86:Use cpu_khz for loops_per_jiffy calculation

Hi!

> > > > > AFAIU in calibrate_delay_direct too we measure the amount by which timer
> > > > > has ticked until DELAY_CALIBRATION_TICKS amount of jiffies has passed.
> > > > > So IMO the code there too assumes that there is one loop per timer
> > > > > cycle ?
> > > >
> > > > On my machine, it reports:
> > > >
> > > > delay using timer specific routine.. 3661.98 BogoMIPS (lpj=7323971)
> > > > ...
> > > > Detected 1828.828 MHz processor.
> > > >
> > > > (/proc/cpuinfo)
> > > > model name      : Genuine Intel(R) CPU           T2400  @ 1.83GHz
> > > > ...
> > > > cpu MHz         : 1000.000
> > > How is it 1000 here ? shouldn't this be 1830.xx
> > 
> > cpufreq effect, I believe.
> > 
> > > > bogomips        : 3657.54
> > > >
> > > > So you'd break it by setting lpj (aka bogomips) to cpu_khz, right?
> > >
> > > We are not setting it to cpu_khz but to tsc_khz, i am assuming that in
> > > this case tsc_khz will be different than cpu_khz. Can you please mail me
> > > the full dmesg log.
> > 
> > Yes, but neither cpu_khz nor tsc_khz will be 3657 bogoMips, right?
> 
> Hi Pavel,
> 
> Thanks for the dmesg. The HZ value that you are using on this system is
> 250, right ?
> 
> If you note the calculations
> > +     lpj = ((u64)tsc_khz * 1000);
> > +     do_div(lpj, HZ);
> 
> We are dividing by HZ over here. So you are right in saying that tsc_khz
> wont be equal to bogoMips but lpj_fine will surely be computed correct
> since we do consider the HZ value.

Ok.

> Please let me know if you still have any doubts.
> Or can i safely assume that you will ACK the patch ;-)

Well, I'm not expert-enough in this subsystem (nor comfortable enough
with your code) to ACK it, sorry.
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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