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

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Date:   Tue, 26 Jan 2021 16:37:59 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     kan.liang@...ux.intel.com
Cc:     acme@...nel.org, mingo@...nel.org, linux-kernel@...r.kernel.org,
        eranian@...gle.com, namhyung@...nel.org, jolsa@...hat.com,
        ak@...ux.intel.com, yao.jin@...ux.intel.com
Subject: Re: [PATCH 03/12] perf/x86/intel: Add perf core PMU support for
 Sapphire Rapids

On Tue, Jan 19, 2021 at 12:38:22PM -0800, kan.liang@...ux.intel.com 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.

Hmmm?