On Tue, May 28, 2019 at 02:23:09PM -0400, Liang, Kan wrote:
> > > 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.
Right, this has what relation to RDPMC ?
> > > @@ -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.
A comment would be even better.
