Re: [PATCH v4 10/14] perf stat: Implement streaming JSON formatting callbacks

[Namhyung Kim <namhyung@kernel.org>]

On Thu, Jul 16, 2026 at 12:02:59AM -0700, Ian Rogers wrote:
> This patch implements streaming JSON formatting callbacks inside
> util/stat-print-json.c, replacing the empty stubs introduced in Commit
> 1.
> 
> Delivers a highly optimized, zero-allocation, and 100% streaming print
> engine for JSON normal and metric-only modes. It bypasses dynamic queue
> events and metric lists entirely, formatting and streaming JSON objects
> directly onto the output file descriptor.
> 
> Utilizes the newly centralized unified aggregation helpers to format CPU
> and thread keys inside the JSON objects, and incorporates full
> interval-mode timestamp printing support.
> 
> Signed-off-by: Ian Rogers <irogers@google.com>
> Assisted-by: Antigravity:gemini-3.5-flash
> Acked-by: Chun-Tse Shao <ctshao@google.com>
> ---
[SNIP]
> +/**
> + * print_aggr_id_json - Print the aggregation prefix for JSON format.
> + *
> + * Copied and adapted from stat-display.c.
> + */
> +static void print_aggr_id_json(const struct perf_stat_config *config, FILE *output,
> +			       struct evsel *evsel, struct aggr_cpu_id id, int aggr_nr)
> +{
> +	switch (config->aggr_mode) {
> +	case AGGR_CORE:
> +		fprintf(output, "\"core\" : \"S%d-D%d-C%d\", \"counters\" : %d, ", id.socket,
> +			id.die, id.core, aggr_nr);
> +		break;
> +	case AGGR_CACHE:
> +		fprintf(output, "\"cache\" : \"S%d-D%d-L%d-ID%d\", \"counters\" : %d, ", id.socket,
> +			id.die, id.cache_lvl, id.cache, aggr_nr);
> +		break;
> +	case AGGR_CLUSTER:
> +		fprintf(output, "\"cluster\" : \"S%d-D%d-CLS%d\", \"counters\" : %d, ", id.socket,
> +			id.die, id.cluster, aggr_nr);
> +		break;
> +	case AGGR_DIE:
> +		fprintf(output, "\"die\" : \"S%d-D%d\", \"counters\" : %d, ", id.socket, id.die,
> +			aggr_nr);
> +		break;
> +	case AGGR_SOCKET:
> +		fprintf(output, "\"socket\" : \"S%d\", \"counters\" : %d, ", id.socket, aggr_nr);
> +		break;
> +	case AGGR_NODE:
> +		fprintf(output, "\"node\" : \"N%d\", \"counters\" : %d, ", id.node, aggr_nr);
> +		break;
> +	case AGGR_NONE:
> +		if (evsel->percore && !config->percore_show_thread)
> +			fprintf(output, "\"core\" : \"S%d-D%d-C%d\", ", id.socket, id.die, id.core);
> +		else if (id.cpu.cpu > -1)
> +			fprintf(output, "\"cpu\" : \"%d\", ", id.cpu.cpu);
> +		break;
> +	case AGGR_THREAD:
> +		fprintf(output, "\"thread\" : \"%s-%d\", ",
> +			perf_thread_map__comm(evsel->core.threads, id.thread_idx),
> +			perf_thread_map__pid(evsel->core.threads, id.thread_idx));
> +		break;
> +	case AGGR_GLOBAL:
> +	case AGGR_UNSET:
> +	case AGGR_MAX:
> +	default:
> +		break;
> +	}

It'd be great if you can use the format-agnostic helpers.


> +}
> +
> +/*
> + * JSON Output Callbacks - Normal Mode (100% Streaming & Zero-Allocation)
> + */
> +
> +static int json_print_start(void *ctx __maybe_unused,
> +			    struct perf_stat_config *config __maybe_unused)
> +{
> +	return 0;
> +}
> +
> +static int json_print_event(void *ctx, struct perf_stat_config *config,
> +			    struct evsel *evsel, int aggr_idx, u64 val,
> +			    u64 ena, u64 run, double stdev_pct __maybe_unused,
> +			    const char *cgrp __maybe_unused)
> +{
> +	struct json_print_state *ps = ctx;
> +	FILE *output = config->output;
> +	bool ok = (run != 0 && ena != 0);
> +	double enabled_percent = 100.0;
> +
> +	fprintf(output, "{");
> +
> +	/* Print interval timestamp first if configured */
> +	if (config->interval && ps && ps->timestamp[0])
> +		fprintf(output, "%s", ps->timestamp);

"Interval" : ?


> +
> +	/* Print aggregation JSON fields if configured */
> +	if (config->aggr_map && aggr_idx >= 0) {
> +		struct aggr_cpu_id id = config->aggr_map->map[aggr_idx];
> +		int aggr_nr = 0;
> +
> +		if (evsel->stats && evsel->stats->aggr)
> +			aggr_nr = evsel->stats->aggr[aggr_idx].nr;
> +
> +		print_aggr_id_json(config, output, evsel, id, aggr_nr);
> +	}
> +
> +	if (ok) {
> +		double sc = evsel->scale;
> +		double avg = val * sc;
> +
> +		fprintf(output, "\"counter-value\" : \"%f\"", avg);
> +	} else {
> +		const char *bad_count = evsel->supported ? CNTR_NOT_COUNTED : CNTR_NOT_SUPPORTED;
> +
> +		fprintf(output, "\"counter-value\" : \"%s\"", bad_count);
> +	}
> +
> +	fprintf(output, ", \"unit\" : \"%s\"", evsel->unit ?: "");
> +	/* Cast away const for legacy evsel__name */
> +	fprintf(output, ", \"event\" : \"%s\"", evsel__name((struct evsel *)evsel));

Cgroups?

> +
> +	if (run != ena)
> +		enabled_percent = 100.0 * run / ena;
> +	fprintf(output, ", \"event-runtime\" : %" PRIu64 ", \"pcnt-running\" : %.2f", run,
> +		enabled_percent);
> +	fprintf(output, "}\n");

Now metrics are printed in a separate object/line?

Thanks,
Namhyung

> +
> +	return 0;
> +}