[core] [3/N] Check cgroupv2 mount status

BUILD.bazel

@@ -1157,6 +1157,30 @@ ray_cc_test(
     ],
 )
 
+ray_cc_library(
+    name = "cgroup_v2_utils",
+    srcs = ["src/ray/raylet/cgroup_v2_utils.cc"],
+    hdrs = ["src/ray/raylet/cgroup_v2_utils.h"],
+    deps = [
+        "@com_google_absl//absl/strings",
+        "//src/ray/util",
+    ],
+)
+
+ray_cc_test(
+    name = "cgroup_v2_utils_test",
+    size = "small",
+    srcs = ["src/ray/raylet/test/cgroup_v2_utils_test.cc"],
+    tags = [
+        "team:core",
+        "manual", # Expect cgroupv2 mount, not intended for unit test and CI.
+    ],
+    deps = [
+        ":cgroup_v2_utils",
+        "@com_google_googletest//:gtest_main",
+    ],
+)
+
 ray_cc_test(
     name = "pull_manager_test",
     size = "small",

src/ray/raylet/cgroup_v2_utils.cc

@@ -0,0 +1,59 @@
+// Copyright 2024 The Ray Authors.
+//
+// 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.
+
+#include "ray/raylet/cgroup_v2_utils.h"
+
+#include <fstream>
+#include <string>
+
+#include "absl/strings/str_split.h"
+#include "absl/strings/strip.h"
+#include "ray/util/logging.h"
+
+namespace ray {
+
+bool IsCgroupV2MountedAsRw() {
+  // Checking all mountpoints directly and parse textually is the easiest way, compared
+  // with mounted filesystem attributes.
+  std::ifstream mounts("/proc/mounts");
+  if (!mounts.is_open()) {
+    return false;
+  }
+
+  // Mount information is formatted as:
+  // <fs_spec> <fs_file> <fs_vfstype> <fs_mntopts> <dump-field> <fsck-field>
+  std::string line;
+  while (std::getline(mounts, line)) {
+    std::vector<std::string_view> mount_info_tokens = absl::StrSplit(line, ' ');
+    RAY_CHECK_EQ(mount_info_tokens.size(), 6UL);
+    // For cgroupv2, `fs_spec` should be `cgroupv2` and there should be only one mount
+    // information item.
+    if (mount_info_tokens[0] != "cgroup2") {
+      continue;
+    }
+    const auto &fs_mntopts = mount_info_tokens[3];
+
+    // Mount options are formatted as: <opt1,opt2,...>.
+    std::vector<std::string_view> mount_opts = absl::StrSplit(fs_mntopts, ',');
+
+    // CgroupV2 has only one mount item, directly returns.
+    return std::any_of(mount_opts.begin(),
+                       mount_opts.end(),
+                       [](const std::string_view cur_opt) { return cur_opt == "rw"; });
+  }
+
+  return false;
+}
+
+}  // namespace ray

src/ray/raylet/cgroup_v2_utils.h

@@ -0,0 +1,24 @@
+// Copyright 2024 The Ray Authors.
+//
+// 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.
+
+// Utils for cgroup v2 related.
+
+#pragma once
+
+namespace ray {
+
+// Return whether cgroup V2 is mounted in read and write mode.
+bool IsCgroupV2MountedAsRw();
+
+}  // namespace ray

src/ray/raylet/test/cgroup_v2_utils_test.cc

@@ -0,0 +1,32 @@
+// Copyright 2024 The Ray Authors.
+//
+// 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.
+
+#include "src/ray/raylet/cgroup_v2_utils.h"
+
+#include <gtest/gtest.h>
+
+namespace ray {
+
+namespace {
+
+// Precondition: cgroup V2 has already been mounted as rw.
+//
+// Setup command:
+// sudo umount /sys/fs/cgroup/unified
+// sudo mount -t cgroup2 cgroup2 /sys/fs/cgroup/unified -o rw
+TEST(CgroupV2UtilsTest, CheckCgroupV2Mount) { EXPECT_TRUE(IsCgroupV2MountedAsRw()); }
+
+}  // namespace
+
+}  // namespace ray