diff --git a/src/runtime/debuglog.go b/src/runtime/debuglog.go
new file mode 100644
index 00000000000..3f8481270a4
--- /dev/null
+++ b/src/runtime/debuglog.go
@@ -0,0 +1,808 @@
+// Copyright 2019 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.
+
+// This file provides an internal debug logging facility. The debug
+// log is a lightweight, in-memory, per-M ring buffer. By default, the
+// runtime prints the debug log on panic.
+//
+// To print something to the debug log, call dlog to obtain a dlogger
+// and use the methods on that to add values. The values will be
+// space-separated in the output (much like println).
+//
+// This facility can be enabled by passing -tags debuglog when
+// building. Without this tag, dlog calls compile to nothing.
+
+package runtime
+
+import (
+	"runtime/internal/atomic"
+	"unsafe"
+)
+
+// debugLogBytes is the size of each per-M ring buffer. This is
+// allocated off-heap to avoid blowing up the M and hence the GC'd
+// heap size.
+const debugLogBytes = 16 << 10
+
+// debugLogStringLimit the the maximum number of bytes in a string.
+// Above this, the string will be truncated with "..(n more bytes).."
+const debugLogStringLimit = debugLogBytes / 8
+
+// dlog returns a debug logger. The caller can use methods on the
+// returned logger to add values, which will be space-separated in the
+// final output, much like println. The caller must call end() to
+// finish the message.
+//
+// dlog can be used from highly-constrained corners of the runtime: it
+// is safe to use in the signal handler, from within the write
+// barrier, from within the stack implementation, and in places that
+// must be recursively nosplit.
+//
+// This will be compiled away if built without the debuglog build tag.
+// However, argument construction may not be. If any of the arguments
+// are not literals or trivial expressions, consider protecting the
+// call with "if dlogEnabled".
+//
+//go:nosplit
+//go:nowritebarrierrec
+func dlog() *dlogger {
+	if !dlogEnabled {
+		return nil
+	}
+
+	// Get the time.
+	tick, nano := uint64(cputicks()), uint64(nanotime())
+
+	// Try to get a cached logger.
+	l := getCachedDlogger()
+
+	// If we couldn't get a cached logger, try to get one from the
+	// global pool.
+	if l == nil {
+		allp := (*uintptr)(unsafe.Pointer(&allDloggers))
+		all := (*dlogger)(unsafe.Pointer(atomic.Loaduintptr(allp)))
+		for l1 := all; l1 != nil; l1 = l1.allLink {
+			if atomic.Load(&l1.owned) == 0 && atomic.Cas(&l1.owned, 0, 1) {
+				l = l1
+				break
+			}
+		}
+	}
+
+	// If that failed, allocate a new logger.
+	if l == nil {
+		l = (*dlogger)(sysAlloc(unsafe.Sizeof(dlogger{}), nil))
+		if l == nil {
+			throw("failed to allocate debug log")
+		}
+		l.w.r.data = &l.w.data
+		l.owned = 1
+
+		// Prepend to allDloggers list.
+		headp := (*uintptr)(unsafe.Pointer(&allDloggers))
+		for {
+			head := atomic.Loaduintptr(headp)
+			l.allLink = (*dlogger)(unsafe.Pointer(head))
+			if atomic.Casuintptr(headp, head, uintptr(unsafe.Pointer(l))) {
+				break
+			}
+		}
+	}
+
+	// If the time delta is getting too high, write a new sync
+	// packet. We set the limit so we don't write more than 6
+	// bytes of delta in the record header.
+	const deltaLimit = 1<<(3*7) - 1 // ~2ms between sync packets
+	if tick-l.w.tick > deltaLimit || nano-l.w.nano > deltaLimit {
+		l.w.writeSync(tick, nano)
+	}
+
+	// Reserve space for framing header.
+	l.w.ensure(debugLogHeaderSize)
+	l.w.write += debugLogHeaderSize
+
+	// Write record header.
+	l.w.uvarint(tick - l.w.tick)
+	l.w.uvarint(nano - l.w.nano)
+	gp := getg()
+	if gp != nil && gp.m != nil && gp.m.p != 0 {
+		l.w.varint(int64(gp.m.p.ptr().id))
+	} else {
+		l.w.varint(-1)
+	}
+
+	return l
+}
+
+// A dlogger writes to the debug log.
+//
+// To obtain a dlogger, call dlog(). When done with the dlogger, call
+// end().
+//
+//go:notinheap
+type dlogger struct {
+	w debugLogWriter
+
+	// allLink is the next dlogger in the allDloggers list.
+	allLink *dlogger
+
+	// owned indicates that this dlogger is owned by an M. This is
+	// accessed atomically.
+	owned uint32
+}
+
+// allDloggers is a list of all dloggers, linked through
+// dlogger.allLink. This is accessed atomically. This is prepend only,
+// so it doesn't need to protect against ABA races.
+var allDloggers *dlogger
+
+//go:nosplit
+func (l *dlogger) end() {
+	if !dlogEnabled {
+		return
+	}
+
+	// Fill in framing header.
+	size := l.w.write - l.w.r.end
+	if !l.w.writeFrameAt(l.w.r.end, size) {
+		throw("record too large")
+	}
+
+	// Commit the record.
+	l.w.r.end = l.w.write
+
+	// Attempt to return this logger to the cache.
+	if putCachedDlogger(l) {
+		return
+	}
+
+	// Return the logger to the global pool.
+	atomic.Store(&l.owned, 0)
+}
+
+const (
+	debugLogUnknown = 1 + iota
+	debugLogBoolTrue
+	debugLogBoolFalse
+	debugLogInt
+	debugLogUint
+	debugLogHex
+	debugLogPtr
+	debugLogString
+	debugLogConstString
+	debugLogStringOverflow
+
+	debugLogPC
+	debugLogTraceback
+)
+
+//go:nosplit
+func (l *dlogger) b(x bool) *dlogger {
+	if !dlogEnabled {
+		return l
+	}
+	if x {
+		l.w.byte(debugLogBoolTrue)
+	} else {
+		l.w.byte(debugLogBoolFalse)
+	}
+	return l
+}
+
+//go:nosplit
+func (l *dlogger) i(x int) *dlogger {
+	return l.i64(int64(x))
+}
+
+//go:nosplit
+func (l *dlogger) i8(x int8) *dlogger {
+	return l.i64(int64(x))
+}
+
+//go:nosplit
+func (l *dlogger) i16(x int16) *dlogger {
+	return l.i64(int64(x))
+}
+
+//go:nosplit
+func (l *dlogger) i32(x int32) *dlogger {
+	return l.i64(int64(x))
+}
+
+//go:nosplit
+func (l *dlogger) i64(x int64) *dlogger {
+	if !dlogEnabled {
+		return l
+	}
+	l.w.byte(debugLogInt)
+	l.w.varint(x)
+	return l
+}
+
+//go:nosplit
+func (l *dlogger) u(x uint) *dlogger {
+	return l.u64(uint64(x))
+}
+
+//go:nosplit
+func (l *dlogger) uptr(x uintptr) *dlogger {
+	return l.u64(uint64(x))
+}
+
+//go:nosplit
+func (l *dlogger) u8(x uint8) *dlogger {
+	return l.u64(uint64(x))
+}
+
+//go:nosplit
+func (l *dlogger) u16(x uint16) *dlogger {
+	return l.u64(uint64(x))
+}
+
+//go:nosplit
+func (l *dlogger) u32(x uint32) *dlogger {
+	return l.u64(uint64(x))
+}
+
+//go:nosplit
+func (l *dlogger) u64(x uint64) *dlogger {
+	if !dlogEnabled {
+		return l
+	}
+	l.w.byte(debugLogUint)
+	l.w.uvarint(x)
+	return l
+}
+
+//go:nosplit
+func (l *dlogger) hex(x uint64) *dlogger {
+	if !dlogEnabled {
+		return l
+	}
+	l.w.byte(debugLogHex)
+	l.w.uvarint(x)
+	return l
+}
+
+//go:nosplit
+func (l *dlogger) p(x interface{}) *dlogger {
+	if !dlogEnabled {
+		return l
+	}
+	l.w.byte(debugLogPtr)
+	if x == nil {
+		l.w.uvarint(0)
+	} else {
+		v := efaceOf(&x)
+		switch v._type.kind & kindMask {
+		case kindChan, kindFunc, kindMap, kindPtr, kindUnsafePointer:
+			l.w.uvarint(uint64(uintptr(v.data)))
+		default:
+			throw("not a pointer type")
+		}
+	}
+	return l
+}
+
+//go:nosplit
+func (l *dlogger) s(x string) *dlogger {
+	if !dlogEnabled {
+		return l
+	}
+	str := stringStructOf(&x)
+	datap := &firstmoduledata
+	if len(x) > 4 && datap.etext <= uintptr(str.str) && uintptr(str.str) < datap.end {
+		// String constants are in the rodata section, which
+		// isn't recorded in moduledata. But it has to be
+		// somewhere between etext and end.
+		l.w.byte(debugLogConstString)
+		l.w.uvarint(uint64(str.len))
+		l.w.uvarint(uint64(uintptr(str.str) - datap.etext))
+	} else {
+		l.w.byte(debugLogString)
+		var b []byte
+		bb := (*slice)(unsafe.Pointer(&b))
+		bb.array = str.str
+		bb.len, bb.cap = str.len, str.len
+		if len(b) > debugLogStringLimit {
+			b = b[:debugLogStringLimit]
+		}
+		l.w.uvarint(uint64(len(b)))
+		l.w.bytes(b)
+		if len(b) != len(x) {
+			l.w.byte(debugLogStringOverflow)
+			l.w.uvarint(uint64(len(x) - len(b)))
+		}
+	}
+	return l
+}
+
+//go:nosplit
+func (l *dlogger) pc(x uintptr) *dlogger {
+	if !dlogEnabled {
+		return l
+	}
+	l.w.byte(debugLogPC)
+	l.w.uvarint(uint64(x))
+	return l
+}
+
+//go:nosplit
+func (l *dlogger) traceback(x []uintptr) *dlogger {
+	if !dlogEnabled {
+		return l
+	}
+	l.w.byte(debugLogTraceback)
+	l.w.uvarint(uint64(len(x)))
+	for _, pc := range x {
+		l.w.uvarint(uint64(pc))
+	}
+	return l
+}
+
+// A debugLogWriter is a ring buffer of binary debug log records.
+//
+// A log record consists of a 2-byte framing header and a sequence of
+// fields. The framing header gives the size of the record as a little
+// endian 16-bit value. Each field starts with a byte indicating its
+// type, followed by type-specific data. If the size in the framing
+// header is 0, it's a sync record consisting of two little endian
+// 64-bit values giving a new time base.
+//
+// Because this is a ring buffer, new records will eventually
+// overwrite old records. Hence, it maintains a reader that consumes
+// the log as it gets overwritten. That reader state is where an
+// actual log reader would start.
+//
+//go:notinheap
+type debugLogWriter struct {
+	write uint64
+	data  debugLogBuf
+
+	// tick and nano are the time bases from the most recently
+	// written sync record.
+	tick, nano uint64
+
+	// r is a reader that consumes records as they get overwritten
+	// by the writer. It also acts as the initial reader state
+	// when printing the log.
+	r debugLogReader
+
+	// buf is a scratch buffer for encoding. This is here to
+	// reduce stack usage.
+	buf [10]byte
+}
+
+//go:notinheap
+type debugLogBuf [debugLogBytes]byte
+
+const (
+	// debugLogHeaderSize is the number of bytes in the framing
+	// header of every dlog record.
+	debugLogHeaderSize = 2
+
+	// debugLogSyncSize is the number of bytes in a sync record.
+	debugLogSyncSize = debugLogHeaderSize + 2*8
+)
+
+//go:nosplit
+func (l *debugLogWriter) ensure(n uint64) {
+	for l.write+n >= l.r.begin+uint64(len(l.data)) {
+		// Consume record at begin.
+		if l.r.skip() == ^uint64(0) {
+			// Wrapped around within a record.
+			//
+			// TODO(austin): It would be better to just
+			// eat the whole buffer at this point, but we
+			// have to communicate that to the reader
+			// somehow.
+			throw("record wrapped around")
+		}
+	}
+}
+
+//go:nosplit
+func (l *debugLogWriter) writeFrameAt(pos, size uint64) bool {
+	l.data[pos%uint64(len(l.data))] = uint8(size)
+	l.data[(pos+1)%uint64(len(l.data))] = uint8(size >> 8)
+	return size <= 0xFFFF
+}
+
+//go:nosplit
+func (l *debugLogWriter) writeSync(tick, nano uint64) {
+	l.tick, l.nano = tick, nano
+	l.ensure(debugLogHeaderSize)
+	l.writeFrameAt(l.write, 0)
+	l.write += debugLogHeaderSize
+	l.writeUint64LE(tick)
+	l.writeUint64LE(nano)
+	l.r.end = l.write
+}
+
+//go:nosplit
+func (l *debugLogWriter) writeUint64LE(x uint64) {
+	var b [8]byte
+	b[0] = byte(x)
+	b[1] = byte(x >> 8)
+	b[2] = byte(x >> 16)
+	b[3] = byte(x >> 24)
+	b[4] = byte(x >> 32)
+	b[5] = byte(x >> 40)
+	b[6] = byte(x >> 48)
+	b[7] = byte(x >> 56)
+	l.bytes(b[:])
+}
+
+//go:nosplit
+func (l *debugLogWriter) byte(x byte) {
+	l.ensure(1)
+	pos := l.write
+	l.write++
+	l.data[pos%uint64(len(l.data))] = x
+}
+
+//go:nosplit
+func (l *debugLogWriter) bytes(x []byte) {
+	l.ensure(uint64(len(x)))
+	pos := l.write
+	l.write += uint64(len(x))
+	for len(x) > 0 {
+		n := copy(l.data[pos%uint64(len(l.data)):], x)
+		pos += uint64(n)
+		x = x[n:]
+	}
+}
+
+//go:nosplit
+func (l *debugLogWriter) varint(x int64) {
+	var u uint64
+	if x < 0 {
+		u = (^uint64(x) << 1) | 1 // complement i, bit 0 is 1
+	} else {
+		u = (uint64(x) << 1) // do not complement i, bit 0 is 0
+	}
+	l.uvarint(u)
+}
+
+//go:nosplit
+func (l *debugLogWriter) uvarint(u uint64) {
+	i := 0
+	for u >= 0x80 {
+		l.buf[i] = byte(u) | 0x80
+		u >>= 7
+		i++
+	}
+	l.buf[i] = byte(u)
+	i++
+	l.bytes(l.buf[:i])
+}
+
+type debugLogReader struct {
+	data *debugLogBuf
+
+	// begin and end are the positions in the log of the beginning
+	// and end of the log data, modulo len(data).
+	begin, end uint64
+
+	// tick and nano are the current time base at begin.
+	tick, nano uint64
+}
+
+//go:nosplit
+func (r *debugLogReader) skip() uint64 {
+	// Read size at pos.
+	if r.begin+debugLogHeaderSize > r.end {
+		return ^uint64(0)
+	}
+	size := uint64(r.readUint16LEAt(r.begin))
+	if size == 0 {
+		// Sync packet.
+		r.tick = r.readUint64LEAt(r.begin + debugLogHeaderSize)
+		r.nano = r.readUint64LEAt(r.begin + debugLogHeaderSize + 8)
+		size = debugLogSyncSize
+	}
+	if r.begin+size > r.end {
+		return ^uint64(0)
+	}
+	r.begin += size
+	return size
+}
+
+//go:nosplit
+func (r *debugLogReader) readUint16LEAt(pos uint64) uint16 {
+	return uint16(r.data[pos%uint64(len(r.data))]) |
+		uint16(r.data[(pos+1)%uint64(len(r.data))])<<8
+}
+
+//go:nosplit
+func (r *debugLogReader) readUint64LEAt(pos uint64) uint64 {
+	var b [8]byte
+	for i := range b {
+		b[i] = r.data[pos%uint64(len(r.data))]
+		pos++
+	}
+	return uint64(b[0]) | uint64(b[1])<<8 |
+		uint64(b[2])<<16 | uint64(b[3])<<24 |
+		uint64(b[4])<<32 | uint64(b[5])<<40 |
+		uint64(b[6])<<48 | uint64(b[7])<<56
+}
+
+func (r *debugLogReader) peek() (tick uint64) {
+	// Consume any sync records.
+	size := uint64(0)
+	for size == 0 {
+		if r.begin+debugLogHeaderSize > r.end {
+			return ^uint64(0)
+		}
+		size = uint64(r.readUint16LEAt(r.begin))
+		if size != 0 {
+			break
+		}
+		if r.begin+debugLogSyncSize > r.end {
+			return ^uint64(0)
+		}
+		// Sync packet.
+		r.tick = r.readUint64LEAt(r.begin + debugLogHeaderSize)
+		r.nano = r.readUint64LEAt(r.begin + debugLogHeaderSize + 8)
+		r.begin += debugLogSyncSize
+	}
+
+	// Peek tick delta.
+	if r.begin+size > r.end {
+		return ^uint64(0)
+	}
+	pos := r.begin + debugLogHeaderSize
+	var u uint64
+	for i := uint(0); ; i += 7 {
+		b := r.data[pos%uint64(len(r.data))]
+		pos++
+		u |= uint64(b&^0x80) << i
+		if b&0x80 == 0 {
+			break
+		}
+	}
+	if pos > r.begin+size {
+		return ^uint64(0)
+	}
+	return r.tick + u
+}
+
+func (r *debugLogReader) header() (end, tick, nano uint64, p int) {
+	// Read size. We've already skipped sync packets and checked
+	// bounds in peek.
+	size := uint64(r.readUint16LEAt(r.begin))
+	end = r.begin + size
+	r.begin += debugLogHeaderSize
+
+	// Read tick, nano, and p.
+	tick = r.uvarint() + r.tick
+	nano = r.uvarint() + r.nano
+	p = int(r.varint())
+
+	return
+}
+
+func (r *debugLogReader) uvarint() uint64 {
+	var u uint64
+	for i := uint(0); ; i += 7 {
+		b := r.data[r.begin%uint64(len(r.data))]
+		r.begin++
+		u |= uint64(b&^0x80) << i
+		if b&0x80 == 0 {
+			break
+		}
+	}
+	return u
+}
+
+func (r *debugLogReader) varint() int64 {
+	u := r.uvarint()
+	var v int64
+	if u&1 == 0 {
+		v = int64(u >> 1)
+	} else {
+		v = ^int64(u >> 1)
+	}
+	return v
+}
+
+func (r *debugLogReader) printVal() bool {
+	typ := r.data[r.begin%uint64(len(r.data))]
+	r.begin++
+
+	switch typ {
+	default:
+		print("<unknown field type ", hex(typ), " pos ", r.begin-1, " end ", r.end, ">\n")
+		return false
+
+	case debugLogUnknown:
+		print("<unknown kind>")
+
+	case debugLogBoolTrue:
+		print(true)
+
+	case debugLogBoolFalse:
+		print(false)
+
+	case debugLogInt:
+		print(r.varint())
+
+	case debugLogUint:
+		print(r.uvarint())
+
+	case debugLogHex, debugLogPtr:
+		print(hex(r.uvarint()))
+
+	case debugLogString:
+		sl := r.uvarint()
+		if r.begin+sl > r.end {
+			r.begin = r.end
+			print("<string length corrupted>")
+			break
+		}
+		for sl > 0 {
+			b := r.data[r.begin%uint64(len(r.data)):]
+			if uint64(len(b)) > sl {
+				b = b[:sl]
+			}
+			r.begin += uint64(len(b))
+			sl -= uint64(len(b))
+			gwrite(b)
+		}
+
+	case debugLogConstString:
+		len, ptr := int(r.uvarint()), uintptr(r.uvarint())
+		ptr += firstmoduledata.etext
+		str := stringStruct{
+			str: unsafe.Pointer(ptr),
+			len: len,
+		}
+		s := *(*string)(unsafe.Pointer(&str))
+		print(s)
+
+	case debugLogStringOverflow:
+		print("..(", r.uvarint(), " more bytes)..")
+
+	case debugLogPC:
+		printDebugLogPC(uintptr(r.uvarint()))
+
+	case debugLogTraceback:
+		n := int(r.uvarint())
+		for i := 0; i < n; i++ {
+			print("\n\t")
+			printDebugLogPC(uintptr(r.uvarint()))
+		}
+	}
+
+	return true
+}
+
+// printDebugLog prints the debug log.
+func printDebugLog() {
+	if !dlogEnabled {
+		return
+	}
+
+	// This function should not panic or throw since it is used in
+	// the fatal panic path and this may deadlock.
+
+	printlock()
+
+	// Get the list of all debug logs.
+	allp := (*uintptr)(unsafe.Pointer(&allDloggers))
+	all := (*dlogger)(unsafe.Pointer(atomic.Loaduintptr(allp)))
+
+	// Count the logs.
+	n := 0
+	for l := all; l != nil; l = l.allLink {
+		n++
+	}
+	if n == 0 {
+		printunlock()
+		return
+	}
+
+	// Prepare read state for all logs.
+	type readState struct {
+		debugLogReader
+		first    bool
+		lost     uint64
+		nextTick uint64
+	}
+	state1 := sysAlloc(unsafe.Sizeof(readState{})*uintptr(n), nil)
+	if state1 == nil {
+		println("failed to allocate read state for", n, "logs")
+		printunlock()
+		return
+	}
+	state := (*[1 << 20]readState)(state1)[:n]
+	{
+		l := all
+		for i := range state {
+			s := &state[i]
+			s.debugLogReader = l.w.r
+			s.first = true
+			s.lost = l.w.r.begin
+			s.nextTick = s.peek()
+			l = l.allLink
+		}
+	}
+
+	// Print records.
+	for {
+		// Find the next record.
+		var best struct {
+			tick uint64
+			i    int
+		}
+		best.tick = ^uint64(0)
+		for i := range state {
+			if state[i].nextTick < best.tick {
+				best.tick = state[i].nextTick
+				best.i = i
+			}
+		}
+		if best.tick == ^uint64(0) {
+			break
+		}
+
+		// Print record.
+		s := &state[best.i]
+		if s.first {
+			print(">> begin log ", best.i)
+			if s.lost != 0 {
+				print("; lost first ", s.lost>>10, "KB")
+			}
+			print(" <<\n")
+			s.first = false
+		}
+
+		end, _, nano, p := s.header()
+		oldEnd := s.end
+		s.end = end
+
+		print("[")
+		var tmpbuf [21]byte
+		pnano := int64(nano) - runtimeInitTime
+		if pnano < 0 {
+			// Logged before runtimeInitTime was set.
+			pnano = 0
+		}
+		print(string(itoaDiv(tmpbuf[:], uint64(pnano), 9)))
+		print(" P ", p, "] ")
+
+		for i := 0; s.begin < s.end; i++ {
+			if i > 0 {
+				print(" ")
+			}
+			if !s.printVal() {
+				// Abort this P log.
+				print("<aborting P log>")
+				end = oldEnd
+				break
+			}
+		}
+		println()
+
+		// Move on to the next record.
+		s.begin = end
+		s.end = oldEnd
+		s.nextTick = s.peek()
+	}
+
+	printunlock()
+}
+
+func printDebugLogPC(pc uintptr) {
+	print(hex(pc))
+	fn := findfunc(pc)
+	if !fn.valid() {
+		print(" [unknown PC]")
+	} else {
+		name := funcname(fn)
+		file, line := funcline(fn, pc)
+		print(" [", name, "+", hex(pc-fn.entry),
+			" ", file, ":", line, "]")
+	}
+}
diff --git a/src/runtime/debuglog_off.go b/src/runtime/debuglog_off.go
new file mode 100644
index 00000000000..bb3e172498e
--- /dev/null
+++ b/src/runtime/debuglog_off.go
@@ -0,0 +1,19 @@
+// Copyright 2019 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.
+
+// +build !debuglog
+
+package runtime
+
+const dlogEnabled = false
+
+type dlogPerM struct{}
+
+func getCachedDlogger() *dlogger {
+	return nil
+}
+
+func putCachedDlogger(l *dlogger) bool {
+	return false
+}
diff --git a/src/runtime/debuglog_on.go b/src/runtime/debuglog_on.go
new file mode 100644
index 00000000000..3d477e8ef5f
--- /dev/null
+++ b/src/runtime/debuglog_on.go
@@ -0,0 +1,45 @@
+// Copyright 2019 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.
+
+// +build debuglog
+
+package runtime
+
+const dlogEnabled = true
+
+// dlogPerM is the per-M debug log data. This is embedded in the m
+// struct.
+type dlogPerM struct {
+	dlogCache *dlogger
+}
+
+// getCachedDlogger returns a cached dlogger if it can do so
+// efficiently, or nil otherwise. The returned dlogger will be owned.
+func getCachedDlogger() *dlogger {
+	mp := acquirem()
+	// We don't return a cached dlogger if we're running on the
+	// signal stack in case the signal arrived while in
+	// get/putCachedDlogger. (Too bad we don't have non-atomic
+	// exchange!)
+	var l *dlogger
+	if getg() != mp.gsignal {
+		l = mp.dlogCache
+		mp.dlogCache = nil
+	}
+	releasem(mp)
+	return l
+}
+
+// putCachedDlogger attempts to return l to the local cache. It
+// returns false if this fails.
+func putCachedDlogger(l *dlogger) bool {
+	mp := acquirem()
+	if getg() != mp.gsignal && mp.dlogCache == nil {
+		mp.dlogCache = l
+		releasem(mp)
+		return true
+	}
+	releasem(mp)
+	return false
+}
diff --git a/src/runtime/debuglog_test.go b/src/runtime/debuglog_test.go
new file mode 100644
index 00000000000..2570e3565bf
--- /dev/null
+++ b/src/runtime/debuglog_test.go
@@ -0,0 +1,158 @@
+// Copyright 2019 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.
+
+// TODO(austin): All of these tests are skipped if the debuglog build
+// tag isn't provided. That means we basically never test debuglog.
+// There are two potential ways around this:
+//
+// 1. Make these tests re-build the runtime test with the debuglog
+// build tag and re-invoke themselves.
+//
+// 2. Always build the whole debuglog infrastructure and depend on
+// linker dead-code elimination to drop it. This is easy for dlog()
+// since there won't be any calls to it. For printDebugLog, we can
+// make panic call a wrapper that is call printDebugLog if the
+// debuglog build tag is set, or otherwise do nothing. Then tests
+// could call printDebugLog directly. This is the right answer in
+// principle, but currently our linker reads in all symbols
+// regardless, so this would slow down and bloat all links. If the
+// linker gets more efficient about this, we should revisit this
+// approach.
+
+package runtime_test
+
+import (
+	"bytes"
+	"fmt"
+	"regexp"
+	"runtime"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"testing"
+)
+
+func skipDebugLog(t *testing.T) {
+	if !runtime.DlogEnabled {
+		t.Skip("debug log disabled (rebuild with -tags debuglog)")
+	}
+}
+
+func dlogCanonicalize(x string) string {
+	begin := regexp.MustCompile(`(?m)^>> begin log \d+ <<\n`)
+	x = begin.ReplaceAllString(x, "")
+	prefix := regexp.MustCompile(`(?m)^\[[^]]+\]`)
+	x = prefix.ReplaceAllString(x, "[]")
+	return x
+}
+
+func TestDebugLog(t *testing.T) {
+	skipDebugLog(t)
+	runtime.ResetDebugLog()
+	runtime.Dlog().S("testing").End()
+	got := dlogCanonicalize(runtime.DumpDebugLog())
+	if want := "[] testing\n"; got != want {
+		t.Fatalf("want %q, got %q", want, got)
+	}
+}
+
+func TestDebugLogTypes(t *testing.T) {
+	skipDebugLog(t)
+	runtime.ResetDebugLog()
+	var varString = strings.Repeat("a", 4)
+	runtime.Dlog().B(true).B(false).I(-42).I16(0x7fff).U64(^uint64(0)).Hex(0xfff).P(nil).S(varString).S("const string").End()
+	got := dlogCanonicalize(runtime.DumpDebugLog())
+	if want := "[] true false -42 32767 18446744073709551615 0xfff 0x0 aaaa const string\n"; got != want {
+		t.Fatalf("want %q, got %q", want, got)
+	}
+}
+
+func TestDebugLogSym(t *testing.T) {
+	skipDebugLog(t)
+	runtime.ResetDebugLog()
+	pc, _, _, _ := runtime.Caller(0)
+	runtime.Dlog().PC(pc).End()
+	got := dlogCanonicalize(runtime.DumpDebugLog())
+	want := regexp.MustCompile(`\[\] 0x[0-9a-f]+ \[runtime_test\.TestDebugLogSym\+0x[0-9a-f]+ .*/debuglog_test\.go:[0-9]+\]\n`)
+	if !want.MatchString(got) {
+		t.Fatalf("want matching %s, got %q", want, got)
+	}
+}
+
+func TestDebugLogInterleaving(t *testing.T) {
+	skipDebugLog(t)
+	runtime.ResetDebugLog()
+	var wg sync.WaitGroup
+	done := int32(0)
+	wg.Add(1)
+	go func() {
+		// Encourage main goroutine to move around to
+		// different Ms and Ps.
+		for atomic.LoadInt32(&done) == 0 {
+			runtime.Gosched()
+		}
+		wg.Done()
+	}()
+	var want bytes.Buffer
+	for i := 0; i < 1000; i++ {
+		runtime.Dlog().I(i).End()
+		fmt.Fprintf(&want, "[] %d\n", i)
+		runtime.Gosched()
+	}
+	atomic.StoreInt32(&done, 1)
+	wg.Wait()
+
+	gotFull := runtime.DumpDebugLog()
+	got := dlogCanonicalize(gotFull)
+	if got != want.String() {
+		// Since the timestamps are useful in understand
+		// failures of this test, we print the uncanonicalized
+		// output.
+		t.Fatalf("want %q, got (uncanonicalized) %q", want.String(), gotFull)
+	}
+}
+
+func TestDebugLogWraparound(t *testing.T) {
+	skipDebugLog(t)
+
+	// Make sure we don't switch logs so it's easier to fill one up.
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	runtime.ResetDebugLog()
+	var longString = strings.Repeat("a", 128)
+	var want bytes.Buffer
+	for i, j := 0, 0; j < 2*runtime.DebugLogBytes; i, j = i+1, j+len(longString) {
+		runtime.Dlog().I(i).S(longString).End()
+		fmt.Fprintf(&want, "[] %d %s\n", i, longString)
+	}
+	log := runtime.DumpDebugLog()
+
+	// Check for "lost" message.
+	lost := regexp.MustCompile(`^>> begin log \d+; lost first \d+KB <<\n`)
+	if !lost.MatchString(log) {
+		t.Fatalf("want matching %s, got %q", lost, log)
+	}
+	idx := lost.FindStringIndex(log)
+	// Strip lost message.
+	log = dlogCanonicalize(log[idx[1]:])
+
+	// Check log.
+	if !strings.HasSuffix(want.String(), log) {
+		t.Fatalf("wrong suffix:\n%s", log)
+	}
+}
+
+func TestDebugLogLongString(t *testing.T) {
+	skipDebugLog(t)
+
+	runtime.ResetDebugLog()
+	var longString = strings.Repeat("a", runtime.DebugLogStringLimit+1)
+	runtime.Dlog().S(longString).End()
+	got := dlogCanonicalize(runtime.DumpDebugLog())
+	want := "[] " + strings.Repeat("a", runtime.DebugLogStringLimit) + " ..(1 more bytes)..\n"
+	if got != want {
+		t.Fatalf("want %q, got %q", want, got)
+	}
+}
diff --git a/src/runtime/export_debuglog_test.go b/src/runtime/export_debuglog_test.go
new file mode 100644
index 00000000000..8cd943b4382
--- /dev/null
+++ b/src/runtime/export_debuglog_test.go
@@ -0,0 +1,46 @@
+// Copyright 2019 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.
+
+// Export debuglog guts for testing.
+
+package runtime
+
+const DlogEnabled = dlogEnabled
+
+const DebugLogBytes = debugLogBytes
+
+const DebugLogStringLimit = debugLogStringLimit
+
+var Dlog = dlog
+
+func (l *dlogger) End()                     { l.end() }
+func (l *dlogger) B(x bool) *dlogger        { return l.b(x) }
+func (l *dlogger) I(x int) *dlogger         { return l.i(x) }
+func (l *dlogger) I16(x int16) *dlogger     { return l.i16(x) }
+func (l *dlogger) U64(x uint64) *dlogger    { return l.u64(x) }
+func (l *dlogger) Hex(x uint64) *dlogger    { return l.hex(x) }
+func (l *dlogger) P(x interface{}) *dlogger { return l.p(x) }
+func (l *dlogger) S(x string) *dlogger      { return l.s(x) }
+func (l *dlogger) PC(x uintptr) *dlogger    { return l.pc(x) }
+
+func DumpDebugLog() string {
+	g := getg()
+	g.writebuf = make([]byte, 0, 1<<20)
+	printDebugLog()
+	buf := g.writebuf
+	g.writebuf = nil
+
+	return string(buf)
+}
+
+func ResetDebugLog() {
+	stopTheWorld("ResetDebugLog")
+	for l := allDloggers; l != nil; l = l.allLink {
+		l.w.write = 0
+		l.w.tick, l.w.nano = 0, 0
+		l.w.r.begin, l.w.r.end = 0, 0
+		l.w.r.tick, l.w.r.nano = 0, 0
+	}
+	startTheWorld()
+}
diff --git a/src/runtime/panic.go b/src/runtime/panic.go
index 543fd23c01c..f39a4bc0a25 100644
--- a/src/runtime/panic.go
+++ b/src/runtime/panic.go
@@ -925,6 +925,8 @@ func dopanic_m(gp *g, pc, sp uintptr) bool {
 		lock(&deadlock)
 	}
 
+	printDebugLog()
+
 	return docrash
 }
 
diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index 0dd2e929a07..cfea1cd45fe 100644
--- a/src/runtime/runtime2.go
+++ b/src/runtime/runtime2.go
@@ -468,6 +468,8 @@ type m struct {
 	vdsoSP uintptr // SP for traceback while in VDSO call (0 if not in call)
 	vdsoPC uintptr // PC for traceback while in VDSO call
 
+	dlogPerM
+
 	mOS
 }
 
diff --git a/src/runtime/signal_sighandler.go b/src/runtime/signal_sighandler.go
index 6e71e41f521..bec46532188 100644
--- a/src/runtime/signal_sighandler.go
+++ b/src/runtime/signal_sighandler.go
@@ -148,5 +148,7 @@ func sighandler(sig uint32, info *siginfo, ctxt unsafe.Pointer, gp *g) {
 		crash()
 	}
 
+	printDebugLog()
+
 	exit(2)
 }