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

[Liang, Kan]

On 1/26/2021 10:37 AM, Peter Zijlstra wrote:
> On Tue, Jan 19, 2021 at 12:38:22PM -0800, kan.liang@xxxxxxxxxxxxxxx wrote:
> > @@ -1577,9 +1668,20 @@ static void setup_pebs_adaptive_sample_data(struct perf_event *event,
> >   	}
> >   
> >   	if (format_size & PEBS_DATACFG_MEMINFO) {
> > +		if (sample_type & PERF_SAMPLE_WEIGHT) {
> > +			u64 weight = meminfo->latency;
> > +
> > +			if (x86_pmu.flags & PMU_FL_INSTR_LATENCY)
> > +				weight >>= PEBS_CACHE_LATENCY_OFFSET;
> > +			data->weight = weight & PEBS_LATENCY_MASK ?:
> >   				intel_get_tsx_weight(meminfo->tsx_tuning);
> > +		}
> > +
> > +		if (sample_type & PERF_SAMPLE_WEIGHT_EXT) {
> > +			data->weight_ext.val = 0;
> > +			if (x86_pmu.flags & PMU_FL_INSTR_LATENCY)
> > +				data->weight_ext.instr_latency = meminfo->latency & PEBS_LATENCY_MASK;
> > +		}
> >   
> >   		if (sample_type & PERF_SAMPLE_DATA_SRC)
> >   			data->data_src.val = get_data_src(event, meminfo->aux);
>
> Talk to me about that SAMPLE_WEIGHT stuff.... I'm not liking it.
>
> Sure you want multiple dimensions, but urgh.
>
> Also, afaict, as proposed you're wasting 80/128 bits. That is, all data
> you want to export fits in a single u64 and yet you're using two, which
> is mighty daft.
>
> Sure, pebs::lat / pebs_meminfo::latency is defined as a u64, but you
> can't tell me that that is ever actually more than 4G cycles. Even the
> TSX block latency is u32.
>
> So how about defining SAMPLE_WEIGHT_STRUCT which uses the exact same
> data as SAMPLE_WEIGHT but unions it with a struct. I'm not sure if we
> want:
>
> union sample_weight {
> 	u64 weight;
>
> 	struct {
> 		u32	low_dword;
> 		u32	high_dword;
> 	};
>
> 	/* or */
>
> 	struct {
> 		u32	low_dword;
> 		u16	high_word;
> 		u16	higher_word;
> 	};
> };
>
> Then have the core code enforce SAMPLE_WEIGHT ^ SAMPLE_WEIGHT_STRUCT and
> make the existing code never set the high dword.

So the kernel will only accept either SAMPLE_WEIGHT type or 
SAMPLE_WEIGHT_STRUCT type. It should error out if both types are set, right?

I will check if u32 is enough for meminfo::latency on the previous 
platforms.

Thanks,
Kan