Cherry-pick #15070 to 7.5: fallback to different CPU calculation if w…

…e have no online_cpu count (#15082)

* [metricbeat] Fallback to different CPU calculation if we have no online_cpu count (#15070)

CHANGELOG.next.asciidoc

@@ -81,6 +81,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Closing handler after verifying the registry key in diskio metricset. {issue}14683[14683] {pull}14759[14759]
 - Fix docker network stats when multiple interfaces are configured. {issue}14586[14586] {pull}14825[14825]
 - Fix ListMetrics pagination in aws module. {issue}14926[14926] {pull}14942[14942]
+- Fix CPU count in docker/cpu in cases where no `online_cpus` are reported {pull}15070[15070]
 
 *Packetbeat*
 

metricbeat/module/docker/cpu/helper.go

@@ -100,7 +100,20 @@ type cpuUsage struct {
 // be updated/initialized with the corresponding value retrieved from Docker API.
 func (u *cpuUsage) CPUs() uint32 {
 	if u.cpus == 0 {
-		u.cpus = u.Stats.CPUStats.OnlineCPUs
+		if u.Stats.CPUStats.OnlineCPUs != 0 {
+			u.cpus = u.Stats.CPUStats.OnlineCPUs
+		} else {
+			//Certain versions of docker don't have `online_cpus`
+			//In addition to this, certain kernel versions will report spurious zeros from the cgroups usage_percpu
+			var realCPUCount uint32
+			for _, rCPUUsage := range u.Stats.CPUStats.CPUUsage.PercpuUsage {
+				if rCPUUsage != 0 {
+					realCPUCount++
+				}
+			}
+			u.cpus = realCPUCount
+		}
+
 	}
 	return u.cpus
 }