Avoid iowait percentage's overflow when counter is reset

metrics/linux/cpuusage.go

@@ -9,6 +9,7 @@ import (
 	"github.com/mackerelio/go-osstat/cpu"
 	"github.com/mackerelio/golib/logging"
 	"github.com/mackerelio/mackerel-agent/metrics"
+	"github.com/mackerelio/mackerel-agent/util"
 )
 
 /*
@@ -55,7 +56,7 @@ func (g *CPUUsageGenerator) Generate() (metrics.Values, error) {
 		"cpu.idle.percentage":   float64(current.Idle-previous.Idle) * cpuCount * 100.0 / totalDiff,
 	}
 	if current.StatCount >= 5 {
-		ret["cpu.iowait.percentage"] = float64(current.Iowait-previous.Iowait) * cpuCount * 100.0 / totalDiff
+		ret["cpu.iowait.percentage"] = float64(util.DiffResettableCounter(current.Iowait, previous.Iowait)) * cpuCount * 100.0 / totalDiff
 	}
 	if current.StatCount >= 6 {
 		ret["cpu.irq.percentage"] = float64(current.Irq-previous.Irq) * cpuCount * 100.0 / totalDiff

util/calc.go

@@ -0,0 +1,11 @@
+package util
+
+// Calculate incremental difference of uint64 counters that can be reset.
+// Avoiding overflow, return current value if it is smaller than previous one.
+func DiffResettableCounter(current, previous uint64) uint64 {
+	if current < previous {
+		// counter has been reset
+		return current
+	}
+	return current - previous
+}

util/calc_test.go

@@ -0,0 +1,25 @@
+package util
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestDiffResettableCounter(t *testing.T) {
+	cases := []struct {
+		inCurrent  uint64
+		inPrevious uint64
+		want       uint64
+	}{
+		{100, 30, 70},
+		{20, 50, 20}, // counter has been reset
+	}
+	for _, tt := range cases {
+		t.Run(fmt.Sprintf("%d - %d", tt.inCurrent, tt.inPrevious), func(t *testing.T) {
+			got := DiffResettableCounter(tt.inCurrent, tt.inPrevious)
+			if got != tt.want {
+				t.Errorf("want=%d, got=%d", tt.want, got)
+			}
+		})
+	}
+}