runtime: use cgo traceback for SIGPROF

If we collected a cgo traceback when entering the SIGPROF signal
handler, record it as part of the profiling stack trace.

This serves as the promised test for https://golang.org/cl/21055 .

Change-Id: I5f60cd6cea1d9b7c3932211483a6bfab60ed21d2
Reviewed-on: https://go-review.googlesource.com/22650
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>

src/runtime/crash_cgo_test.go

@@ -7,6 +7,7 @@
 package runtime_test
 
 import (
+	"bytes"
 	"fmt"
 	"internal/testenv"
 	"os/exec"
@@ -232,3 +233,32 @@ func TestCgoTracebackContext(t *testing.T) {
 		t.Errorf("expected %q got %v", want, got)
 	}
 }
+
+func TestCgoPprof(t *testing.T) {
+	if runtime.GOOS != "linux" || runtime.GOARCH != "amd64" {
+		t.Skipf("not yet supported on %s/%s", runtime.GOOS, runtime.GOARCH)
+	}
+	testenv.MustHaveGoRun(t)
+
+	exe, err := buildTestProg(t, "testprogcgo")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	got, err := testEnv(exec.Command(exe, "CgoPprof")).CombinedOutput()
+	if err != nil {
+		t.Fatal(err)
+	}
+	fn := strings.TrimSpace(string(got))
+
+	top, err := exec.Command("go", "tool", "pprof", "-top", "-nodecount=1", exe, fn).CombinedOutput()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	t.Logf("%s", top)
+
+	if !bytes.Contains(top, []byte("cpuHog")) {
+		t.Error("missing cpuHog in pprof output")
+	}
+}

src/runtime/proc.go

@@ -3085,12 +3085,24 @@ func sigprof(pc, sp, lr uintptr, gp *g, mp *m) {
 	var haveStackLock *g
 	n := 0
 	if mp.ncgo > 0 && mp.curg != nil && mp.curg.syscallpc != 0 && mp.curg.syscallsp != 0 {
-		// Cgo, we can't unwind and symbolize arbitrary C code,
-		// so instead collect Go stack that leads to the cgo call.
-		// This is especially important on windows, since all syscalls are cgo calls.
+		cgoOff := 0
+		// Check cgoCallersUse to make sure that we are not
+		// interrupting other code that is fiddling with
+		// cgoCallers.  We are running in a signal handler
+		// with all signals blocked, so we don't have to worry
+		// about any other code interrupting us.
+		if atomic.Load(&mp.cgoCallersUse) == 0 && mp.cgoCallers != nil && mp.cgoCallers[0] != 0 {
+			for cgoOff < len(mp.cgoCallers) && mp.cgoCallers[cgoOff] != 0 {
+				cgoOff++
+			}
+			copy(stk[:], mp.cgoCallers[:cgoOff])
+			mp.cgoCallers[0] = 0
+		}
+
+		// Collect Go stack that leads to the cgo call.
 		if gcTryLockStackBarriers(mp.curg) {
 			haveStackLock = mp.curg
-			n = gentraceback(mp.curg.syscallpc, mp.curg.syscallsp, 0, mp.curg, 0, &stk[0], len(stk), nil, nil, 0)
+			n = gentraceback(mp.curg.syscallpc, mp.curg.syscallsp, 0, mp.curg, 0, &stk[cgoOff], len(stk)-cgoOff, nil, nil, 0)
 		}
 	} else if traceback {
 		var flags uint = _TraceTrap

src/runtime/testdata/testprogcgo/pprof.go

@@ -0,0 +1,96 @@
+// Copyright 2016 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
+
+// Run a slow C function saving a CPU profile.
+
+/*
+#include <stdint.h>
+
+int salt1;
+int salt2;
+
+void cpuHog() {
+	int foo = salt1;
+	int i;
+
+	for (i = 0; i < 100000; i++) {
+		if (foo > 0) {
+			foo *= foo;
+		} else {
+			foo *= foo + 1;
+		}
+	}
+	salt2 = foo;
+}
+
+static int cpuHogCount;
+
+struct cgoTracebackArg {
+	uintptr_t  context;
+	uintptr_t* buf;
+	uintptr_t  max;
+};
+
+// pprofCgoTraceback is passed to runtime.SetCgoTraceback.
+// For testing purposes it pretends that all CPU hits in C code are in cpuHog.
+void pprofCgoTraceback(void* parg) {
+	struct cgoTracebackArg* arg = (struct cgoTracebackArg*)(parg);
+	arg->buf[0] = (uintptr_t)(cpuHog) + 0x10;
+	arg->buf[1] = 0;
+	++cpuHogCount;
+}
+
+// getCpuHogCount fetches the number of times we've seen cpuHog in the
+// traceback.
+int getCpuHogCount() {
+	return cpuHogCount;
+}
+*/
+import "C"
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"runtime"
+	"runtime/pprof"
+	"time"
+	"unsafe"
+)
+
+func init() {
+	register("CgoPprof", CgoPprof)
+}
+
+func CgoPprof() {
+	runtime.SetCgoTraceback(0, unsafe.Pointer(C.pprofCgoTraceback), nil, nil)
+
+	f, err := ioutil.TempFile("", "prof")
+	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()
+	for C.getCpuHogCount() < 2 && time.Since(t0) < time.Second {
+		C.cpuHog()
+	}
+
+	pprof.StopCPUProfile()
+
+	name := f.Name()
+	if err := f.Close(); err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(2)
+	}
+
+	fmt.Println(name)
+}