From 40f76ed636176fe301570692f25bb1a322c9330f Mon Sep 17 00:00:00 2001
From: Davanum Srinivas <davanum@gmail.com>
Date: Thu, 25 Jun 2026 21:09:55 -0400
Subject: [PATCH] cpuset: add NumCPU to read the host's online CPU count from
 sysfs

NumCPU reads /sys/devices/system/cpu/online and returns the number of online
CPUs. Unlike runtime.NumCPU it is not affected by the calling process's CPU
affinity (its cgroup cpuset), so it reflects the whole machine, which is what
callers want when sizing a host-wide resource by CPU count.

Signed-off-by: Davanum Srinivas <davanum@gmail.com>
---
 cpuset/numcpu.go      | 50 ++++++++++++++++++++++++++++++
 cpuset/numcpu_test.go | 72 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 122 insertions(+)
 create mode 100644 cpuset/numcpu.go
 create mode 100644 cpuset/numcpu_test.go

diff --git a/cpuset/numcpu.go b/cpuset/numcpu.go
new file mode 100644
index 00000000..40e7bd3a
--- /dev/null
+++ b/cpuset/numcpu.go
@@ -0,0 +1,50 @@
+/*
+Copyright 2026 The Kubernetes 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.
+*/
+
+package cpuset
+
+import (
+	"fmt"
+	"os"
+	"strings"
+)
+
+// onlineCPUsPath is the sysfs file listing the host's online CPUs. It is a
+// variable so tests can point it at a fixture.
+var onlineCPUsPath = "/sys/devices/system/cpu/online"
+
+// NumCPU returns the number of CPUs that are online on the host, read from
+// /sys/devices/system/cpu/online. The file holds a cpu list such as "0-3,8-11";
+// see https://docs.kernel.org/admin-guide/cputopology.html for the format.
+//
+// Unlike runtime.NumCPU, the result is not affected by the calling process's
+// CPU affinity (its cgroup cpuset), so it reflects the whole machine. This is
+// what you want when sizing a host-wide resource by CPU count.
+//
+// NumCPU returns an error if the online file cannot be read or parsed, for
+// example on non-Linux platforms. Callers that need a value regardless should
+// fall back to runtime.NumCPU.
+func NumCPU() (int, error) {
+	data, err := os.ReadFile(onlineCPUsPath)
+	if err != nil {
+		return 0, err
+	}
+	set, err := Parse(strings.TrimSpace(string(data)))
+	if err != nil {
+		return 0, fmt.Errorf("parsing %q: %w", onlineCPUsPath, err)
+	}
+	return set.Size(), nil
+}
diff --git a/cpuset/numcpu_test.go b/cpuset/numcpu_test.go
new file mode 100644
index 00000000..6f4df0f1
--- /dev/null
+++ b/cpuset/numcpu_test.go
@@ -0,0 +1,72 @@
+/*
+Copyright 2026 The Kubernetes 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.
+*/
+
+package cpuset
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestNumCPU(t *testing.T) {
+	orig := onlineCPUsPath
+	t.Cleanup(func() { onlineCPUsPath = orig })
+
+	dir := t.TempDir()
+
+	tests := []struct {
+		name    string
+		content string
+		want    int
+		wantErr bool
+	}{
+		{name: "single", content: "0\n", want: 1},
+		{name: "range", content: "0-3\n", want: 4},
+		{name: "list and ranges", content: "0-1,4-7\n", want: 6},
+		{name: "no trailing newline", content: "0-15", want: 16},
+		{name: "malformed", content: "nope\n", wantErr: true},
+	}
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			path := filepath.Join(dir, tc.name)
+			if err := os.WriteFile(path, []byte(tc.content), 0o600); err != nil {
+				t.Fatal(err)
+			}
+			onlineCPUsPath = path
+			got, err := NumCPU()
+			if tc.wantErr {
+				if err == nil {
+					t.Fatalf("NumCPU() = %d, want error", got)
+				}
+				return
+			}
+			if err != nil {
+				t.Fatalf("NumCPU() unexpected error: %v", err)
+			}
+			if got != tc.want {
+				t.Errorf("NumCPU() = %d, want %d", got, tc.want)
+			}
+		})
+	}
+
+	t.Run("missing file", func(t *testing.T) {
+		onlineCPUsPath = filepath.Join(dir, "absent")
+		if _, err := NumCPU(); err == nil {
+			t.Error("NumCPU() with missing file: want error, got nil")
+		}
+	})
+}