Re: [PATCH] perf/x86: fix event counter update issue

[Peter Zijlstra]

On Tue, Nov 29, 2016 at 05:06:36PM +0000, Liang, Kan wrote:
> 
> 
> > On Tue, Nov 29, 2016 at 02:46:14PM +0000, Liang, Kan wrote:
> > > > And note that we _ALWAYS_ set the IN bits, even for !sampling events.
> > > > Also note we set max_period to (1<<31) - 1, so we should never
> > > > exceed 31 bits.
> > > >
> > >
> > > The max_period is 0xfffffffff.
> > >
> > > The limit is breaked by this patch.
> > > 069e0c3c4058 ("perf/x86/intel: Support full width counting")
> > > https://patchwork.kernel.org/patch/2784191/
> > >
> > > 	/* Support full width counters using alternative MSR range */
> > > 	if (x86_pmu.intel_cap.full_width_write) {
> > > 		x86_pmu.max_period = x86_pmu.cntval_mask;
> > > 		x86_pmu.perfctr = MSR_IA32_PMC0;
> > > 		pr_cont("full-width counters, ");
> > > 	}
> > >
> > 
> > Wth do KNL/SLM have full_width_write set if they have short counters?
> 
> Yes, the full_width_write is set on KNL/SLM.
> 
> > I should the whole point of the full_wdith thing was that in that case the
> > counters were actually 64bit wide.
> 
> Even for big core, the counter width is 48 bit.
> AFAIK, no Intel platform has 64bit wide counter.

Then yes, that patch is very broken.