Date: Tue, 29 Nov 2005 17:48:58 +0100
From: Andi Kleen <ak@suse.de>
To: thockin@hockin.org
Cc: Andi Kleen <ak@suse.de>, Steven Rostedt <rostedt@goodmis.org>,
       Ingo Molnar <mingo@elte.hu>, john stultz <johnstul@us.ibm.com>,
       LKML <linux-kernel@vger.kernel.org>
Subject: Re: [RT] read_tsc: ACK! TSC went backward! Unsynced TSCs?
Message-ID: <20051129164858.GL19515@wotan.suse.de>
References: <1133179554.11491.3.camel@localhost.localdomain> <20051128173040.GA32547@hockin.org> <1133199568.7416.31.camel@mindpipe> <20051128183517.GA4549@hockin.org> <p73r78zh7kv.fsf@verdi.suse.de> <20051129165219.GA12687@hockin.org>
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On Tue, Nov 29, 2005 at 08:52:19AM -0800, thockin@hockin.org wrote:
> On Tue, Nov 29, 2005 at 07:06:24AM -0700, Andi Kleen wrote:
> > But I'm surprised you're saying 2.6.11 broke. At least 2.6.11 64bit should
> > have always used HPET in this case. I only broke it around 2.6.13
> > where I added an overeager optimization for single socket DC on my side based
> > on a misunderstanding. Earlier and later kernels should have been ok.
> 
> we didn't have HPET enabled in BIOS until recently.  Turning that on made
> all the TSC gettimeofday() crap disappear.  Now to find and kill any
> straggling users of rdtsc

The newer kernels work around this too by using pmtimer when needed.
Of course it's slow. 

Regarding straggling users of rdtsc - one way would be to optionally
trap them and log them in the kernel. That would work in ring 3 at least.

-Andi

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