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

[Liang, Kan]

On 5/29/2019 3:57 AM, Peter Zijlstra wrote:
> On Tue, May 28, 2019 at 02:24:56PM -0400, Liang, Kan wrote:
> > On 5/28/2019 9:48 AM, Peter Zijlstra wrote:
> > > On Tue, May 21, 2019 at 02:40:50PM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:
> > > > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > > > index b980b9e95d2a..0d7081434d1d 100644
> > > > --- a/include/linux/perf_event.h
> > > > +++ b/include/linux/perf_event.h
> > > > @@ -133,6 +133,11 @@ struct hw_perf_event {
> > > >    			struct hw_perf_event_extra extra_reg;
> > > >    			struct hw_perf_event_extra branch_reg;
> > > > +
> > > > +			u64		saved_metric;
> > > > +			u64		saved_slots;
> > > > +			u64		last_slots;
> > > > +			u64		last_metric;
> > >
> > > This is really sad, and I'm thinking much of that really isn't needed
> > > anyway, due to how you're not using some of the other fields.
> >
> > If we don't cache the value, we have to update all metrics events when
> > reading any metrics event. I think that could bring high overhead.
>
> Since you don't support sampling, or even 'normal' functionality on this
> FIXED3/SLOTS thing, you'll not use prev_count, sample_period,
> last_period, period_left, interrupts_seq, interrupts, freq_time_stamp
> and freq_count_stamp.
>
> So why do you then need to grow the data structure with 4 more nonsense
> fields?
>
> Also, I'm not sure what you need those last things for, you reset the
> value to 0 every time you read them, there is no delta to track.

The 'last things' are only for per-thread Topdown RDPMC.
We have to save/restore slots and metrics value in context switch.
It's used to calculate the delta between thread shced in and current 
read/sched out.


I will split the patch into several patches, and try to make things clear.

Thanks,
Kan