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    SubjectRe: clocksource changes in 2.6.31 - possible regression
    On Mon, 17 Aug 2009 11:15:54 -0700
    john stultz <johnstul@us.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@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.
    >
    > 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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