Re: [PATCH 4/9] perf/x86/intel: Support hardware TopDown metrics

[Liang, Kan]

On 5/28/2019 8:43 AM, Peter Zijlstra wrote:
> On Tue, May 21, 2019 at 02:40:50PM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:
> > The 8bit metrics ratio values lose precision when the measurement period
> > gets longer.
> >
> > To avoid this we always reset the metric value when reading, as we
> > already accumulate the count in the perf count value.
> >
> > For a long period read, low precision is acceptable.
> > For a short period read, the register will be reset often enough that it
> > is not a problem.
>
> > The PERF_METRICS may report wrong value if its delta was less than 1/255
> > of SLOTS (Fixed counter 3).
> >
> > To avoid this, the PERF_METRICS and SLOTS registers have to be reset
> > simultaneously. The slots value has to be cached as well.
>
> That doesn't sound like it is NMI-safe.
>  >
>
> > RDPMC
> > =========
> > The TopDown can be collected per thread/process. To use TopDown
> > through RDPMC in applications on Icelake, the metrics and slots values
> > have to be saved/restored during context switching.
> >
> > Add specific set_period() to specially handle the slots and metrics
> > event. Because,
> >   - The initial value must be 0.
> >   - Only need to restore the value in context switch. For other cases,
> >     the counters have been cleared after read.
>
> So the above claims to explain RDPMC, but doesn't mention that magic
> value below at all. In fact, I don't see how the above relates to RDPMC
> at all.

Current perf only support per-core Topdown RDPMC. On Icelake, it can be 
extended to per-thread Topdown RDPMC.
It tries to explain the extra work for per-thread topdown RDPMC, e.g. 
save/restore slots and metrics value in context switch.


> > @@ -2141,7 +2157,9 @@ static int x86_pmu_event_idx(struct perf_event *event)
> >   	if (!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED))
> >   		return 0;
> >   
> > -	if (x86_pmu.num_counters_fixed && idx >= INTEL_PMC_IDX_FIXED) {
> > +	if (is_metric_idx(idx))
> > +		idx = 1 << 29;
>
> I can't find this in the SDM RDPMC description. What does it return?

It will return the value of PERF_METRICS. I will add it in the changelog.

Thanks,
Kan
> > +	else if (x86_pmu.num_counters_fixed && idx >= INTEL_PMC_IDX_FIXED) {
> >   		idx -= INTEL_PMC_IDX_FIXED;
> >   		idx |= 1 << 30;
> >   	}