Re: clocksource changes in 2.6.31 - possible regression

[Stephen Hemminger <shemminger@vyatta.com>]

On Mon, 17 Aug 2009 10:48:57 -0700
john stultz <johnstul@us.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.

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