runtime: make systemstack tail call if already switched

Currently systemstack always calls its argument, even if we're already
on the system stack. Unfortunately, traceback with _TraceJump stops at
the first systemstack it sees, which often cuts off runtime stacks
early in profiles.

Fix this by performing a tail call if we're already on the system
stack. This eliminates it from the traceback entirely, so it won't
stop prematurely (or all get mushed into a single node in the profile
graph).

Change-Id: Ibc69e8765e899f8d3806078517b8c7314da196f4
Reviewed-on: https://go-review.googlesource.com/74050
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Keith Randall <khr@golang.org>

1 files changed, 33 insertions, 0 deletions

diff --git a/src/runtime/stack_test.go b/src/runtime/stack_test.go
index c9b84be066..8e7c7d47a8 100644
--- a/src/runtime/stack_test.go
+++ b/src/runtime/stack_test.go
@@ -5,6 +5,7 @@
 package runtime_test
 
 import (
+	"bytes"
 	"fmt"
 	. "runtime"
 	"strings"
@@ -685,3 +686,35 @@ func TestStackWrapperStack(t *testing.T) {
 		t.Fatalf("<autogenerated> appears in stack trace:\n%s", stk)
 	}
 }
+
+func TestTracebackSystemstack(t *testing.T) {
+	if GOARCH == "ppc64" || GOARCH == "ppc64le" {
+		t.Skip("systemstack tail call not implemented on ppc64x")
+	}
+
+	// Test that profiles correctly jump over systemstack,
+	// including nested systemstack calls.
+	pcs := make([]uintptr, 20)
+	pcs = pcs[:TracebackSystemstack(pcs, 5)]
+	// Check that runtime.TracebackSystemstack appears five times
+	// and that we see TestTracebackSystemstack.
+	countIn, countOut := 0, 0
+	frames := CallersFrames(pcs)
+	var tb bytes.Buffer
+	for {
+		frame, more := frames.Next()
+		fmt.Fprintf(&tb, "\n%s+0x%x %s:%d", frame.Function, frame.PC-frame.Entry, frame.File, frame.Line)
+		switch frame.Function {
+		case "runtime.TracebackSystemstack":
+			countIn++
+		case "runtime_test.TestTracebackSystemstack":
+			countOut++
+		}
+		if !more {
+			break
+		}
+	}
+	if countIn != 5 || countOut != 1 {
+		t.Fatalf("expected 5 calls to TracebackSystemstack and 1 call to TestTracebackSystemstack, got:%s", tb.String())
+	}
+}