From 3565316d7e306d60ad74248291eaabb3410f2972 Mon Sep 17 00:00:00 2001
From: Ben Kochie <superq@gmail.com>
Date: Sat, 23 May 2020 21:46:54 +0200
Subject: Linux CPU: Cache CPU metrics

Cache CPU metrics to avoid counters (ie iowait) jumping backwards.

Fixes: https://github.com/prometheus/node_exporter/issues/1686

Signed-off-by: Ben Kochie <superq@gmail.com>
---
 .circleci/config.yml   |  2 +-
 CHANGELOG.md           |  2 +-
 collector/cpu_linux.go | 85 +++++++++++++++++++++++++++++++++++++++++++++++++-
 3 files changed, 86 insertions(+), 3 deletions(-)

diff --git a/.circleci/config.yml b/.circleci/config.yml
index 19c0f1b..018b8b1 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -28,7 +28,7 @@ jobs:
     steps:
     - checkout
     - run: sudo pip install codespell
-    - run: codespell --skip=".git,./vendor,ttar,go.mod,go.sum,*pem" -L uint,packages\',uptodate
+    - run: codespell --skip=".git,./vendor,ttar,go.mod,go.sum,*pem,./collector/fixtures" -L uint,packages\',uptodate
 
   build:
     machine:
diff --git a/CHANGELOG.md b/CHANGELOG.md
index a5d884f..9e05d0f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,7 +3,7 @@
 * [CHANGE]
 * [FEATURE]
 * [ENHANCEMENT]
-* [BUGFIX]
+* [BUGFIX] Linux CPU: Cache CPU metrics to make them monotonically increasing #1711
 
 ## 1.0.0-rc.1 / 2020-05-14
 
diff --git a/collector/cpu_linux.go b/collector/cpu_linux.go
index ae8ee53..dfa4d4a 100644
--- a/collector/cpu_linux.go
+++ b/collector/cpu_linux.go
@@ -19,6 +19,7 @@ import (
 	"fmt"
 	"path/filepath"
 	"strconv"
+	"sync"
 
 	"github.com/go-kit/kit/log"
 	"github.com/go-kit/kit/log/level"
@@ -35,6 +36,8 @@ type cpuCollector struct {
 	cpuCoreThrottle    *prometheus.Desc
 	cpuPackageThrottle *prometheus.Desc
 	logger             log.Logger
+	cpuStats           []procfs.CPUStat
+	cpuStatsMutex      sync.Mutex
 }
 
 var (
@@ -203,7 +206,12 @@ func (c *cpuCollector) updateStat(ch chan<- prometheus.Metric) error {
 		return err
 	}
 
-	for cpuID, cpuStat := range stats.CPU {
+	c.updateCPUStats(stats.CPU)
+
+	// Acquire a lock to read the stats.
+	c.cpuStatsMutex.Lock()
+	defer c.cpuStatsMutex.Unlock()
+	for cpuID, cpuStat := range c.cpuStats {
 		cpuNum := strconv.Itoa(cpuID)
 		ch <- prometheus.MustNewConstMetric(c.cpu, prometheus.CounterValue, cpuStat.User, cpuNum, "user")
 		ch <- prometheus.MustNewConstMetric(c.cpu, prometheus.CounterValue, cpuStat.Nice, cpuNum, "nice")
@@ -221,3 +229,78 @@ func (c *cpuCollector) updateStat(ch chan<- prometheus.Metric) error {
 
 	return nil
 }
+
+// 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()
+
+	// Reset the cache if the list of CPUs has changed.
+	if len(c.cpuStats) != len(newStats) {
+		c.cpuStats = make([]procfs.CPUStat, len(newStats))
+	}
+
+	for i, n := range newStats {
+		// If idle jumps backwards, assume we had a hotplug event and reset the stats for this CPU.
+		if n.Idle < c.cpuStats[i].Idle {
+			level.Warn(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)
+			c.cpuStats[i] = procfs.CPUStat{}
+		}
+		c.cpuStats[i].Idle = n.Idle
+
+		if n.User >= c.cpuStats[i].User {
+			c.cpuStats[i].User = n.User
+		} else {
+			level.Warn(c.logger).Log("msg", "CPU User counter jumped backwards", "cpu", i, "old_value", c.cpuStats[i].User, "new_value", n.User)
+		}
+
+		if n.Nice >= c.cpuStats[i].Nice {
+			c.cpuStats[i].Nice = n.Nice
+		} else {
+			level.Warn(c.logger).Log("msg", "CPU Nice counter jumped backwards", "cpu", i, "old_value", c.cpuStats[i].Nice, "new_value", n.Nice)
+		}
+
+		if n.System >= c.cpuStats[i].System {
+			c.cpuStats[i].System = n.System
+		} else {
+			level.Warn(c.logger).Log("msg", "CPU System counter jumped backwards", "cpu", i, "old_value", c.cpuStats[i].System, "new_value", n.System)
+		}
+
+		if n.Iowait >= c.cpuStats[i].Iowait {
+			c.cpuStats[i].Iowait = n.Iowait
+		} else {
+			level.Warn(c.logger).Log("msg", "CPU Iowait counter jumped backwards", "cpu", i, "old_value", c.cpuStats[i].Iowait, "new_value", n.Iowait)
+		}
+
+		if n.IRQ >= c.cpuStats[i].IRQ {
+			c.cpuStats[i].IRQ = n.IRQ
+		} else {
+			level.Warn(c.logger).Log("msg", "CPU IRQ counter jumped backwards", "cpu", i, "old_value", c.cpuStats[i].IRQ, "new_value", n.IRQ)
+		}
+
+		if n.SoftIRQ >= c.cpuStats[i].SoftIRQ {
+			c.cpuStats[i].SoftIRQ = n.SoftIRQ
+		} else {
+			level.Warn(c.logger).Log("msg", "CPU SoftIRQ counter jumped backwards", "cpu", i, "old_value", c.cpuStats[i].SoftIRQ, "new_value", n.SoftIRQ)
+		}
+
+		if n.Steal >= c.cpuStats[i].Steal {
+			c.cpuStats[i].Steal = n.Steal
+		} else {
+			level.Warn(c.logger).Log("msg", "CPU Steal counter jumped backwards", "cpu", i, "old_value", c.cpuStats[i].Steal, "new_value", n.Steal)
+		}
+
+		if n.Guest >= c.cpuStats[i].Guest {
+			c.cpuStats[i].Guest = n.Guest
+		} else {
+			level.Warn(c.logger).Log("msg", "CPU Guest counter jumped backwards", "cpu", i, "old_value", c.cpuStats[i].Guest, "new_value", n.Guest)
+		}
+
+		if n.GuestNice >= c.cpuStats[i].GuestNice {
+			c.cpuStats[i].GuestNice = n.GuestNice
+		} else {
+			level.Warn(c.logger).Log("msg", "CPU GuestNice counter jumped backwards", "cpu", i, "old_value", c.cpuStats[i].GuestNice, "new_value", n.GuestNice)
+		}
+	}
+}
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