perf python: Add evlist compute_metric

Add a compute_metric function that computes a metric double value for a
given evlist, metric name, CPU and thread. For example:
```
>>> import perf
>>> x = perf.parse_metrics("TopdownL1")
>>> x.open()
>>> x.enable()
>>> x.disable()
>>> x.metrics()
['tma_bad_speculation', 'tma_frontend_bound', 'tma_backend_bound', 'tma_retiring']
>>> x.compute_metric('tma_bad_speculation', 0, -1)
0.08605342847131037
```

Committer notes:

Initialize thread_idx and cpu_idx to zero as albeit them not possibly
coming out unitialized from the loop as mexp would be not NULL only if
they were initialized, some older compilers don't notice that and error
with:

    GEN     /tmp/build/perf/python/perf.cpython-36m-x86_64-linux-gnu.so
  /git/perf-6.17.0-rc3/tools/perf/util/python.c: In function ‘pyrf_evlist__compute_metric’:
  /git/perf-6.17.0-rc3/tools/perf/util/python.c:1363:3: error: ‘thread_idx’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
     evsel__read_counter(metric_events[i], cpu_idx, thread_idx);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  /git/perf-6.17.0-rc3/tools/perf/util/python.c:1389:41: note: ‘thread_idx’ was declared here
    int ret, cpu = 0, cpu_idx, thread = 0, thread_idx;
                                           ^~~~~~~~~~
  /git/perf-6.17.0-rc3/tools/perf/util/python.c:1363:3: error: ‘cpu_idx’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
     evsel__read_counter(metric_events[i], cpu_idx, thread_idx);
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  /git/perf-6.17.0-rc3/tools/perf/util/python.c:1389:20: note: ‘cpu_idx’ was declared here
    int ret, cpu = 0, cpu_idx, thread = 0, thread_idx;
                      ^~~~~~~
  /git/perf-6.17.0-rc3/tools/perf/util/python.c: At top level:
  cc1: error: unrecognized command line option ‘-Wno-cast-function-type’ [-Werror]
  cc1: all warnings being treated as errors
  error: command 'gcc' failed with exit status 1
  cp: cannot stat '/tmp/build/perf/python_ext_build/lib/perf*.so': No such file or directory

Reviewed-by: Howard Chu <howardchu95@gmail.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Chun-Tse Shao <ctshao@google.com>
Cc: Collin Funk <collin.funk1@gmail.com>
Cc: Dr. David Alan Gilbert <linux@treblig.org>
Cc: Gautam Menghani <gautam@linux.ibm.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Falcon <thomas.falcon@intel.com>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Cc: Tiezhu Yang <yangtiezhu@loongson.cn>
Cc: Weilin Wang <weilin.wang@intel.com>
Cc: Xu Yang <xu.yang_2@nxp.com>
Link: https://lore.kernel.org/r/20250819013941.209033-9-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

1 files changed, 125 insertions, 0 deletions

diff --git a/tools/perf/util/python.c b/tools/perf/util/python.c
index 31089f8e5519..ad8a5446ae48 100644
--- a/tools/perf/util/python.c
+++ b/tools/perf/util/python.c
@@ -14,6 +14,7 @@
 #include "evlist.h"
 #include "evsel.h"
 #include "event.h"
+#include "expr.h"
 #include "print_binary.h"
 #include "record.h"
 #include "strbuf.h"
@@ -1330,6 +1331,124 @@ static PyObject *pyrf_evlist__metrics(struct pyrf_evlist *pevlist)
 	return list;
 }
 
+static int prepare_metric(const struct metric_expr *mexp,
+			  const struct evsel *evsel,
+			  struct expr_parse_ctx *pctx,
+			  int cpu_idx, int thread_idx)
+{
+	struct evsel * const *metric_events = mexp->metric_events;
+	struct metric_ref *metric_refs = mexp->metric_refs;
+
+	for (int i = 0; metric_events[i]; i++) {
+		char *n = strdup(evsel__metric_id(metric_events[i]));
+		double val, ena, run;
+		int source_count = evsel__source_count(metric_events[i]);
+		int ret;
+		struct perf_counts_values *old_count, *new_count;
+
+		if (!n)
+			return -ENOMEM;
+
+		if (source_count == 0)
+			source_count = 1;
+
+		ret = evsel__ensure_counts(metric_events[i]);
+		if (ret)
+			return ret;
+
+		/* Set up pointers to the old and newly read counter values. */
+		old_count = perf_counts(metric_events[i]->prev_raw_counts, cpu_idx, thread_idx);
+		new_count = perf_counts(metric_events[i]->counts, cpu_idx, thread_idx);
+		/* Update the value in metric_events[i]->counts. */
+		evsel__read_counter(metric_events[i], cpu_idx, thread_idx);
+
+		val = new_count->val - old_count->val;
+		ena = new_count->ena - old_count->ena;
+		run = new_count->run - old_count->run;
+
+		if (ena != run && run != 0)
+			val = val * ena / run;
+		ret = expr__add_id_val_source_count(pctx, n, val, source_count);
+		if (ret)
+			return ret;
+	}
+
+	for (int i = 0; metric_refs && metric_refs[i].metric_name; i++) {
+		int ret = expr__add_ref(pctx, &metric_refs[i]);
+
+		if (ret)
+			return ret;
+	}
+
+	return 0;
+}
+
+static PyObject *pyrf_evlist__compute_metric(struct pyrf_evlist *pevlist,
+					     PyObject *args, PyObject *kwargs)
+{
+	int ret, cpu = 0, cpu_idx = 0, thread = 0, thread_idx = 0;
+	const char *metric;
+	struct rb_node *node;
+	struct metric_expr *mexp = NULL;
+	struct expr_parse_ctx *pctx;
+	double result = 0;
+
+	if (!PyArg_ParseTuple(args, "sii", &metric, &cpu, &thread))
+		return NULL;
+
+	for (node = rb_first_cached(&pevlist->evlist.metric_events.entries);
+	     mexp == NULL && node;
+	     node = rb_next(node)) {
+		struct metric_event *me = container_of(node, struct metric_event, nd);
+		struct list_head *pos;
+
+		list_for_each(pos, &me->head) {
+			struct metric_expr *e = container_of(pos, struct metric_expr, nd);
+
+			if (strcmp(e->metric_name, metric))
+				continue;
+
+			if (e->metric_events[0] == NULL)
+				continue;
+
+			cpu_idx = perf_cpu_map__idx(e->metric_events[0]->core.cpus,
+						    (struct perf_cpu){.cpu = cpu});
+			if (cpu_idx < 0)
+				continue;
+
+			thread_idx = perf_thread_map__idx(e->metric_events[0]->core.threads,
+							  thread);
+			if (thread_idx < 0)
+				continue;
+
+			mexp = e;
+			break;
+		}
+	}
+	if (!mexp) {
+		PyErr_Format(PyExc_TypeError, "Unknown metric '%s' for CPU '%d' and thread '%d'",
+			     metric, cpu, thread);
+		return NULL;
+	}
+
+	pctx = expr__ctx_new();
+	if (!pctx)
+		return PyErr_NoMemory();
+
+	ret = prepare_metric(mexp, mexp->metric_events[0], pctx, cpu_idx, thread_idx);
+	if (ret) {
+		expr__ctx_free(pctx);
+		errno = -ret;
+		PyErr_SetFromErrno(PyExc_OSError);
+		return NULL;
+	}
+	if (expr__parse(&result, pctx, mexp->metric_expr))
+		result = 0.0;
+
+	expr__ctx_free(pctx);
+	return PyFloat_FromDouble(result);
+}
+
 static PyObject *pyrf_evlist__mmap(struct pyrf_evlist *pevlist,
 				   PyObject *args, PyObject *kwargs)
 {
@@ -1565,6 +1684,12 @@ static PyMethodDef pyrf_evlist__methods[] = {
 		.ml_doc	  = PyDoc_STR("List of metric names within the evlist.")
 	},
 	{
+		.ml_name  = "compute_metric",
+		.ml_meth  = (PyCFunction)pyrf_evlist__compute_metric,
+		.ml_flags = METH_VARARGS | METH_KEYWORDS,
+		.ml_doc	  = PyDoc_STR("compute metric for given name, cpu and thread")
+	},
+	{
 		.ml_name  = "mmap",
 		.ml_meth  = (PyCFunction)pyrf_evlist__mmap,
 		.ml_flags = METH_VARARGS | METH_KEYWORDS,