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

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Message-ID: <20080411075554.GA6834@elte.hu>
Date:	Fri, 11 Apr 2008 09:55:54 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Karsten Wiese <fzu@...gehoertderstaat.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: set_cyc2ns_scale() remove tsc_now and ns_now

* 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 
the affected installed base is still significant to warrant a fix.

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

	Ingo
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