gh-154032: Fix differential flamegraph scaling

Author: pablogsal

Lib/profiling/sampling/_flamegraph_assets/flamegraph.js

@@ -99,6 +99,11 @@ function getDisplayName(moduleName, filename) {
   return filename;
 }
 
+function samplesToMilliseconds(samples, data) {
+  const intervalUsec = data?.stats?.sample_interval_usec ?? 1000;
+  return samples * intervalUsec / 1000;
+}
+
 function selectFlamegraphData() {
   const baseData = isShowingElided ? elidedFlamegraphData : normalData;
 
@@ -246,12 +251,13 @@ function setupLogos() {
 // Status Bar
 // ============================================================================
 
-function updateStatusBar(nodeData, rootValue) {
+function updateStatusBar(nodeData, data) {
   const funcname = resolveString(nodeData.funcname) || resolveString(nodeData.name) || "--";
   const filename = resolveString(nodeData.filename) || "";
   const moduleName = resolveString(nodeData.module) || "";
   const lineno = nodeData.lineno;
-  const timeMs = (nodeData.value / 1000).toFixed(2);
+  const timeMs = samplesToMilliseconds(nodeData.value, data).toFixed(2);
+  const rootValue = data.value;
   const percent = rootValue > 0 ? ((nodeData.value / rootValue) * 100).toFixed(1) : "0.0";
 
   const brandEl = document.getElementById('status-brand');
@@ -313,9 +319,9 @@ function createPythonTooltip(data) {
         .style("opacity", 0);
     }
 
-    const timeMs = (d.data.value / 1000).toFixed(2);
+    const timeMs = samplesToMilliseconds(d.data.value, data).toFixed(2);
     const selfSamples = d.data.self || 0;
-    const selfMs = (selfSamples / 1000).toFixed(2);
+    const selfMs = samplesToMilliseconds(selfSamples, data).toFixed(2);
     const percentage = ((d.data.value / data.value) * 100).toFixed(2);
     const relativePercentage = Math.min(100, ((d.data.value / (zoomedNodeValue ?? data.value)) * 100)).toFixed(2);
     const calls = d.data.calls || 0;
@@ -399,9 +405,9 @@ function createPythonTooltip(data) {
     // Differential stats section
     let diffSection = "";
     if (d.data.diff !== undefined && d.data.baseline !== undefined) {
-      const baselineSelf = (d.data.baseline / 1000).toFixed(2);
-      const currentSelf = ((d.data.self_time || 0) / 1000).toFixed(2);
-      const diffMs = (d.data.diff / 1000).toFixed(2);
+      const baselineSelf = samplesToMilliseconds(d.data.baseline, data).toFixed(2);
+      const currentSelf = samplesToMilliseconds(d.data.self_time || 0, data).toFixed(2);
+      const diffMs = samplesToMilliseconds(d.data.diff, data).toFixed(2);
       const diffPct = d.data.diff_pct;
       const sign = d.data.diff >= 0 ? "+" : "";
       const diffClass = d.data.diff > 0 ? "regression" : (d.data.diff < 0 ? "improvement" : "neutral");
@@ -499,7 +505,7 @@ function createPythonTooltip(data) {
       .style("opacity", 1);
 
     // Update status bar
-    updateStatusBar(d.data, data.value);
+    updateStatusBar(d.data, data);
   };
 
   pythonTooltip.hide = function () {

Lib/profiling/sampling/stack_collector.py

@@ -70,7 +70,7 @@ def export(self, filename):
 class FlamegraphCollector(StackTraceCollector):
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
-        self.stats = {}
+        self.stats = {"sample_interval_usec": self.sample_interval_usec}
         self._root = {"samples": 0, "children": {}, "threads": set()}
         self._total_samples = 0
         self._sample_count = 0  # Track actual number of samples (not thread traces)
@@ -550,16 +550,19 @@ def _convert_to_flamegraph_format(self):
         current_stats = self._aggregate_path_samples(self._root)
         baseline_stats = self._aggregate_path_samples(self._baseline_collector._root)
 
-        # Scale baseline values to make them comparable, accounting for both
-        # sample count differences and sample interval differences.
+        # Scale baseline samples to the number of samples in the current
+        # profile. The sample interval is only needed when converting samples
+        # to time for display.
         baseline_total = self._baseline_collector._total_samples
         if baseline_total > 0 and self._total_samples > 0:
-            current_time = self._total_samples * self.sample_interval_usec
-            baseline_time = baseline_total * self._baseline_collector.sample_interval_usec
-            scale = current_time / baseline_time
+            scale = self._total_samples / baseline_total
         elif baseline_total > 0:
-            # Current profile is empty - use interval-based scale for elided display
-            scale = self.sample_interval_usec / self._baseline_collector.sample_interval_usec
+            # Express baseline samples in units of the current sample interval
+            # for the elided display.
+            scale = (
+                self._baseline_collector.sample_interval_usec
+                / self.sample_interval_usec
+            )
         else:
             scale = 1.0
 
@@ -653,6 +656,7 @@ def _build_elided_flamegraph(self, baseline_stats, scale):
         if not self._extract_elided_nodes(baseline_data, path=()):
             return None
 
+        self._scale_flamegraph_values(baseline_data, scale)
         self._add_elided_metadata(baseline_data, baseline_stats, scale, path=())
 
         # Merge only profiling metadata, not thread-level stats
@@ -666,6 +670,13 @@ def _build_elided_flamegraph(self, baseline_stats, scale):
 
         return baseline_data
 
+    def _scale_flamegraph_values(self, node, scale):
+        """Express flamegraph values in units of the current sample interval."""
+        node["value"] = node.get("value", 0) * scale
+        node["self"] = node.get("self", 0) * scale
+        for child in node.get("children", ()):
+            self._scale_flamegraph_values(child, scale)
+
     def _extract_elided_nodes(self, node, path):
         """Remove non-elided nodes and recalculate values bottom-up."""
         if not node:

Lib/test/test_profiling/test_sampling_profiler/mocks.py

@@ -93,20 +93,24 @@ def __repr__(self):
         return f"MockAwaitedInfo(thread_id={self.thread_id}, awaited_by={len(self.awaited_by)} tasks)"
 
 
-def make_diff_collector_with_mock_baseline(baseline_samples):
+def make_diff_collector_with_mock_baseline(
+    baseline_samples, *, baseline_interval=1000, current_interval=1000
+):
     """Create a DiffFlamegraphCollector with baseline injected directly,
     skipping the binary round-trip that _load_baseline normally does."""
     from profiling.sampling.stack_collector import (
         DiffFlamegraphCollector,
         FlamegraphCollector,
     )
 
-    baseline = FlamegraphCollector(1000)
+    baseline = FlamegraphCollector(baseline_interval)
     for sample in baseline_samples:
         baseline.collect(sample)
 
     # Path is unused since we inject _baseline_collector directly;
     # use __file__ as a dummy path that passes the existence check.
-    diff = DiffFlamegraphCollector(1000, baseline_binary_path=__file__)
+    diff = DiffFlamegraphCollector(
+        current_interval, baseline_binary_path=__file__
+    )
     diff._baseline_collector = baseline
     return diff

Lib/test/test_profiling/test_sampling_profiler/test_collectors.py

@@ -505,6 +505,7 @@ def test_flamegraph_collector_basic(self):
         self.assertIn("func1 (file.py:10)", resolve_name(child, strings))
         self.assertEqual(child["value"], 1)
         self.assertEqual(child["self"], 1)  # leaf: all time is self
+        self.assertEqual(data["stats"]["sample_interval_usec"], 1000)
 
     def test_flamegraph_collector_export(self):
         """Test flamegraph HTML export functionality."""
@@ -1556,6 +1557,57 @@ def test_diff_flamegraph_scale_factor(self):
         self.assertAlmostEqual(func1_node["diff"], 0.0)
         self.assertAlmostEqual(func1_node["diff_pct"], 0.0)
 
+    def test_diff_flamegraph_scale_factor_with_different_intervals(self):
+        """Scale factor normalizes profiles sampled at different rates."""
+        frames = [
+            MockInterpreterInfo(0, [
+                MockThreadInfo(1, [MockFrameInfo("file.py", 10, "func1")])
+            ])
+        ]
+
+        diff = make_diff_collector_with_mock_baseline(
+            [frames] * 10,
+            baseline_interval=1000,
+            current_interval=10000,
+        )
+        diff.collect(frames)
+
+        data = diff._convert_to_flamegraph_format()
+        self.assertAlmostEqual(data["stats"]["baseline_scale"], 0.1)
+        self.assertEqual(data["stats"]["sample_interval_usec"], 10000)
+        self.assertAlmostEqual(data["baseline"], 1.0)
+        self.assertEqual(data["self_time"], 1)
+        self.assertAlmostEqual(data["diff"], 0.0)
+        self.assertAlmostEqual(data["diff_pct"], 0.0)
+
+    def test_diff_flamegraph_elided_scale_with_different_intervals(self):
+        """Elided values use the current profile's sample units."""
+        baseline_frames = [
+            MockInterpreterInfo(0, [
+                MockThreadInfo(1, [MockFrameInfo("file.py", 10, "old_func")])
+            ])
+        ]
+        current_frames = [
+            MockInterpreterInfo(0, [
+                MockThreadInfo(1, [MockFrameInfo("file.py", 20, "new_func")])
+            ])
+        ]
+
+        diff = make_diff_collector_with_mock_baseline(
+            [baseline_frames] * 10,
+            baseline_interval=1000,
+            current_interval=10000,
+        )
+        diff.collect(current_frames)
+
+        data = diff._convert_to_flamegraph_format()
+        elided = data["stats"]["elided_flamegraph"]
+        self.assertEqual(elided["stats"]["sample_interval_usec"], 10000)
+        self.assertAlmostEqual(elided["value"], 1.0)
+        self.assertAlmostEqual(elided["self"], 1.0)
+        self.assertAlmostEqual(elided["baseline"], 1.0)
+        self.assertAlmostEqual(elided["diff"], -1.0)
+
     def test_diff_flamegraph_elided_stacks(self):
         """Paths in baseline but not current produce elided stacks."""
         baseline_frames_1 = [

Misc/NEWS.d/next/Library/2026-07-19-00-00-00.gh-issue-154032.d1ffFg.rst

@@ -0,0 +1,2 @@
+Fix differential flamegraphs incorrectly reporting changes when the baseline
+and current profiles use different sampling rates.