RE: [PATCH 1/2] perf/x86/intel: enable CPU ref_cycles for GP counter

[Liang, Kan]

> 
> On Mon, May 22, 2017 at 12:23 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> wrote:
> > On Mon, May 22, 2017 at 04:55:47PM +0000, Liang, Kan wrote:
> >>
> >>
> >> > On Fri, May 19, 2017 at 10:06:21AM -0700, kan.liang@xxxxxxxxx wrote:
> >> > > diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> >> > > index
> >> > > 580b60f..e8b2326 100644
> >> > > --- a/arch/x86/events/core.c
> >> > > +++ b/arch/x86/events/core.c
> >> > > @@ -101,6 +101,10 @@ u64 x86_perf_event_update(struct
> perf_event
> >> > *event)
> >> > >   delta = (new_raw_count << shift) - (prev_raw_count << shift);
> >> > >   delta >>= shift;
> >> > >
> >> > > + /* Correct the count number if applying ref_cycles replacement
> >> > > + */ if (!is_sampling_event(event) &&
> >> > > +     (hwc->flags & PERF_X86_EVENT_REF_CYCLES_REP))
> >> > > +         delta *= x86_pmu.ref_cycles_factor;
> >> >
> >> > That condition seems wrong, why only correct for !sampling events?
> >> >
> >>
> >> For sampling, it's either fixed freq mode or fixed period mode.
> >>  - In the fixed freq mode, we should do nothing, because the adaptive
> >>    frequency algorithm will handle it.
> >>  - In the fixed period mode, we have already adjusted the period in
> >>     ref_cycles_rep().
> >> Therefore, we should only handle !sampling events here.
> >
> > How so? For sampling events the actual event count should also be
> > accurate.
> 
> Yes, it must be. Because you can reconstruct the total number of occurrences
> of the event by adding all the periods recorded in each sample. So the period
> in each sample must reflect user event and not kernel event.

Peter and Stephane, you are right.
After adjusting the period, I can only make sure the number of samples for
the bus_cycles event is the same as that for ref cycles event.
I still need to adjust the number of occurrences of the event accordingly,
to make it accurate.
I will change it in next version.

Thanks,
Kan