linux-kernel - Re: Performance overhead of get_cycles

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Message-ID: <20071211170334.GA30144@elte.hu>
Date:	Tue, 11 Dec 2007 18:03:34 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	dor.laor@...ranet.com, tglx@...utronix.de,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kvm-devel <kvm-devel@...ts.sourceforge.net>
Subject: Re: Performance overhead of get_cycles_sync


* Arjan van de Ven <arjan@...radead.org> wrote:

> > * Dor Laor <dor.laor@...il.com> wrote:
> > 
> > The patch below should resolve this - could you please test and Ack 
> > it? But this CPUID was present in v2.6.23 too, so why did it only 
> > show up in 2.6.24-rc for you?
> 
> isn't this probably wrong since this code is also used in the vsyscall 
> code..

the TSC clocksource (and hence the vsyscall code) is turned off on 
systems that fail the TOD/CLOCK portion of this test:

  http://people.redhat.com/mingo/time-warp-test/time-warp-test.c

i.e. on the majority of systems in place.

	Ingo
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