Add basic benchmark scheduler

This patch introduces the BenchmarkScheduler interface that sets the CPU
affinity mask for the current process and gets a benchmark core for use
by llvm-exegesis. This patch additional introduces a BenchmarkScheduler
implementation that simply does nothing, which will preserve the
behavior that we have currently where we do not explicitly set CPU
masks.

Pull Request: google#270

gematria/datasets/pipelines/BUILD.bazel

@@ -81,3 +81,17 @@ gematria_py_test(
         "//gematria/proto:execution_annotation_py_pb2",
     ],
 )
+
+gematria_py_library(
+    name = "benchmark_cpu_scheduler",
+    srcs = ["benchmark_cpu_scheduler.py"],
+)
+
+gematria_py_test(
+    name = "benchmark_cpu_scheduler_test",
+    size = "small",
+    srcs = ["benchmark_cpu_scheduler_test.py"],
+    deps = [
+        ":benchmark_cpu_scheduler",
+    ],
+)

gematria/datasets/pipelines/benchmark_cpu_scheduler.py

@@ -0,0 +1,143 @@
+# Copyright 2024 Google Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import abc
+from typing_extensions import override
+from collections.abc import Iterable
+import os
+import re
+
+
+class BenchmarkScheduler(metaclass=abc.ABCMeta):
+  """Schedules a benchmark and the parent process to reduce noise.
+
+  BenchmarkScheduler is an abstraction that provides two main pieces of
+  functionality. Firstly, it provides a function
+  (setup_and_get_benchmark_core) that allows an implementation to perform any
+  necessary setup in the parent process and provide a core ID that should be
+  used to perform any benchmarking. Additionally, implementations are
+  intended to hold state to verify that the expected state is maintained and
+  not changed by external software.
+  """
+
+  @abc.abstractmethod
+  def setup_and_get_benchmark_core(self) -> int | None:
+    """Sets up the parent process and chooses a benchmark core.
+
+    This function will perform any relevant setup in the parent process,
+    and return a core ID that can be used to run benchmarks on.
+
+    Returns:
+      Returns an integer core ID to specify a core that should be used for
+      running benchmarks, or None to indicate that any core can be used.
+    """
+
+  @abc.abstractmethod
+  def verify(self):
+    """Verifies that conditions match what is expected.
+
+    This function allows for implementations to verify that the original
+    setup created in setup_and_get_benchmark_core is maintained for every
+    benchmark that is run.
+    """
+
+
+class NoSchedulingBenchmarkScheduler(BenchmarkScheduler):
+  """A basic BenchmarkScheduler implementation that does nothing.
+
+  This BenchmarkScheduler implementation does nothing. It leaves scheduling
+  of the benchmarking process to the operating system by specifying any core
+  can be used for benchmarking, performs no setup in the parent process, and
+  performs no verification.
+  """
+
+  @override
+  def setup_and_get_benchmark_core(self) -> int | None:
+    return None
+
+  @override
+  def verify(self):
+    pass
+
+
+class DefaultBenchmarkScheduler(BenchmarkScheduler):
+  """A BenchmarkScheduler that schedules processes separately.
+
+  DefaultBenchmarkScheduler schedules the main process and the benchmark
+  subprocess on separate cores, making sure to reserve the second hyperthread
+  on the benchmarking core to prevent interference. It expects that the main
+  process is initially given a CPU Mask with three active threads, additionally
+  assuming that two of the threads are neighboring (part of the same core).
+  Errors are raised if these conditions are not met. The benchmarking core
+  returned is one of the two neighboring threads. The main process has its
+  COU mask limited to the thread that neighbors neither of the other threads.
+  """
+
+  def __init__(self):
+    self._cpu_mask = []
+
+  @staticmethod
+  def _get_neighboring_threads(cpu_index: int) -> list[int]:
+    with open(
+        f'/sys/devices/system/cpu/cpu{cpu_index}/topology/thread_siblings_list'
+    ) as thread_sibling_list_handle:
+      neighboring_threads_strings = re.split(
+          r'[-,]+', thread_sibling_list_handle.read().strip()
+      )
+      neighboring_threads = [
+          int(cpu_index_str) for cpu_index_str in neighboring_threads_strings
+      ]
+    return neighboring_threads
+
+  def _get_aux_core_and_hyperthread_pair(
+      self,
+      cpu_mask: Iterable[int],
+  ) -> tuple[int, list[int]]:
+    for cpu_index in cpu_mask:
+      neighboring_threads = self._get_neighboring_threads(cpu_index)
+      if len(neighboring_threads) != 2:
+        raise ValueError('Expected two hyperthreads per CPU.')
+
+      if (
+          neighboring_threads[0] in cpu_mask
+          and neighboring_threads[1] in cpu_mask
+      ):
+        cpus = list(cpu_mask)
+        cpus.remove(neighboring_threads[0])
+        cpus.remove(neighboring_threads[1])
+        return (cpus[0], [neighboring_threads[0], neighboring_threads[1]])
+    raise ValueError(
+        'Expected a pair of neighboring hyperthreads in the CPU mask.'
+    )
+
+  @override
+  def setup_and_get_benchmark_core(self) -> int | None:
+    cpu_mask = os.sched_getaffinity(0)
+
+    if len(cpu_mask) != 3:
+      raise ValueError('Expected to have three CPUs.')
+
+    aux_core, hyperthread_pair = self._get_aux_core_and_hyperthread_pair(
+        cpu_mask
+    )
+    os.sched_setaffinity(0, [aux_core])
+    self._cpu_mask = [aux_core]
+
+    return hyperthread_pair[0]
+
+  @override
+  def verify(self):
+    cpu_mask = list(os.sched_getaffinity(0))
+    if self._cpu_mask != cpu_mask:
+      raise ValueError('Expected the CPU mask to not change.')

gematria/datasets/pipelines/benchmark_cpu_scheduler_test.py

@@ -0,0 +1,128 @@
+# Copyright 2024 Google Inc.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import os
+from collections.abc import Iterable
+
+from absl.testing import absltest
+
+from gematria.datasets.pipelines import benchmark_cpu_scheduler
+
+
+class BenchmarkSchedulerTests(absltest.TestCase):
+
+  def test_no_scheduling(self):
+    scheduler = benchmark_cpu_scheduler.NoSchedulingBenchmarkScheduler()
+    self.assertIsNone(scheduler.setup_and_get_benchmark_core())
+    scheduler.verify()
+
+  def test_default_scheduler_get_neighboring_threads(self):
+    scheduler = benchmark_cpu_scheduler.DefaultBenchmarkScheduler()
+    neighboring_threads = scheduler._get_neighboring_threads(0)
+
+    # Just check that we get two CPU ids back that are not the same. We cannot
+    # do much more without knowing more about the system topology, and this
+    # should be a reasonable enough test.
+    self.assertLen(neighboring_threads, 2)
+    self.assertNotEqual(neighboring_threads[0], neighboring_threads[1])
+
+  @staticmethod
+  def _set_normal_affinity():
+    cpu_mask = os.sched_getaffinity(0)
+    cpu_mask_list = list(cpu_mask)
+    aux_cpu = cpu_mask.pop()
+    hyperthread_pair_part = cpu_mask.pop()
+    hyperthread_pair = benchmark_cpu_scheduler.DefaultBenchmarkScheduler._get_neighboring_threads(
+        hyperthread_pair_part
+    )
+    new_cpu_mask = [aux_cpu, *hyperthread_pair]
+
+    os.sched_setaffinity(0, new_cpu_mask)
+    return (aux_cpu, hyperthread_pair, cpu_mask_list)
+
+  @staticmethod
+  def _reset_cpu_affinity(cpu_mask: Iterable[int]):
+    os.sched_setaffinity(0, cpu_mask)
+
+  def test_default_scheduler_get_cores(self):
+    expected_aux_cpu, expected_hyperthread_pair, old_cpu_mask = (
+        self._set_normal_affinity()
+    )
+    scheduler = benchmark_cpu_scheduler.DefaultBenchmarkScheduler()
+    cpu_mask = os.sched_getaffinity(0)
+    aux_cpu, hyperthread_pair = scheduler._get_aux_core_and_hyperthread_pair(
+        cpu_mask
+    )
+    self.assertEqual(aux_cpu, expected_aux_cpu)
+    self.assertContainsSubsequence(hyperthread_pair, expected_hyperthread_pair)
+    self._reset_cpu_affinity(old_cpu_mask)
+
+  def test_default_scheduler_get_cores_no_neighboring_threads(self):
+    cpu_mask = os.sched_getaffinity(0)
+    three_cores = [cpu_mask.pop(), cpu_mask.pop(), cpu_mask.pop()]
+
+    scheduler = benchmark_cpu_scheduler.DefaultBenchmarkScheduler()
+    with self.assertRaises(ValueError):
+      scheduler._get_aux_core_and_hyperthread_pair(three_cores)
+
+  def test_default_scheduler_setup(self):
+    expected_aux_cpu, expected_hyperthread_pair, old_cpu_mask = (
+        self._set_normal_affinity()
+    )
+
+    scheduler = benchmark_cpu_scheduler.DefaultBenchmarkScheduler()
+    benchmark_core = scheduler.setup_and_get_benchmark_core()
+    self.assertIn(benchmark_core, expected_hyperthread_pair)
+    set_cpu_mask = os.sched_getaffinity(0)
+    self.assertLen(set_cpu_mask, 1)
+    self.assertEqual(set_cpu_mask.pop(), expected_aux_cpu)
+
+    self._reset_cpu_affinity(old_cpu_mask)
+
+  def test_default_scheduler_not_three_cpus(self):
+    old_cpu_mask = os.sched_getaffinity(0)
+    cpu_mask_list = list(old_cpu_mask)
+    os.sched_setaffinity(0, cpu_mask_list[0:2])
+
+    scheduler = benchmark_cpu_scheduler.DefaultBenchmarkScheduler()
+    with self.assertRaises(ValueError):
+      scheduler.setup_and_get_benchmark_core()
+
+    os.sched_setaffinity(0, old_cpu_mask)
+
+  def test_default_scheduler_verify(self):
+    _, _, old_cpu_mask = self._set_normal_affinity()
+
+    scheduler = benchmark_cpu_scheduler.DefaultBenchmarkScheduler()
+    scheduler.setup_and_get_benchmark_core()
+    scheduler.verify()
+
+    self._reset_cpu_affinity(old_cpu_mask)
+
+  def test_default_scheduler_verify_mask_changed(self):
+    _, _, old_cpu_mask = self._set_normal_affinity()
+
+    scheduler = benchmark_cpu_scheduler.DefaultBenchmarkScheduler()
+    scheduler.setup_and_get_benchmark_core()
+
+    cpu_mask_list = list(old_cpu_mask)
+    os.sched_setaffinity(0, cpu_mask_list[1:3])
+    with self.assertRaises(ValueError):
+      scheduler.verify()
+
+    self._reset_cpu_affinity(old_cpu_mask)
+
+
+if __name__ == '__main__':
+  absltest.main()