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

[Peter Zijlstra <peterz@infradead.org>]

On Tue, May 28, 2019 at 02:56:15PM +0200, Peter Zijlstra wrote:
> On Tue, May 21, 2019 at 02:40:50PM -0700, kan.liang@linux.intel.com wrote:
> 
> > diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> > index e9075d57853d..07ecfe75f0e6 100644
> > --- a/arch/x86/events/core.c
> > +++ b/arch/x86/events/core.c
> > @@ -91,16 +91,20 @@ u64 x86_perf_event_update(struct perf_event *event)
> >  					new_raw_count) != prev_raw_count)
> >  		goto again;

AFAICT, you don't actually need that cmpxchg loop for the metric crud.

> >  
> > -	/*
> > -	 * Now we have the new raw value and have updated the prev
> > -	 * timestamp already. We can now calculate the elapsed delta
> > -	 * (event-)time and add that to the generic event.
> > -	 *
> > -	 * Careful, not all hw sign-extends above the physical width
> > -	 * of the count.
> > -	 */
> > -	delta = (new_raw_count << shift) - (prev_raw_count << shift);
> > -	delta >>= shift;
> > +	if (unlikely(hwc->flags & PERF_X86_EVENT_UPDATE))
> > +		delta = x86_pmu.metric_update_event(event, new_raw_count);
> > +	else {
> > +		/*
> > +		 * Now we have the new raw value and have updated the prev
> > +		 * timestamp already. We can now calculate the elapsed delta
> > +		 * (event-)time and add that to the generic event.
> > +		 *
> > +		 * Careful, not all hw sign-extends above the physical width
> > +		 * of the count.
> > +		 */
> > +		delta = (new_raw_count << shift) - (prev_raw_count << shift);
> > +		delta >>= shift;
> > +	}
> >  
> >  	local64_add(delta, &event->count);
> >  	local64_sub(delta, &hwc->period_left);
> 
> > @@ -1186,6 +1194,9 @@ int x86_perf_event_set_period(struct perf_event *event)
> >  	if (idx == INTEL_PMC_IDX_FIXED_BTS)
> >  		return 0;
> >  
> > +	if (x86_pmu.set_period && x86_pmu.set_period(event))
> > +		goto update_userpage;
> > +
> >  	/*
> >  	 * If we are way outside a reasonable range then just skip forward:
> >  	 */
> > @@ -1234,6 +1245,7 @@ int x86_perf_event_set_period(struct perf_event *event)
> >  			(u64)(-left) & x86_pmu.cntval_mask);
> >  	}
> >  
> > +update_userpage:
> >  	perf_event_update_userpage(event);
> >  
> >  	return ret;
> 
> 
> *groan*.... that's pretty terrible.