cgo/misc/test: burn CPU to improve sleep accuracy

Fixes #4008.

Run a background goroutine that wastes CPU to trick the
power management into raising the CPU frequency which,
by side effect, makes sleep more accurate on arm.

=== RUN TestParallelSleep
--- PASS: TestParallelSleep (1.30 seconds)
_cgo_gotypes.go:772:    sleep(1) slept for 1.000458s

R=minux.ma, r
CC=golang-dev
https://golang.org/cl/6498060

diff --git a/misc/cgo/test/issue1560.go b/misc/cgo/test/issue1560.go
index 7bc1dd6df490af26145f44de13975d541eee93e1..ac897aa33bf217a8637b2352178b0a260f69ea1a 100644 (file)
--- a/misc/cgo/test/issue1560.go
+++ b/misc/cgo/test/issue1560.go
@@ -35,16 +35,35 @@ func BackgroundSleep(n int) {
        }()
 }
 
+// wasteCPU starts a background goroutine to waste CPU 
+// to cause the power management to raise the CPU frequency. 
+// On ARM this has the side effect of making sleep more accurate.
+func wasteCPU() chan struct{} {
+       done := make(chan struct{})
+       go func() {
+               for {
+                       select {
+                       case <-done:
+                               return
+                       default:
+                       }
+               }
+       }()
+       // pause for a short amount of time to allow the
+       // power management to recognise load has risen.
+       <-time.After(300 * time.Millisecond)
+       return done
+}
+
 func testParallelSleep(t *testing.T) {
+       defer runtime.GOMAXPROCS(runtime.GOMAXPROCS(2))
+       defer close(wasteCPU())
+
        sleepSec := 1
-       if runtime.GOARCH == "arm" {
-               // on ARM, the 1.3s deadline is frequently missed,
-               // so increase sleep time to 2s
-               sleepSec = 2
-       }
        start := time.Now()
        parallelSleep(sleepSec)
-       dt := time.Now().Sub(start)
+       dt := time.Since(start)
+       t.Logf("sleep(%d) slept for %v", sleepSec, dt)
        // bug used to run sleeps in serial, producing a 2*sleepSec-second delay.
        if dt >= time.Duration(sleepSec)*1300*time.Millisecond {
                t.Fatalf("parallel %d-second sleeps slept for %f seconds", sleepSec, dt.Seconds())