Two-socket machines: Hardware and Top show each CPU, not one with every core

Hardware counts CPUs, cores and threads per CPU. Top groups threads under a row
per socket, reads each core's temperature from its own socket's coretemp chip,
and the CPU temperature is the hotter package.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/metrics/cputemp_test.go

@@ -0,0 +1,16 @@
+package metrics
+
+import "testing"
+
+// a two-socket board: two coretemp chips, and the CPU's temperature is the hotter package
+func TestCPUTempHottestPackage(t *testing.T) {
+	temps := []Temp{
+		{Chip: "coretemp", Label: "Core 0", C: 31},
+		{Chip: "coretemp", Label: "Core 0", C: 44, Pkg: 1},
+		{Chip: "coretemp", Label: "Package id 0", C: 35},
+		{Chip: "coretemp", Label: "Package id 1", C: 41, Pkg: 1},
+	}
+	if got := cpuTemp(temps); got != 41 {
+		t.Fatalf("cpuTemp = %v, want 41", got)
+	}
+}

internal/metrics/metrics.go

@@ -60,6 +60,7 @@ type Temp struct {
 	C     float64 `json:"c"`
 	Max   float64 `json:"max,omitempty"`
 	Crit  float64 `json:"crit,omitempty"`
+	Pkg   int     `json:"pkg,omitempty"` // coretemp: the CPU package the chip belongs to — a two-socket board has two chips, both with a "Core 0"
 }
 
 type Fan struct {
@@ -544,6 +545,19 @@ func hwmonDir(d string) (chip string, temps []Temp, fans []Fan) {
 			fans = append(fans, Fan{Chip: chip, Label: label, RPM: int(raw)})
 		}
 	}
+	// coretemp is one chip per CPU package, and only its "Package id N" says which
+	if chip == "coretemp" {
+		for _, t := range temps {
+			if n, ok := strings.CutPrefix(t.Label, "Package id "); ok {
+				if pkg, err := strconv.Atoi(n); err == nil {
+					for i := range temps {
+						temps[i].Pkg = pkg
+					}
+				}
+				break
+			}
+		}
+	}
 	return
 }
 
@@ -559,7 +573,10 @@ func hwmon() ([]Temp, []Fan) {
 		if temps[i].Chip != temps[j].Chip {
 			return temps[i].Chip < temps[j].Chip
 		}
-		return temps[i].Label < temps[j].Label
+		if temps[i].Label != temps[j].Label {
+			return temps[i].Label < temps[j].Label
+		}
+		return temps[i].Pkg < temps[j].Pkg
 	})
 	return temps, fans
 }
@@ -1112,6 +1129,38 @@ func coreIDs(n int) []int {
 	return coreIDsVal[:n]
 }
 
+// corePkgs maps logical CPUs to the CPU package (socket) each is on, and is nil on a
+// one-socket machine: there is nothing to tell apart.
+var (
+	corePkgsOnce sync.Once
+	corePkgsVal  []int
+)
+
+func corePkgs(n int) []int {
+	corePkgsOnce.Do(func() {
+		seen := map[int]bool{}
+		var out []int
+		for i := 0; ; i++ {
+			b, err := os.ReadFile("/sys/devices/system/cpu/cpu" + strconv.Itoa(i) + "/topology/physical_package_id")
+			if err != nil {
+				break
+			}
+			id, err := strconv.Atoi(strings.TrimSpace(string(b)))
+			if err != nil {
+				break
+			}
+			out, seen[id] = append(out, id), true
+		}
+		if len(seen) > 1 {
+			corePkgsVal = out
+		}
+	})
+	if len(corePkgsVal) < n {
+		return nil
+	}
+	return corePkgsVal[:n]
+}
+
 // ProcTable is the Processes tab payload.
 type ProcTable struct {
 	T         int64      `json:"t"`
@@ -1123,7 +1172,8 @@ type ProcTable struct {
 	CPUMaxMHz float64    `json:"cpu_max_mhz,omitempty"`
 	CPUPowerW float64    `json:"cpu_power_w,omitempty"`
 	CPUTemp   float64    `json:"cpu_temp,omitempty"`
-	CoreIDs   []int      `json:"core_ids,omitempty"` // for each logical CPU, the physical core it is a thread of (what the temperature sensors are named by)
+	CoreIDs   []int      `json:"core_ids,omitempty"`  // for each logical CPU, the physical core it is a thread of (what the temperature sensors are named by)
+	CorePkgs  []int      `json:"core_pkgs,omitempty"` // for each logical CPU, the socket it is on; only on a machine with more than one
 	Load      [3]float64 `json:"load"`
 	Mem       Mem        `json:"mem"`
 	Swap      Mem        `json:"swap"`
@@ -1182,18 +1232,24 @@ func cpuMHz() float64 {
 
 // cpuTemp picks the sensor that best represents the CPU package.
 func cpuTemp(temps []Temp) float64 {
-	best, max := 0.0, 0.0
+	// the hottest package: a two-socket board has two, and the hotter one is the one to watch
+	pkg, max := 0.0, 0.0
 	for _, t := range temps {
 		l := strings.ToLower(t.Label)
 		if t.Chip == "coretemp" && strings.HasPrefix(l, "package") || t.Chip == "k10temp" && (l == "tctl" || l == "tdie") ||
 			t.Chip == "thermal" && strings.Contains(l, "cpu") || t.Chip == "cpu_thermal" || t.Chip == "soc_thermal" {
-			return t.C
+			if t.C > pkg {
+				pkg = t.C
+			}
 		}
 		if (t.Chip == "coretemp" || t.Chip == "k10temp") && t.C > max {
-			max, best = t.C, t.C
+			max = t.C
 		}
 	}
-	return best
+	if pkg > 0 {
+		return pkg
+	}
+	return max
 }
 
 var bootTime = func() int64 {
@@ -1323,6 +1379,7 @@ func (c *Collector) Processes() ProcTable {
 		last := c.ring[n-1]
 		t.CPU, t.Cores, t.Net, t.CPUTemp, t.CPUPowerW = last.CPU, last.Cores, last.Net, cpuTemp(last.Temps), last.CPUPower
 		t.CoreIDs = coreIDs(len(last.Cores))
+		t.CorePkgs = corePkgs(len(last.Cores))
 		from := n - 90
 		if from < 0 {
 			from = 0