diff --git a/Lib/test/test_tools/test_ftscalingbench.py b/Lib/test/test_tools/test_ftscalingbench.py
new file mode 100644
index 00000000000000..81fe925c8d64fe
--- /dev/null
+++ b/Lib/test/test_tools/test_ftscalingbench.py
@@ -0,0 +1,75 @@
+"""Tests for Tools/ftscalingbench/ftscalingbench.py."""
+
+import sys
+import unittest
+from unittest import mock
+
+from test.test_tools import skip_if_missing, imports_under_tool
+
+skip_if_missing('ftscalingbench')
+
+with imports_under_tool('ftscalingbench'):
+    import ftscalingbench
+
+
+def lscpu(rows):
+    """Build `lscpu -p=cpu,node,core,MAXMHZ` output from (cpu, node, core, mhz)."""
+    lines = ['# cpu,node,core,MAXMHZ']
+    lines += [f'{cpu},{node},{core},{mhz}' for cpu, node, core, mhz in rows]
+    return '\n'.join(lines) + '\n'
+
+
+def smt_rows(count, mhz, first_cpu=0, first_core=0):
+    """Rows for `count` cores with two hardware threads each."""
+    rows = []
+    for i in range(count):
+        cpu = first_cpu + i * 2
+        rows.append((cpu, 0, first_core + i, mhz))
+        rows.append((cpu + 1, 0, first_core + i, mhz))
+    return rows
+
+
+class DetermineAffinityTests(unittest.TestCase):
+
+    def select(self, output):
+        with (mock.patch('subprocess.check_output', return_value=output),
+              mock.patch.object(sys, 'platform', 'linux')):
+            return ftscalingbench.determine_num_threads_and_affinity()
+
+    def test_performance_cores_binned_at_different_clocks(self):
+        # Two of the eight performance cores clock higher than the rest.
+        rows = smt_rows(4, '5000.0000')
+        rows += smt_rows(2, '5200.0000', first_cpu=8, first_core=4)
+        rows += smt_rows(2, '5000.0000', first_cpu=12, first_core=6)
+        rows += [(16 + i, 0, 8 + i, '3700.0000') for i in range(8)]
+        self.assertEqual(self.select(lscpu(rows)),
+                         [0, 2, 4, 6, 8, 10, 12, 14])
+
+    def test_efficiency_cores_are_skipped(self):
+        rows = smt_rows(4, '4800.0000')
+        rows += [(8 + i, 0, 4 + i, '3600.0000') for i in range(4)]
+        self.assertEqual(self.select(lscpu(rows)), [0, 2, 4, 6])
+
+    def test_one_thread_per_physical_core(self):
+        self.assertEqual(self.select(lscpu(smt_rows(8, '3700.0000'))),
+                         [0, 2, 4, 6, 8, 10, 12, 14])
+
+    def test_missing_max_clock(self):
+        # MAXMHZ is empty on some kernels and in many virtual machines.
+        rows = [(i, 0, i, '') for i in range(4)]
+        self.assertEqual(self.select(lscpu(rows)), [0, 1, 2, 3])
+
+    def test_second_numa_node_is_ignored(self):
+        rows = [(i, 0, i, '3000.0000') for i in range(4)]
+        rows += [(4 + i, 1, 4 + i, '3000.0000') for i in range(4)]
+        self.assertEqual(self.select(lscpu(rows)), [0, 1, 2, 3])
+
+    def test_lscpu_missing(self):
+        with (mock.patch('subprocess.check_output', side_effect=FileNotFoundError),
+              mock.patch.object(sys, 'platform', 'linux')):
+            cpus = ftscalingbench.determine_num_threads_and_affinity()
+        self.assertTrue(all(cpu is None for cpu in cpus))
+
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/Misc/NEWS.d/next/Tools-Demos/2026-08-02-11-42-15.gh-issue-155090.Kq7mZt.rst b/Misc/NEWS.d/next/Tools-Demos/2026-08-02-11-42-15.gh-issue-155090.Kq7mZt.rst
new file mode 100644
index 00000000000000..0c32dbc1a1b012
--- /dev/null
+++ b/Misc/NEWS.d/next/Tools-Demos/2026-08-02-11-42-15.gh-issue-155090.Kq7mZt.rst
@@ -0,0 +1,4 @@
+``Tools/ftscalingbench`` now uses every performance core on processors that
+clock some of them higher than others, such as Intel parts with Turbo Boost
+Max 3.0. It previously kept only the fastest cores and ran with fewer threads
+than the machine offered.
diff --git a/Tools/ftscalingbench/ftscalingbench.py b/Tools/ftscalingbench/ftscalingbench.py
index a79242e740371b..f0e9e0574825aa 100644
--- a/Tools/ftscalingbench/ftscalingbench.py
+++ b/Tools/ftscalingbench/ftscalingbench.py
@@ -403,10 +403,17 @@ def determine_num_threads_and_affinity():
     cpus = []
     cores = set()
     max_mhz_all = max(row[3] for row in table)
+    min_mhz_all = min(row[3] for row in table)
+    # Performance cores are not always binned to the same clock, so split them
+    # from the efficiency cores at the midpoint rather than at the maximum.
+    if max_mhz_all != min_mhz_all:
+        min_mhz_wanted = (max_mhz_all + min_mhz_all) / 2
+    else:
+        min_mhz_wanted = 0
     for cpu, node, core, maxmhz in table:
         # Choose only CPUs on the same node, unique cores, and try to avoid
         # "efficiency" cores.
-        if node == 0 and core not in cores and maxmhz == max_mhz_all:
+        if node == 0 and core not in cores and maxmhz >= min_mhz_wanted:
             cpus.append(cpu)
             cores.add(core)
     return cpus