linux-kernel - Re: [PATCH] x86: set_cyc2ns_scale() remove tsc_now and ns

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Message-ID: <20080411080617.GP10019@one.firstfloor.org>
Date:	Fri, 11 Apr 2008 10:06:17 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Karsten Wiese <fzu@...gehoertderstaat.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: set_cyc2ns_scale() remove tsc_now and ns_now

On Fri, Apr 11, 2008 at 09:55:54AM +0200, Ingo Molnar wrote:
> 
> * Andi Kleen <andi@...stfloor.org> wrote:
> 
> > (actually it is still the wrong time -- really needs a grace period 
> > during which the TSC is not used 
> > ftp://firstfloor.org/pub/ak/quilt/patches/sched-clock implemented some 
> > of these ideas against an older kernel)
> 
> recent CPUs have constant-freq TSCs so it's mostly a legacy issue, but 

Actually there millions of non constant freq TSC CPUs shipped each
quarter ... 

> we dont really have to worry about complications like grace periods - 
> higher layers in the scheduler protect against temporary sched_clock() 
> outliers.

But you still get scheduling hickups even with the sanity check. If the 
scheduler depends on a smooth time that is not good and my (admittedly much less 
than yours) understanding of CFS is that it relies on that. Especially ondemand 
can cause quite a lot of cpufreq changes on some workloads.

> So i think this can all be done much simpler. Just get rid of 
> the global cpu_khz notion, sched_clock() should simply follow the ->freq 
> value - and that's it.

At some point you have to generate an offset to something and that
offset must be different for different frequencies, otherwise
you get large systematic errors

(<imagine complicated mathematical proof why this is so, but it should
be fairly obvious>)

-Andi

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