Saving boot time during ProcessesWithContext call (linux).

On Linux, the system boot time is used when calculating process creation times
(expressed as number of seconds since the epoch). Obtaining the boot time
involves parsing the /proc/stat file, which can incur siginificant overhead
on systems with many CPU cores.

Permanently caching the boot time is not an option since it can change when the
system clock is updated (e.g. ntpd).

In this changeset we will save the boot time during one invocation of the
ProcessesWithContext function (listing all running processes).

process/process_linux.go

@@ -30,6 +30,8 @@ const prioProcess = 0 // linux/resource.h
 
 var clockTicks = 100 // default value
 
+const bootTimeContextKey = "host_boot_time"
+
 func init() {
 	clkTck, err := sysconf.Sysconf(sysconf.SC_CLK_TCK)
 	// ignore errors
@@ -1072,7 +1074,13 @@ func (p *Process) fillFromTIDStatWithContext(ctx context.Context, tid int32) (ui
 		Iowait: iotime / float64(clockTicks),
 	}
 
-	bootTime, _ := common.BootTimeWithContext(ctx)
+	var bootTime uint64
+	if bt, ok := ctx.Value(bootTimeContextKey).(uint64); ok {
+		bootTime = bt
+	} else {
+		bootTime, _ = common.BootTimeWithContext(ctx)
+	}
+
 	t, err := strconv.ParseUint(fields[22], 10, 64)
 	if err != nil {
 		return 0, 0, nil, 0, 0, 0, nil, err
@@ -1138,6 +1146,12 @@ func ProcessesWithContext(ctx context.Context) ([]*Process, error) {
 		return out, err
 	}
 
+	if bootTime, err := common.BootTimeWithContext(ctx); err == nil {
+		// Save the current value of boot time in the context -- to be used in
+		// fillFromTIDStatWithContext for each PID.
+		ctx = context.WithValue(ctx, bootTimeContextKey, bootTime)
+	}
+
 	for _, pid := range pids {
 		p, err := NewProcessWithContext(ctx, pid)
 		if err != nil {