Re: [PATCH 03/12] perf/x86/intel: Add perf core PMU support for Sapphire Rapids

[Liang, Kan]

On 1/26/2021 9:44 AM, Peter Zijlstra wrote:
> On Tue, Jan 19, 2021 at 12:38:22PM -0800, kan.liang@xxxxxxxxxxxxxxx wrote:
> > @@ -3671,6 +3853,31 @@ static int intel_pmu_hw_config(struct perf_event *event)
> >   		}
> >   	}
> >   
> > +	/*
> > +	 * To retrieve complete Memory Info of the load latency event, an
> > +	 * auxiliary event has to be enabled simultaneously. Add a check for
> > +	 * the load latency event.
> > +	 *
> > +	 * In a group, the auxiliary event must be in front of the load latency
> > +	 * event. The rule is to simplify the implementation of the check.
> > +	 * That's because perf cannot have a complete group at the moment.
> > +	 */
> > +	if (x86_pmu.flags & PMU_FL_MEM_LOADS_AUX &&
> > +	    (event->attr.sample_type & PERF_SAMPLE_DATA_SRC) &&
> > +	    is_mem_loads_event(event)) {
> > +		struct perf_event *leader = event->group_leader;
> > +		struct perf_event *sibling = NULL;
> > +
> > +		if (!is_mem_loads_aux_event(leader)) {
> > +			for_each_sibling_event(sibling, leader) {
> > +				if (is_mem_loads_aux_event(sibling))
> > +					break;
> > +			}
> > +			if (list_entry_is_head(sibling, &leader->sibling_list, sibling_list))
> > +				return -ENODATA;
> > +		}
> > +	}
> > +
> >   	if (!(event->attr.config & ARCH_PERFMON_EVENTSEL_ANY))
> >   		return 0;
> >   
>
> I have vague memories of this getting mentioned in a call at some point.
> Pretend I don't know anything and tell me more.

Adding the auxiliary event is for the new data source fields, data block 
& address block. If perf only samples the load latency event, the value 
of the data block & address block fields in a sample is not correct. To 
get the correct value, we have to sample both the auxiliary event and 
the load latency together on SPR. So I add the check in the kernel. I 
also modify the perf mem in the perf tool accordingly.

Thanks,
Kan