Handle small backwards jumps in CPU idle

The Linux CPU idle stat can also jump backwards slightly in some cases.
Allow the jump back up to 3 seconds before we attempt to reset the CPU
counter cache.

Fixes: https://github.com/prometheus/node_exporter/issues/1903

Signed-off-by: Ben Kochie <superq@gmail.com>

collector/cpu_linux.go

@@ -46,10 +46,14 @@ type cpuCollector struct {
 	cpuBugsIncludeRegexp  *regexp.Regexp
 }
 
+// Idle jump back limit in seconds.
+const jumpBackSeconds = 3.0
+
 var (
-	enableCPUInfo = kingpin.Flag("collector.cpu.info", "Enables metric cpu_info").Bool()
+	enableCPUInfo        = kingpin.Flag("collector.cpu.info", "Enables metric cpu_info").Bool()
-	flagsInclude  = kingpin.Flag("collector.cpu.info.flags-include", "Filter the `flags` field in cpuInfo with a value that must be a regular expression").String()
+	flagsInclude         = kingpin.Flag("collector.cpu.info.flags-include", "Filter the `flags` field in cpuInfo with a value that must be a regular expression").String()
-	bugsInclude   = kingpin.Flag("collector.cpu.info.bugs-include", "Filter the `bugs` field in cpuInfo with a value that must be a regular expression").String()
+	bugsInclude          = kingpin.Flag("collector.cpu.info.bugs-include", "Filter the `bugs` field in cpuInfo with a value that must be a regular expression").String()
+	jumpBackDebugMessage = fmt.Sprintf("CPU Idle counter jumped backwards more than %f seconds, possible hotplug event, resetting CPU stats", jumpBackSeconds)
 )
 
 func init() {
@@ -302,6 +306,7 @@ func (c *cpuCollector) updateStat(ch chan<- prometheus.Metric) error {
 
 // updateCPUStats updates the internal cache of CPU stats.
 func (c *cpuCollector) updateCPUStats(newStats []procfs.CPUStat) {
+
 	// Acquire a lock to update the stats.
 	c.cpuStatsMutex.Lock()
 	defer c.cpuStatsMutex.Unlock()
@@ -312,12 +317,17 @@ func (c *cpuCollector) updateCPUStats(newStats []procfs.CPUStat) {
 	}
 
 	for i, n := range newStats {
-		// If idle jumps backwards, assume we had a hotplug event and reset the stats for this CPU.
+		// If idle jumps backwards by more than X seconds, assume we had a hotplug event and reset the stats for this CPU.
-		if n.Idle < c.cpuStats[i].Idle {
+		if (c.cpuStats[i].Idle - n.Idle) >= jumpBackSeconds {
-			level.Debug(c.logger).Log("msg", "CPU Idle counter jumped backwards, possible hotplug event, resetting CPU stats", "cpu", i, "old_value", c.cpuStats[i].Idle, "new_value", n.Idle)
+			level.Debug(c.logger).Log("msg", jumpBackDebugMessage, "cpu", i, "old_value", c.cpuStats[i].Idle, "new_value", n.Idle)
 			c.cpuStats[i] = procfs.CPUStat{}
 		}
-		c.cpuStats[i].Idle = n.Idle
+
+		if n.Idle >= c.cpuStats[i].Idle {
+			c.cpuStats[i].Idle = n.Idle
+		} else {
+			level.Debug(c.logger).Log("msg", "CPU Idle counter jumped backwards", "cpu", i, "old_value", c.cpuStats[i].Idle, "new_value", n.Idle)
+		}
 
 		if n.User >= c.cpuStats[i].User {
 			c.cpuStats[i].User = n.User

collector/cpu_linux_test.go

@@ -0,0 +1,105 @@
+// Copyright 2021 The Prometheus 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.
+
+// +build !nocpu
+
+package collector
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/go-kit/log"
+	"github.com/prometheus/procfs"
+)
+
+func makeTestCPUCollector(s []procfs.CPUStat) *cpuCollector {
+	dup := make([]procfs.CPUStat, len(s))
+	copy(dup, s)
+	return &cpuCollector{
+		logger:   log.NewNopLogger(),
+		cpuStats: dup,
+	}
+}
+
+func TestCPU(t *testing.T) {
+	firstCPUStat := []procfs.CPUStat{{
+		User:      100.0,
+		Nice:      100.0,
+		System:    100.0,
+		Idle:      100.0,
+		Iowait:    100.0,
+		IRQ:       100.0,
+		SoftIRQ:   100.0,
+		Steal:     100.0,
+		Guest:     100.0,
+		GuestNice: 100.0,
+	}}
+
+	c := makeTestCPUCollector(firstCPUStat)
+	want := []procfs.CPUStat{{
+		User:      101.0,
+		Nice:      101.0,
+		System:    101.0,
+		Idle:      101.0,
+		Iowait:    101.0,
+		IRQ:       101.0,
+		SoftIRQ:   101.0,
+		Steal:     101.0,
+		Guest:     101.0,
+		GuestNice: 101.0,
+	}}
+	c.updateCPUStats(want)
+	got := c.cpuStats
+	if !reflect.DeepEqual(want, got) {
+		t.Fatalf("should have %v CPU Stat: got %v", want, got)
+	}
+
+	c = makeTestCPUCollector(firstCPUStat)
+	jumpBack := []procfs.CPUStat{{
+		User:      99.9,
+		Nice:      99.9,
+		System:    99.9,
+		Idle:      99.9,
+		Iowait:    99.9,
+		IRQ:       99.9,
+		SoftIRQ:   99.9,
+		Steal:     99.9,
+		Guest:     99.9,
+		GuestNice: 99.9,
+	}}
+	c.updateCPUStats(jumpBack)
+	got = c.cpuStats
+	if reflect.DeepEqual(jumpBack, got) {
+		t.Fatalf("should have %v CPU Stat: got %v", firstCPUStat, got)
+	}
+
+	c = makeTestCPUCollector(firstCPUStat)
+	resetIdle := []procfs.CPUStat{{
+		User:      102.0,
+		Nice:      102.0,
+		System:    102.0,
+		Idle:      1.0,
+		Iowait:    102.0,
+		IRQ:       102.0,
+		SoftIRQ:   102.0,
+		Steal:     102.0,
+		Guest:     102.0,
+		GuestNice: 102.0,
+	}}
+	c.updateCPUStats(resetIdle)
+	got = c.cpuStats
+	if !reflect.DeepEqual(resetIdle, got) {
+		t.Fatalf("should have %v CPU Stat: got %v", resetIdle, got)
+	}
+}