runtime: use frame pointer unwinding for block and mutex profilers

Use frame pointer unwinding, where supported, to collect call stacks for
the block, and mutex profilers. This method of collecting call stacks is
typically an order of magnitude faster than callers/tracebackPCs. The
marginal benefit for these profile types is likely small compared to
using frame pointer unwinding for the execution tracer. However, the
block profiler can have noticeable overhead unless the sampling rate is
very high. Additionally, using frame pointer unwinding in more places
helps ensure more testing/support, which benefits systems like the
execution tracer which rely on frame pointer unwinding to be practical
to use.

Change-Id: I4b36c90cd2df844645fd275a41b247352d635727
Reviewed-on: https://go-review.googlesource.com/c/go/+/533258
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Cherry Mui <cherryyz@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

1 files changed, 43 insertions, 12 deletions

diff --git a/src/runtime/mprof.go b/src/runtime/mprof.go
index d4d5e285fd..b51edcbcab 100644
--- a/src/runtime/mprof.go
+++ b/src/runtime/mprof.go
@@ -43,7 +43,10 @@ const (
 	// Note that it's only used internally as a guard against
 	// wildly out-of-bounds slicing of the PCs that come after
 	// a bucket struct, and it could increase in the future.
-	maxStack = 32
+	// The "+ 1" is to account for the first stack entry being
+	// taken up by a "skip" sentinel value for profilers which
+	// defer inline frame expansion until the profile is reported.
+	maxStack = 32 + 1
 )
 
 type bucketType int
@@ -502,14 +505,40 @@ func blocksampled(cycles, rate int64) bool {
 	return true
 }
 
+// saveblockevent records a profile event of the type specified by which.
+// cycles is the quantity associated with this event and rate is the sampling rate,
+// used to adjust the cycles value in the manner determined by the profile type.
+// skip is the number of frames to omit from the traceback associated with the event.
+// The traceback will be recorded from the stack of the goroutine associated with the current m.
+// skip should be positive if this event is recorded from the current stack
+// (e.g. when this is not called from a system stack)
 func saveblockevent(cycles, rate int64, skip int, which bucketType) {
-	var nstk int
 	gp := getg()
 	mp := acquirem() // we must not be preempted while accessing profstack
-	if gp.m.curg == nil || gp.m.curg == gp {
-		nstk = callers(skip, mp.profStack)
+	nstk := 1
+	if tracefpunwindoff() || gp.m.hasCgoOnStack() {
+		mp.profStack[0] = logicalStackSentinel
+		if gp.m.curg == nil || gp.m.curg == gp {
+			nstk = callers(skip, mp.profStack[1:])
+		} else {
+			nstk = gcallers(gp.m.curg, skip, mp.profStack[1:])
+		}
 	} else {
-		nstk = gcallers(gp.m.curg, skip, mp.profStack)
+		mp.profStack[0] = uintptr(skip)
+		if gp.m.curg == nil || gp.m.curg == gp {
+			if skip > 0 {
+				// We skip one fewer frame than the provided value for frame
+				// pointer unwinding because the skip value includes the current
+				// frame, whereas the saved frame pointer will give us the
+				// caller's return address first (so, not including
+				// saveblockevent)
+				mp.profStack[0] -= 1
+			}
+			nstk += fpTracebackPCs(unsafe.Pointer(getfp()), mp.profStack[1:])
+		} else {
+			mp.profStack[1] = gp.m.curg.sched.pc
+			nstk += 1 + fpTracebackPCs(unsafe.Pointer(gp.m.curg.sched.bp), mp.profStack[2:])
+		}
 	}
 
 	saveBlockEventStack(cycles, rate, mp.profStack[:nstk], which)
@@ -689,9 +718,10 @@ func (prof *mLockProfile) captureStack() {
 	}
 	prof.pending = 0
 
+	prof.stack[0] = logicalStackSentinel
 	if debug.runtimeContentionStacks.Load() == 0 {
-		prof.stack[0] = abi.FuncPCABIInternal(_LostContendedRuntimeLock) + sys.PCQuantum
-		prof.stack[1] = 0
+		prof.stack[1] = abi.FuncPCABIInternal(_LostContendedRuntimeLock) + sys.PCQuantum
+		prof.stack[2] = 0
 		return
 	}
 
@@ -702,7 +732,7 @@ func (prof *mLockProfile) captureStack() {
 	systemstack(func() {
 		var u unwinder
 		u.initAt(pc, sp, 0, gp, unwindSilentErrors|unwindJumpStack)
-		nstk = tracebackPCs(&u, skip, prof.stack)
+		nstk = 1 + tracebackPCs(&u, skip, prof.stack[1:])
 	})
 	if nstk < len(prof.stack) {
 		prof.stack[nstk] = 0
@@ -732,6 +762,7 @@ func (prof *mLockProfile) store() {
 	saveBlockEventStack(cycles, rate, prof.stack[:nstk], mutexProfile)
 	if lost > 0 {
 		lostStk := [...]uintptr{
+			logicalStackSentinel,
 			abi.FuncPCABIInternal(_LostContendedRuntimeLock) + sys.PCQuantum,
 		}
 		saveBlockEventStack(lost, rate, lostStk[:], mutexProfile)
@@ -952,8 +983,8 @@ func record(r *MemProfileRecord, b *bucket) {
 	if asanenabled {
 		asanwrite(unsafe.Pointer(&r.Stack0[0]), unsafe.Sizeof(r.Stack0))
 	}
-	copy(r.Stack0[:], b.stk())
-	clear(r.Stack0[b.nstk:])
+	i := copy(r.Stack0[:], b.stk())
+	clear(r.Stack0[i:])
 }
 
 func iterate_memprof(fn func(*bucket, uintptr, *uintptr, uintptr, uintptr, uintptr)) {
@@ -1008,7 +1039,7 @@ func BlockProfile(p []BlockProfileRecord) (n int, ok bool) {
 			if asanenabled {
 				asanwrite(unsafe.Pointer(&r.Stack0[0]), unsafe.Sizeof(r.Stack0))
 			}
-			i := copy(r.Stack0[:], b.stk())
+			i := fpunwindExpand(r.Stack0[:], b.stk())
 			clear(r.Stack0[i:])
 			p = p[1:]
 		}
@@ -1036,7 +1067,7 @@ func MutexProfile(p []BlockProfileRecord) (n int, ok bool) {
 			r := &p[0]
 			r.Count = int64(bp.count)
 			r.Cycles = bp.cycles
-			i := copy(r.Stack0[:], b.stk())
+			i := fpunwindExpand(r.Stack0[:], b.stk())
 			clear(r.Stack0[i:])
 			p = p[1:]
 		}