Correct CPU usage calculation in presence of offline CPUs and newer Linux

In https://github.com/torvalds/linux/commit/5ca3726 (released in v4.7-rc1) the
content of the `cpuacct.usage_percpu` file in sysfs was changed to include both
online and offline cpus. This broke the arithmetic in the stats helpers used by
`docker stats`, since it was using the length of the PerCPUUsage array as a
proxy for the number of online CPUs.

Add current number of online CPUs to types.StatsJSON and use it in the
calculation.

Keep a fallback to `len(v.CPUStats.CPUUsage.PercpuUsage)` so this code
continues to work when talking to an older daemon. An old client talking to a
new daemon will ignore the new field and behave as before.

Fixes #28941.

Signed-off-by: Ian Campbell <ian.campbell@docker.com>

daemon/stats/collector.go

@@ -80,6 +80,12 @@ func (s *Collector) Run() {
 			continue
 		}
 
+		onlineCPUs, err := s.getNumberOnlineCPUs()
+		if err != nil {
+			logrus.Errorf("collecting system online cpu count: %v", err)
+			continue
+		}
+
 		for _, pair := range pairs {
 			stats, err := s.supervisor.GetContainerStats(pair.container)
 			if err != nil {
@@ -97,6 +103,7 @@ func (s *Collector) Run() {
 			}
 			// FIXME: move to containerd on Linux (not Windows)
 			stats.CPUStats.SystemUsage = systemUsage
+			stats.CPUStats.OnlineCPUs = onlineCPUs
 
 			pair.publisher.Publish(*stats)
 		}

daemon/stats/collector_unix.go

@@ -11,6 +11,11 @@ import (
 	"github.com/opencontainers/runc/libcontainer/system"
 )
 
+/*
+#include <unistd.h>
+*/
+import "C"
+
 // platformNewStatsCollector performs platform specific initialisation of the
 // Collector structure.
 func platformNewStatsCollector(s *Collector) {
@@ -64,3 +69,11 @@ func (s *Collector) getSystemCPUUsage() (uint64, error) {
 	}
 	return 0, fmt.Errorf("invalid stat format. Error trying to parse the '/proc/stat' file")
 }
+
+func (s *Collector) getNumberOnlineCPUs() (uint32, error) {
+	i, err := C.sysconf(C._SC_NPROCESSORS_ONLN)
+	if err != nil {
+		return 0, err
+	}
+	return uint32(i), nil
+}

daemon/stats/collector_windows.go

@@ -13,3 +13,7 @@ func platformNewStatsCollector(s *Collector) {
 func (s *Collector) getSystemCPUUsage() (uint64, error) {
 	return 0, nil
 }
+
+func (s *Collector) getNumberOnlineCPUs() (uint32, error) {
+	return 0, nil
+}