Use cgo to get systems clock ticks for metrics

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)

pkg/cgroups/fs/cpuacct.go

@@ -14,7 +14,10 @@ import (
 	"github.com/dotcloud/docker/pkg/system"
 )
 
-var cpuCount = float64(runtime.NumCPU())
+var (
+	cpuCount   = float64(runtime.NumCPU())
+	clockTicks = float64(system.GetClockTicks())
+)
 
 type cpuacctGroup struct {
 }
@@ -58,7 +61,7 @@ func (s *cpuacctGroup) Stats(d *data) (map[string]float64, error) {
 		deltaSystem = lastSystem - startSystem
 	)
 	if deltaSystem > 0.0 {
-		percentage = ((deltaProc / deltaSystem) * 100.0) * cpuCount
+		percentage = ((deltaProc / deltaSystem) * clockTicks) * cpuCount
 	}
 	// NOTE: a percentage over 100% is valid for POSIX because that means the
 	// processes is using multiple cores

pkg/system/sysconfig.go

@@ -0,0 +1,13 @@
+// +build linux,cgo
+
+package system
+
+/*
+#include <unistd.h>
+int get_hz(void) { return sysconf(_SC_CLK_TCK); }
+*/
+import "C"
+
+func GetClockTicks() int {
+	return int(C.get_hz())
+}

pkg/system/sysconfig_nocgo.go

@@ -0,0 +1,9 @@
+// +build linux,!cgo
+
+package system
+
+func GetClockTicks() int {
+	// when we cannot call out to C to get the sysconf it is fairly safe to
+	// just return 100
+	return 100
+}

pkg/system/unsupported.go

@@ -17,3 +17,9 @@ func UsetCloseOnExec(fd uintptr) error {
 func Gettid() int {
 	return 0
 }
+
+func GetClockTicks() int {
+	// when we cannot call out to C to get the sysconf it is fairly safe to
+	// just return 100
+	return 100
+}