NAK: [SRU][wily] Fix KNL Intel P State driver divide by zero

[Brad Figg]

On Wed, Oct 21, 2015 at 01:33:16AM +0000, Zhang, Xiong Y wrote:
> > On 10/20/2015 02:40 AM, Zhang, Xiong Y wrote:
> > > The following commit is from 4.3 kernel which fix devide by zero in
> > > intel P state driver on KNL.
> > >
> > > commit 8e601a9f97a00bab031980de34f9a81891c1f82f
> > > Author: Srinivas Pandruvada <srinivas.pandruvada at linux.intel.com>
> > > Date:   Thu Oct 15 12:34:21 2015 -0700
> > >
> > >     cpufreq: intel_pstate: Fix divide by zero on Knights Landing (KNL)
> > >
> > >     This is a workaround for KNL platform, where in some cases MPERF
> > counter
> > >     will not have updated value before next read of MSR_IA32_MPERF. In
> > this
> > >     case divide by zero will occur. This change ignores current sample for
> > >     busy calculation in this case.
> > >
> > >     Fixes: b34ef932d79a (intel_pstate: Knights Landing support)
> > >     Signed-off-by: Srinivas Pandruvada
> > <srinivas.pandruvada at linux.intel.com>
> > >     Acked-by: Kristen Carlson Accardi <kristen at linux.intel.com>
> > >     Cc: 4.1+ <stable at vger.kernel.org> # 4.1+
> > >     Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki at intel.com>
> > >
> > > diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
> > > index 3af9dd7..aa33b92 100644
> > > --- a/drivers/cpufreq/intel_pstate.c
> > > +++ b/drivers/cpufreq/intel_pstate.c
> > > @@ -776,6 +776,11 @@ static inline void intel_pstate_sample(struct
> > > cpudata *cpu)
> > >         local_irq_save(flags);
> > >         rdmsrl(MSR_IA32_APERF, aperf);
> > >         rdmsrl(MSR_IA32_MPERF, mperf);
> > > +       if (cpu->prev_mperf == mperf) {
> > > +               local_irq_restore(flags);
> > > +               return;
> > > +       }
> > > +
> > >         tsc = rdtsc();
> > >         local_irq_restore(flags);
> > >
> > > thanks
> > >
> > >
> > >
> > 
> > Wily is now subject to SRU policies. Do you have a bug report for this
> > patch ?
> [Zhang, Xiong Y] We only have a bug report internally and don't have one on launchpad. Because I don't have KNL hardware, so I couldn't fill one on launchpad and couldn't supply the verification.

You don't need the specific HW in order to create a bug for this. However,
someone needs to be able to verify the kernel that has the fix does the
right thing. You need to find someone with the HW in order to verify the
fix.

Brad

> > 
> > rtg
> > 
> > 
> > --
> > Tim Gardner tim.gardner at canonical.com
> 
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-- 
Brad Figg brad.figg at canonical.com http://www.canonical.com