linux-kernel - Re: [patch 13/19] GTOD: Mark TSC unusable for highres timers

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Date:	Fri, 10 Nov 2006 09:57:28 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...l.org>
Cc:	tglx@...utronix.de, Andi Kleen <ak@...e.de>,
	john stultz <johnstul@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, Len Brown <lenb@...nel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Roman Zippel <zippel@...ux-m68k.org>
Subject: Re: [patch 13/19] GTOD: Mark TSC unusable for highres timers

* Andrew Morton <akpm@...l.org> wrote:

> If so, could that function use the PIT/pmtimer/etc for working out if 
> the TSC is bust, rather than directly using jiffies?

there's no realiable way to figure out the TSC is bust: some CPUs have a 
slight 'skew' between cores for example. On some systems the TSC might 
skew between sockets. A CPU might break its TSC only once some 
powersaving mode has been activated - which might be long after bootup. 
The whole TSC business is a nightmare and cannot be supported reliably. 
AFAIK Windows doesnt use it, so it's a continuous minefield for new 
hardware to break.

We should wait until CPU makers get their act together and implement a 
TSC variant that is /architecturally promised/ to have constant 
frequency (system bus frequency or whatever) and which never stops.

	Ingo
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