linux-kernel - Re: clocksource changes in 2.6.31

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Message-ID: <20090817112704.2b4b2987@nehalam>
Date:	Mon, 17 Aug 2009 11:27:04 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	john stultz <johnstul@...ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: clocksource changes in 2.6.31 - possible regression

On Mon, 17 Aug 2009 11:15:54 -0700
john stultz <johnstul@...ibm.com> wrote:

> On Mon, 2009-08-17 at 11:01 -0700, Stephen Hemminger wrote:
> > On Mon, 17 Aug 2009 10:48:57 -0700
> > john stultz <johnstul@...ibm.com> wrote:
> > 
> > > On Mon, 2009-08-17 at 09:03 -0700, Stephen Hemminger wrote:
> > > > The following commit causes a change for kernels built with HRT but
> > > > not actually using HRT.  I typically use the generic kernel we ship
> > > > on test machines, and that kernel has NOHZ and HRT (for power savings/virt
> > > > and HRT for QoS), but I want to be able to enable TSC as a clock source
> > > > when doing performance tests with pktgen.
> > > > 
> > > > The machine in question is a several year old Opteron box, that
> > > > normally reports clocksources: acpi_pm jiffies tsc
> > > > but now with 2.6.31-rc6, it only has acpi_pm.
> > > 
> > > I might need to review the patch again, but I believe we just don't
> > > allow you to switch to non HRT compatible clocksources (like jiffies) if
> > > we're already in HRT mode (and thus would hang when switched). 
> > > 
> > > 
> > > The behavior you describe where you can't switch to the TSC, may be due
> > > to the TSC disqualification code marking it as non HRT compatible
> > > (again, I need to double check). While I'm not sure that's really
> > > correct, as the TSC is fine for HRT, in this case on your box, the TSC
> > > has been marked as unstable (likely due to being unsynced on old AMD SMP
> > > systems). There is a real chance that the timekeeping code on your
> > > system could see the TSC go backwards, calculate a negative time
> > > interval, and then end up hanging. 
> > > 
> > 
> > TSC was alway stable on this box, and worked fine.  There was no
> > message in log about TSC instability. The change was bisected
> > down to that one commit.
> 
> But just to clarify, the TSC was never selected as the default
> clocksource on the box either, right?

correct.

I am okay with turning it off on boot command line for my tests,
but it might be an issue for other users.
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