runtime/pprof: check if PC is reused for inlining

When describing call stacks that include inlined function calls, the
runtime uses "fake" PCs to represent the frames that inlining removed.
Those PCs correspond to real NOP instructions that the compiler inserts
for this purpose.

Describing the call stack in a protobuf-formatted profile requires the
runtime/pprof package to collapse any sequences of fake call sites back
into single PCs, removing the NOPs but retaining their line info.

But because the NOP instructions are part of the function, they can
appear as leaf nodes in a CPU profile. That results in an address that
should sometimes be ignored (when it appears as a call site) and that
sometimes should be present in the profile (when it is observed
consuming CPU time).

When processing a PC address, consider it first as a fake PC to add to
the current inlining deck, and then as a previously-seen (real) PC.

Fixes #50996

Change-Id: I80802369978bd7ac9969839ecfc9995ea4f84ab4
Reviewed-on: https://go-review.googlesource.com/c/go/+/384239
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: rhysh

src/runtime/pprof/proto.go

@@ -244,6 +244,10 @@ type locInfo struct {
 	// to represent inlined functions
 	// https://github.com/golang/go/blob/d6f2f833c93a41ec1c68e49804b8387a06b131c5/src/runtime/traceback.go#L347-L368
 	pcs []uintptr
+
+	// results of allFrames call for this PC
+	frames          []runtime.Frame
+	symbolizeResult symbolizeFlag
 }
 
 // newProfileBuilder returns a new profileBuilder.
@@ -399,6 +403,24 @@ func (b *profileBuilder) appendLocsForStack(locs []uint64, stk []uintptr) (newLo
 	for len(stk) > 0 {
 		addr := stk[0]
 		if l, ok := b.locs[addr]; ok {
+			// When generating code for an inlined function, the compiler adds
+			// NOP instructions to the outermost function as a placeholder for
+			// each layer of inlining. When the runtime generates tracebacks for
+			// stacks that include inlined functions, it uses the addresses of
+			// those NOPs as "fake" PCs on the stack as if they were regular
+			// function call sites. But if a profiling signal arrives while the
+			// CPU is executing one of those NOPs, its PC will show up as a leaf
+			// in the profile with its own Location entry. So, always check
+			// whether addr is a "fake" PC in the context of the current call
+			// stack by trying to add it to the inlining deck before assuming
+			// that the deck is complete.
+			if len(b.deck.pcs) > 0 {
+				if added := b.deck.tryAdd(addr, l.frames, l.symbolizeResult); added {
+					stk = stk[1:]
+					continue
+				}
+			}
+
 			// first record the location if there is any pending accumulated info.
 			if id := b.emitLocation(); id > 0 {
 				locs = append(locs, id)
@@ -451,6 +473,27 @@ func (b *profileBuilder) appendLocsForStack(locs []uint64, stk []uintptr) (newLo
 	return locs
 }
 
+// Here's an example of how Go 1.17 writes out inlined functions, compiled for
+// linux/amd64. The disassembly of main.main shows two levels of inlining: main
+// calls b, b calls a, a does some work.
+//
+//   inline.go:9   0x4553ec  90              NOPL                 // func main()    { b(v) }
+//   inline.go:6   0x4553ed  90              NOPL                 // func b(v *int) { a(v) }
+//   inline.go:5   0x4553ee  48c7002a000000  MOVQ $0x2a, 0(AX)    // func a(v *int) { *v = 42 }
+//
+// If a profiling signal arrives while executing the MOVQ at 0x4553ee (for line
+// 5), the runtime will report the stack as the MOVQ frame being called by the
+// NOPL at 0x4553ed (for line 6) being called by the NOPL at 0x4553ec (for line
+// 9).
+//
+// The role of pcDeck is to collapse those three frames back into a single
+// location at 0x4553ee, with file/line/function symbolization info representing
+// the three layers of calls. It does that via sequential calls to pcDeck.tryAdd
+// starting with the leaf-most address. The fourth call to pcDeck.tryAdd will be
+// for the caller of main.main. Because main.main was not inlined in its caller,
+// the deck will reject the addition, and the fourth PC on the stack will get
+// its own location.
+
 // pcDeck is a helper to detect a sequence of inlined functions from
 // a stack trace returned by the runtime.
 //
@@ -535,7 +578,12 @@ func (b *profileBuilder) emitLocation() uint64 {
 	newFuncs := make([]newFunc, 0, 8)
 
 	id := uint64(len(b.locs)) + 1
-	b.locs[addr] = locInfo{id: id, pcs: append([]uintptr{}, b.deck.pcs...)}
+	b.locs[addr] = locInfo{
+		id:              id,
+		pcs:             append([]uintptr{}, b.deck.pcs...),
+		symbolizeResult: b.deck.symbolizeResult,
+		frames:          append([]runtime.Frame{}, b.deck.frames...),
+	}
 
 	start := b.pb.startMessage()
 	b.pb.uint64Opt(tagLocation_ID, id)