runtime: get traceback from VDSO code

Currently if a profiling signal arrives while executing within a VDSO
the profiler will report _ExternalCode, which is needlessly confusing
for a pure Go program. Change the VDSO calling code to record the
caller's PC/SP, so that we can do a traceback from that point. If that
fails for some reason, report _VDSO rather than _ExternalCode, which
should at least point in the right direction.

This adds some instructions to the code that calls the VDSO, but the
slowdown is reasonably negligible:

name                                  old time/op  new time/op  delta
ClockVDSOAndFallbackPaths/vDSO-8      40.5ns ± 2%  41.3ns ± 1%  +1.85%  (p=0.002 n=10+10)
ClockVDSOAndFallbackPaths/Fallback-8  41.9ns ± 1%  43.5ns ± 1%  +3.84%  (p=0.000 n=9+9)
TimeNow-8                             41.5ns ± 3%  41.5ns ± 2%    ~     (p=0.723 n=10+10)

Fixes #24142

Change-Id: Iacd935db3c4c782150b3809aaa675a71799b1c9c
Reviewed-on: https://go-review.googlesource.com/97315
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

1 files changed, 46 insertions, 0 deletions

diff --git a/src/runtime/testdata/testprog/timeprof.go b/src/runtime/testdata/testprog/timeprof.go
new file mode 100644
index 0000000000..0702885369
--- /dev/null
+++ b/src/runtime/testdata/testprog/timeprof.go
@@ -0,0 +1,46 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"runtime/pprof"
+	"time"
+)
+
+func init() {
+	register("TimeProf", TimeProf)
+}
+
+func TimeProf() {
+	f, err := ioutil.TempFile("", "timeprof")
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(2)
+	}
+
+	if err := pprof.StartCPUProfile(f); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(2)
+	}
+
+	t0 := time.Now()
+	// We should get a profiling signal 100 times a second,
+	// so running for 1/10 second should be sufficient.
+	for time.Since(t0) < time.Second/10 {
+	}
+
+	pprof.StopCPUProfile()
+
+	name := f.Name()
+	if err := f.Close(); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(2)
+	}
+
+	fmt.Println(name)
+}