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>

src/runtime/asm_386.s

@@ -474,11 +474,12 @@ switch:
 	RET
 
 noswitch:
-	// already on system stack, just call directly
+	// already on system stack; tail call the function
+	// Using a tail call here cleans up tracebacks since we won't stop
+	// at an intermediate systemstack.
 	MOVL	DI, DX
 	MOVL	0(DI), DI
-	CALL	DI
+	JMP	DI
-	RET
 
 /*
  * support for morestack

src/runtime/asm_amd64.s

@@ -419,11 +419,12 @@ switch:
 	RET
 
 noswitch:
-	// already on m stack, just call directly
+	// already on m stack; tail call the function
+	// Using a tail call here cleans up tracebacks since we won't stop
+	// at an intermediate systemstack.
 	MOVQ	DI, DX
 	MOVQ	0(DI), DI
-	CALL	DI
+	JMP	DI
-	RET
 
 /*
  * support for morestack

src/runtime/asm_amd64p32.s

@@ -306,10 +306,11 @@ switch:
 
 noswitch:
 	// already on m stack, just call directly
+	// Using a tail call here cleans up tracebacks since we won't stop
+	// at an intermediate systemstack.
 	MOVL	DI, DX
 	MOVL	0(DI), DI
-	CALL	DI
+	JMP	DI
-	RET
 
 /*
  * support for morestack

src/runtime/asm_arm.s

@@ -358,10 +358,12 @@ switch:
 	RET
 
 noswitch:
+	// Using a tail call here cleans up tracebacks since we won't stop
+	// at an intermediate systemstack.
 	MOVW	R0, R7
 	MOVW	0(R0), R0
-	BL	(R0)
+	MOVW.P	4(R13), R14	// restore LR
-	RET
+	B	(R0)
 
 /*
  * support for morestack

src/runtime/asm_arm64.s

@@ -239,9 +239,11 @@ switch:
 
 noswitch:
 	// already on m stack, just call directly
+	// Using a tail call here cleans up tracebacks since we won't stop
+	// at an intermediate systemstack.
 	MOVD	0(R26), R3	// code pointer
-	BL	(R3)
+	MOVD.P	16(RSP), R30	// restore LR
-	RET
+	B	(R3)
 
 /*
  * support for morestack

src/runtime/asm_mips64x.s

@@ -214,9 +214,12 @@ switch:
 
 noswitch:
 	// already on m stack, just call directly
+	// Using a tail call here cleans up tracebacks since we won't stop
+	// at an intermediate systemstack.
 	MOVV	0(REGCTXT), R4	// code pointer
-	JAL	(R4)
+	MOVV	0(R29), R31	// restore LR
-	RET
+	ADDV	$8, R29
+	JMP	(R4)
 
 /*
  * support for morestack

src/runtime/asm_mipsx.s

@@ -215,9 +215,12 @@ switch:
 
 noswitch:
 	// already on m stack, just call directly
+	// Using a tail call here cleans up tracebacks since we won't stop
+	// at an intermediate systemstack.
 	MOVW	0(REGCTXT), R4	// code pointer
-	JAL	(R4)
+	MOVW	0(R29), R31	// restore LR
-	RET
+	ADD	$4, R29
+	JMP	(R4)
 
 /*
  * support for morestack

src/runtime/asm_ppc64x.s

@@ -265,6 +265,9 @@ switch:
 
 noswitch:
 	// already on m stack, just call directly
+	// On other arches we do a tail call here, but it appears to be
+	// impossible to tail call a function pointer in shared mode on
+	// ppc64 because the caller is responsible for restoring the TOC.
 	MOVD	0(R11), R12	// code pointer
 	MOVD	R12, CTR
 	BL	(CTR)

src/runtime/asm_s390x.s

@@ -224,9 +224,12 @@ switch:
 
 noswitch:
 	// already on m stack, just call directly
+	// Using a tail call here cleans up tracebacks since we won't stop
+	// at an intermediate systemstack.
 	MOVD	0(R12), R3	// code pointer
-	BL	(R3)
+	MOVD	0(R15), LR	// restore LR
-	RET
+	ADD	$8, R15
+	BR	(R3)
 
 /*
  * support for morestack

src/runtime/export_test.go

@@ -395,3 +395,16 @@ func LockOSCounts() (external, internal uint32) {
 	}
 	return g.m.lockedExt, g.m.lockedInt
 }
+
+//go:noinline
+func TracebackSystemstack(stk []uintptr, i int) int {
+	if i == 0 {
+		pc, sp := getcallerpc(), getcallersp(unsafe.Pointer(&stk))
+		return gentraceback(pc, sp, 0, getg(), 0, &stk[0], len(stk), nil, nil, _TraceJumpStack)
+	}
+	n := 0
+	systemstack(func() {
+		n = TracebackSystemstack(stk, i-1)
+	})
+	return n
+}

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())
+	}
+}