debuglog.go

Documentation: runtime

     1  // Copyright 2019 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  // This file provides an internal debug logging facility. The debug
     6  // log is a lightweight, in-memory, per-M ring buffer. By default, the
     7  // runtime prints the debug log on panic.
     8  //
     9  // To print something to the debug log, call dlog to obtain a dlogger
    10  // and use the methods on that to add values. The values will be
    11  // space-separated in the output (much like println).
    12  //
    13  // This facility can be enabled by passing -tags debuglog when
    14  // building. Without this tag, dlog calls compile to nothing.
    15  
    16  package runtime
    17  
    18  import (
    19  	"runtime/internal/atomic"
    20  	"unsafe"
    21  )
    22  
    23  // debugLogBytes is the size of each per-M ring buffer. This is
    24  // allocated off-heap to avoid blowing up the M and hence the GC'd
    25  // heap size.
    26  const debugLogBytes = 16 << 10
    27  
    28  // debugLogStringLimit is the maximum number of bytes in a string.
    29  // Above this, the string will be truncated with "..(n more bytes).."
    30  const debugLogStringLimit = debugLogBytes / 8
    31  
    32  // dlog returns a debug logger. The caller can use methods on the
    33  // returned logger to add values, which will be space-separated in the
    34  // final output, much like println. The caller must call end() to
    35  // finish the message.
    36  //
    37  // dlog can be used from highly-constrained corners of the runtime: it
    38  // is safe to use in the signal handler, from within the write
    39  // barrier, from within the stack implementation, and in places that
    40  // must be recursively nosplit.
    41  //
    42  // This will be compiled away if built without the debuglog build tag.
    43  // However, argument construction may not be. If any of the arguments
    44  // are not literals or trivial expressions, consider protecting the
    45  // call with "if dlogEnabled".
    46  //
    47  //go:nosplit
    48  //go:nowritebarrierrec
    49  func dlog() *dlogger {
    50  	if !dlogEnabled {
    51  		return nil
    52  	}
    53  
    54  	// Get the time.
    55  	tick, nano := uint64(cputicks()), uint64(nanotime())
    56  
    57  	// Try to get a cached logger.
    58  	l := getCachedDlogger()
    59  
    60  	// If we couldn't get a cached logger, try to get one from the
    61  	// global pool.
    62  	if l == nil {
    63  		allp := (*uintptr)(unsafe.Pointer(&allDloggers))
    64  		all := (*dlogger)(unsafe.Pointer(atomic.Loaduintptr(allp)))
    65  		for l1 := all; l1 != nil; l1 = l1.allLink {
    66  			if atomic.Load(&l1.owned) == 0 && atomic.Cas(&l1.owned, 0, 1) {
    67  				l = l1
    68  				break
    69  			}
    70  		}
    71  	}
    72  
    73  	// If that failed, allocate a new logger.
    74  	if l == nil {
    75  		// Use sysAllocOS instead of sysAlloc because we want to interfere
    76  		// with the runtime as little as possible, and sysAlloc updates accounting.
    77  		l = (*dlogger)(sysAllocOS(unsafe.Sizeof(dlogger{})))
    78  		if l == nil {
    79  			throw("failed to allocate debug log")
    80  		}
    81  		l.w.r.data = &l.w.data
    82  		l.owned = 1
    83  
    84  		// Prepend to allDloggers list.
    85  		headp := (*uintptr)(unsafe.Pointer(&allDloggers))
    86  		for {
    87  			head := atomic.Loaduintptr(headp)
    88  			l.allLink = (*dlogger)(unsafe.Pointer(head))
    89  			if atomic.Casuintptr(headp, head, uintptr(unsafe.Pointer(l))) {
    90  				break
    91  			}
    92  		}
    93  	}
    94  
    95  	// If the time delta is getting too high, write a new sync
    96  	// packet. We set the limit so we don't write more than 6
    97  	// bytes of delta in the record header.
    98  	const deltaLimit = 1<<(3*7) - 1 // ~2ms between sync packets
    99  	if tick-l.w.tick > deltaLimit || nano-l.w.nano > deltaLimit {
   100  		l.w.writeSync(tick, nano)
   101  	}
   102  
   103  	// Reserve space for framing header.
   104  	l.w.ensure(debugLogHeaderSize)
   105  	l.w.write += debugLogHeaderSize
   106  
   107  	// Write record header.
   108  	l.w.uvarint(tick - l.w.tick)
   109  	l.w.uvarint(nano - l.w.nano)
   110  	gp := getg()
   111  	if gp != nil && gp.m != nil && gp.m.p != 0 {
   112  		l.w.varint(int64(gp.m.p.ptr().id))
   113  	} else {
   114  		l.w.varint(-1)
   115  	}
   116  
   117  	return l
   118  }
   119  
   120  // A dlogger writes to the debug log.
   121  //
   122  // To obtain a dlogger, call dlog(). When done with the dlogger, call
   123  // end().
   124  //
   125  //go:notinheap
   126  type dlogger struct {
   127  	w debugLogWriter
   128  
   129  	// allLink is the next dlogger in the allDloggers list.
   130  	allLink *dlogger
   131  
   132  	// owned indicates that this dlogger is owned by an M. This is
   133  	// accessed atomically.
   134  	owned uint32
   135  }
   136  
   137  // allDloggers is a list of all dloggers, linked through
   138  // dlogger.allLink. This is accessed atomically. This is prepend only,
   139  // so it doesn't need to protect against ABA races.
   140  var allDloggers *dlogger
   141  
   142  //go:nosplit
   143  func (l *dlogger) end() {
   144  	if !dlogEnabled {
   145  		return
   146  	}
   147  
   148  	// Fill in framing header.
   149  	size := l.w.write - l.w.r.end
   150  	if !l.w.writeFrameAt(l.w.r.end, size) {
   151  		throw("record too large")
   152  	}
   153  
   154  	// Commit the record.
   155  	l.w.r.end = l.w.write
   156  
   157  	// Attempt to return this logger to the cache.
   158  	if putCachedDlogger(l) {
   159  		return
   160  	}
   161  
   162  	// Return the logger to the global pool.
   163  	atomic.Store(&l.owned, 0)
   164  }
   165  
   166  const (
   167  	debugLogUnknown = 1 + iota
   168  	debugLogBoolTrue
   169  	debugLogBoolFalse
   170  	debugLogInt
   171  	debugLogUint
   172  	debugLogHex
   173  	debugLogPtr
   174  	debugLogString
   175  	debugLogConstString
   176  	debugLogStringOverflow
   177  
   178  	debugLogPC
   179  	debugLogTraceback
   180  )
   181  
   182  //go:nosplit
   183  func (l *dlogger) b(x bool) *dlogger {
   184  	if !dlogEnabled {
   185  		return l
   186  	}
   187  	if x {
   188  		l.w.byte(debugLogBoolTrue)
   189  	} else {
   190  		l.w.byte(debugLogBoolFalse)
   191  	}
   192  	return l
   193  }
   194  
   195  //go:nosplit
   196  func (l *dlogger) i(x int) *dlogger {
   197  	return l.i64(int64(x))
   198  }
   199  
   200  //go:nosplit
   201  func (l *dlogger) i8(x int8) *dlogger {
   202  	return l.i64(int64(x))
   203  }
   204  
   205  //go:nosplit
   206  func (l *dlogger) i16(x int16) *dlogger {
   207  	return l.i64(int64(x))
   208  }
   209  
   210  //go:nosplit
   211  func (l *dlogger) i32(x int32) *dlogger {
   212  	return l.i64(int64(x))
   213  }
   214  
   215  //go:nosplit
   216  func (l *dlogger) i64(x int64) *dlogger {
   217  	if !dlogEnabled {
   218  		return l
   219  	}
   220  	l.w.byte(debugLogInt)
   221  	l.w.varint(x)
   222  	return l
   223  }
   224  
   225  //go:nosplit
   226  func (l *dlogger) u(x uint) *dlogger {
   227  	return l.u64(uint64(x))
   228  }
   229  
   230  //go:nosplit
   231  func (l *dlogger) uptr(x uintptr) *dlogger {
   232  	return l.u64(uint64(x))
   233  }
   234  
   235  //go:nosplit
   236  func (l *dlogger) u8(x uint8) *dlogger {
   237  	return l.u64(uint64(x))
   238  }
   239  
   240  //go:nosplit
   241  func (l *dlogger) u16(x uint16) *dlogger {
   242  	return l.u64(uint64(x))
   243  }
   244  
   245  //go:nosplit
   246  func (l *dlogger) u32(x uint32) *dlogger {
   247  	return l.u64(uint64(x))
   248  }
   249  
   250  //go:nosplit
   251  func (l *dlogger) u64(x uint64) *dlogger {
   252  	if !dlogEnabled {
   253  		return l
   254  	}
   255  	l.w.byte(debugLogUint)
   256  	l.w.uvarint(x)
   257  	return l
   258  }
   259  
   260  //go:nosplit
   261  func (l *dlogger) hex(x uint64) *dlogger {
   262  	if !dlogEnabled {
   263  		return l
   264  	}
   265  	l.w.byte(debugLogHex)
   266  	l.w.uvarint(x)
   267  	return l
   268  }
   269  
   270  //go:nosplit
   271  func (l *dlogger) p(x any) *dlogger {
   272  	if !dlogEnabled {
   273  		return l
   274  	}
   275  	l.w.byte(debugLogPtr)
   276  	if x == nil {
   277  		l.w.uvarint(0)
   278  	} else {
   279  		v := efaceOf(&x)
   280  		switch v._type.kind & kindMask {
   281  		case kindChan, kindFunc, kindMap, kindPtr, kindUnsafePointer:
   282  			l.w.uvarint(uint64(uintptr(v.data)))
   283  		default:
   284  			throw("not a pointer type")
   285  		}
   286  	}
   287  	return l
   288  }
   289  
   290  //go:nosplit
   291  func (l *dlogger) s(x string) *dlogger {
   292  	if !dlogEnabled {
   293  		return l
   294  	}
   295  	str := stringStructOf(&x)
   296  	datap := &firstmoduledata
   297  	if len(x) > 4 && datap.etext <= uintptr(str.str) && uintptr(str.str) < datap.end {
   298  		// String constants are in the rodata section, which
   299  		// isn't recorded in moduledata. But it has to be
   300  		// somewhere between etext and end.
   301  		l.w.byte(debugLogConstString)
   302  		l.w.uvarint(uint64(str.len))
   303  		l.w.uvarint(uint64(uintptr(str.str) - datap.etext))
   304  	} else {
   305  		l.w.byte(debugLogString)
   306  		var b []byte
   307  		bb := (*slice)(unsafe.Pointer(&b))
   308  		bb.array = str.str
   309  		bb.len, bb.cap = str.len, str.len
   310  		if len(b) > debugLogStringLimit {
   311  			b = b[:debugLogStringLimit]
   312  		}
   313  		l.w.uvarint(uint64(len(b)))
   314  		l.w.bytes(b)
   315  		if len(b) != len(x) {
   316  			l.w.byte(debugLogStringOverflow)
   317  			l.w.uvarint(uint64(len(x) - len(b)))
   318  		}
   319  	}
   320  	return l
   321  }
   322  
   323  //go:nosplit
   324  func (l *dlogger) pc(x uintptr) *dlogger {
   325  	if !dlogEnabled {
   326  		return l
   327  	}
   328  	l.w.byte(debugLogPC)
   329  	l.w.uvarint(uint64(x))
   330  	return l
   331  }
   332  
   333  //go:nosplit
   334  func (l *dlogger) traceback(x []uintptr) *dlogger {
   335  	if !dlogEnabled {
   336  		return l
   337  	}
   338  	l.w.byte(debugLogTraceback)
   339  	l.w.uvarint(uint64(len(x)))
   340  	for _, pc := range x {
   341  		l.w.uvarint(uint64(pc))
   342  	}
   343  	return l
   344  }
   345  
   346  // A debugLogWriter is a ring buffer of binary debug log records.
   347  //
   348  // A log record consists of a 2-byte framing header and a sequence of
   349  // fields. The framing header gives the size of the record as a little
   350  // endian 16-bit value. Each field starts with a byte indicating its
   351  // type, followed by type-specific data. If the size in the framing
   352  // header is 0, it's a sync record consisting of two little endian
   353  // 64-bit values giving a new time base.
   354  //
   355  // Because this is a ring buffer, new records will eventually
   356  // overwrite old records. Hence, it maintains a reader that consumes
   357  // the log as it gets overwritten. That reader state is where an
   358  // actual log reader would start.
   359  //
   360  //go:notinheap
   361  type debugLogWriter struct {
   362  	write uint64
   363  	data  debugLogBuf
   364  
   365  	// tick and nano are the time bases from the most recently
   366  	// written sync record.
   367  	tick, nano uint64
   368  
   369  	// r is a reader that consumes records as they get overwritten
   370  	// by the writer. It also acts as the initial reader state
   371  	// when printing the log.
   372  	r debugLogReader
   373  
   374  	// buf is a scratch buffer for encoding. This is here to
   375  	// reduce stack usage.
   376  	buf [10]byte
   377  }
   378  
   379  //go:notinheap
   380  type debugLogBuf [debugLogBytes]byte
   381  
   382  const (
   383  	// debugLogHeaderSize is the number of bytes in the framing
   384  	// header of every dlog record.
   385  	debugLogHeaderSize = 2
   386  
   387  	// debugLogSyncSize is the number of bytes in a sync record.
   388  	debugLogSyncSize = debugLogHeaderSize + 2*8
   389  )
   390  
   391  //go:nosplit
   392  func (l *debugLogWriter) ensure(n uint64) {
   393  	for l.write+n >= l.r.begin+uint64(len(l.data)) {
   394  		// Consume record at begin.
   395  		if l.r.skip() == ^uint64(0) {
   396  			// Wrapped around within a record.
   397  			//
   398  			// TODO(austin): It would be better to just
   399  			// eat the whole buffer at this point, but we
   400  			// have to communicate that to the reader
   401  			// somehow.
   402  			throw("record wrapped around")
   403  		}
   404  	}
   405  }
   406  
   407  //go:nosplit
   408  func (l *debugLogWriter) writeFrameAt(pos, size uint64) bool {
   409  	l.data[pos%uint64(len(l.data))] = uint8(size)
   410  	l.data[(pos+1)%uint64(len(l.data))] = uint8(size >> 8)
   411  	return size <= 0xFFFF
   412  }
   413  
   414  //go:nosplit
   415  func (l *debugLogWriter) writeSync(tick, nano uint64) {
   416  	l.tick, l.nano = tick, nano
   417  	l.ensure(debugLogHeaderSize)
   418  	l.writeFrameAt(l.write, 0)
   419  	l.write += debugLogHeaderSize
   420  	l.writeUint64LE(tick)
   421  	l.writeUint64LE(nano)
   422  	l.r.end = l.write
   423  }
   424  
   425  //go:nosplit
   426  func (l *debugLogWriter) writeUint64LE(x uint64) {
   427  	var b [8]byte
   428  	b[0] = byte(x)
   429  	b[1] = byte(x >> 8)
   430  	b[2] = byte(x >> 16)
   431  	b[3] = byte(x >> 24)
   432  	b[4] = byte(x >> 32)
   433  	b[5] = byte(x >> 40)
   434  	b[6] = byte(x >> 48)
   435  	b[7] = byte(x >> 56)
   436  	l.bytes(b[:])
   437  }
   438  
   439  //go:nosplit
   440  func (l *debugLogWriter) byte(x byte) {
   441  	l.ensure(1)
   442  	pos := l.write
   443  	l.write++
   444  	l.data[pos%uint64(len(l.data))] = x
   445  }
   446  
   447  //go:nosplit
   448  func (l *debugLogWriter) bytes(x []byte) {
   449  	l.ensure(uint64(len(x)))
   450  	pos := l.write
   451  	l.write += uint64(len(x))
   452  	for len(x) > 0 {
   453  		n := copy(l.data[pos%uint64(len(l.data)):], x)
   454  		pos += uint64(n)
   455  		x = x[n:]
   456  	}
   457  }
   458  
   459  //go:nosplit
   460  func (l *debugLogWriter) varint(x int64) {
   461  	var u uint64
   462  	if x < 0 {
   463  		u = (^uint64(x) << 1) | 1 // complement i, bit 0 is 1
   464  	} else {
   465  		u = (uint64(x) << 1) // do not complement i, bit 0 is 0
   466  	}
   467  	l.uvarint(u)
   468  }
   469  
   470  //go:nosplit
   471  func (l *debugLogWriter) uvarint(u uint64) {
   472  	i := 0
   473  	for u >= 0x80 {
   474  		l.buf[i] = byte(u) | 0x80
   475  		u >>= 7
   476  		i++
   477  	}
   478  	l.buf[i] = byte(u)
   479  	i++
   480  	l.bytes(l.buf[:i])
   481  }
   482  
   483  type debugLogReader struct {
   484  	data *debugLogBuf
   485  
   486  	// begin and end are the positions in the log of the beginning
   487  	// and end of the log data, modulo len(data).
   488  	begin, end uint64
   489  
   490  	// tick and nano are the current time base at begin.
   491  	tick, nano uint64
   492  }
   493  
   494  //go:nosplit
   495  func (r *debugLogReader) skip() uint64 {
   496  	// Read size at pos.
   497  	if r.begin+debugLogHeaderSize > r.end {
   498  		return ^uint64(0)
   499  	}
   500  	size := uint64(r.readUint16LEAt(r.begin))
   501  	if size == 0 {
   502  		// Sync packet.
   503  		r.tick = r.readUint64LEAt(r.begin + debugLogHeaderSize)
   504  		r.nano = r.readUint64LEAt(r.begin + debugLogHeaderSize + 8)
   505  		size = debugLogSyncSize
   506  	}
   507  	if r.begin+size > r.end {
   508  		return ^uint64(0)
   509  	}
   510  	r.begin += size
   511  	return size
   512  }
   513  
   514  //go:nosplit
   515  func (r *debugLogReader) readUint16LEAt(pos uint64) uint16 {
   516  	return uint16(r.data[pos%uint64(len(r.data))]) |
   517  		uint16(r.data[(pos+1)%uint64(len(r.data))])<<8
   518  }
   519  
   520  //go:nosplit
   521  func (r *debugLogReader) readUint64LEAt(pos uint64) uint64 {
   522  	var b [8]byte
   523  	for i := range b {
   524  		b[i] = r.data[pos%uint64(len(r.data))]
   525  		pos++
   526  	}
   527  	return uint64(b[0]) | uint64(b[1])<<8 |
   528  		uint64(b[2])<<16 | uint64(b[3])<<24 |
   529  		uint64(b[4])<<32 | uint64(b[5])<<40 |
   530  		uint64(b[6])<<48 | uint64(b[7])<<56
   531  }
   532  
   533  func (r *debugLogReader) peek() (tick uint64) {
   534  	// Consume any sync records.
   535  	size := uint64(0)
   536  	for size == 0 {
   537  		if r.begin+debugLogHeaderSize > r.end {
   538  			return ^uint64(0)
   539  		}
   540  		size = uint64(r.readUint16LEAt(r.begin))
   541  		if size != 0 {
   542  			break
   543  		}
   544  		if r.begin+debugLogSyncSize > r.end {
   545  			return ^uint64(0)
   546  		}
   547  		// Sync packet.
   548  		r.tick = r.readUint64LEAt(r.begin + debugLogHeaderSize)
   549  		r.nano = r.readUint64LEAt(r.begin + debugLogHeaderSize + 8)
   550  		r.begin += debugLogSyncSize
   551  	}
   552  
   553  	// Peek tick delta.
   554  	if r.begin+size > r.end {
   555  		return ^uint64(0)
   556  	}
   557  	pos := r.begin + debugLogHeaderSize
   558  	var u uint64
   559  	for i := uint(0); ; i += 7 {
   560  		b := r.data[pos%uint64(len(r.data))]
   561  		pos++
   562  		u |= uint64(b&^0x80) << i
   563  		if b&0x80 == 0 {
   564  			break
   565  		}
   566  	}
   567  	if pos > r.begin+size {
   568  		return ^uint64(0)
   569  	}
   570  	return r.tick + u
   571  }
   572  
   573  func (r *debugLogReader) header() (end, tick, nano uint64, p int) {
   574  	// Read size. We've already skipped sync packets and checked
   575  	// bounds in peek.
   576  	size := uint64(r.readUint16LEAt(r.begin))
   577  	end = r.begin + size
   578  	r.begin += debugLogHeaderSize
   579  
   580  	// Read tick, nano, and p.
   581  	tick = r.uvarint() + r.tick
   582  	nano = r.uvarint() + r.nano
   583  	p = int(r.varint())
   584  
   585  	return
   586  }
   587  
   588  func (r *debugLogReader) uvarint() uint64 {
   589  	var u uint64
   590  	for i := uint(0); ; i += 7 {
   591  		b := r.data[r.begin%uint64(len(r.data))]
   592  		r.begin++
   593  		u |= uint64(b&^0x80) << i
   594  		if b&0x80 == 0 {
   595  			break
   596  		}
   597  	}
   598  	return u
   599  }
   600  
   601  func (r *debugLogReader) varint() int64 {
   602  	u := r.uvarint()
   603  	var v int64
   604  	if u&1 == 0 {
   605  		v = int64(u >> 1)
   606  	} else {
   607  		v = ^int64(u >> 1)
   608  	}
   609  	return v
   610  }
   611  
   612  func (r *debugLogReader) printVal() bool {
   613  	typ := r.data[r.begin%uint64(len(r.data))]
   614  	r.begin++
   615  
   616  	switch typ {
   617  	default:
   618  		print("<unknown field type ", hex(typ), " pos ", r.begin-1, " end ", r.end, ">\n")
   619  		return false
   620  
   621  	case debugLogUnknown:
   622  		print("<unknown kind>")
   623  
   624  	case debugLogBoolTrue:
   625  		print(true)
   626  
   627  	case debugLogBoolFalse:
   628  		print(false)
   629  
   630  	case debugLogInt:
   631  		print(r.varint())
   632  
   633  	case debugLogUint:
   634  		print(r.uvarint())
   635  
   636  	case debugLogHex, debugLogPtr:
   637  		print(hex(r.uvarint()))
   638  
   639  	case debugLogString:
   640  		sl := r.uvarint()
   641  		if r.begin+sl > r.end {
   642  			r.begin = r.end
   643  			print("<string length corrupted>")
   644  			break
   645  		}
   646  		for sl > 0 {
   647  			b := r.data[r.begin%uint64(len(r.data)):]
   648  			if uint64(len(b)) > sl {
   649  				b = b[:sl]
   650  			}
   651  			r.begin += uint64(len(b))
   652  			sl -= uint64(len(b))
   653  			gwrite(b)
   654  		}
   655  
   656  	case debugLogConstString:
   657  		len, ptr := int(r.uvarint()), uintptr(r.uvarint())
   658  		ptr += firstmoduledata.etext
   659  		str := stringStruct{
   660  			str: unsafe.Pointer(ptr),
   661  			len: len,
   662  		}
   663  		s := *(*string)(unsafe.Pointer(&str))
   664  		print(s)
   665  
   666  	case debugLogStringOverflow:
   667  		print("..(", r.uvarint(), " more bytes)..")
   668  
   669  	case debugLogPC:
   670  		printDebugLogPC(uintptr(r.uvarint()), false)
   671  
   672  	case debugLogTraceback:
   673  		n := int(r.uvarint())
   674  		for i := 0; i < n; i++ {
   675  			print("\n\t")
   676  			// gentraceback PCs are always return PCs.
   677  			// Convert them to call PCs.
   678  			//
   679  			// TODO(austin): Expand inlined frames.
   680  			printDebugLogPC(uintptr(r.uvarint()), true)
   681  		}
   682  	}
   683  
   684  	return true
   685  }
   686  
   687  // printDebugLog prints the debug log.
   688  func printDebugLog() {
   689  	if !dlogEnabled {
   690  		return
   691  	}
   692  
   693  	// This function should not panic or throw since it is used in
   694  	// the fatal panic path and this may deadlock.
   695  
   696  	printlock()
   697  
   698  	// Get the list of all debug logs.
   699  	allp := (*uintptr)(unsafe.Pointer(&allDloggers))
   700  	all := (*dlogger)(unsafe.Pointer(atomic.Loaduintptr(allp)))
   701  
   702  	// Count the logs.
   703  	n := 0
   704  	for l := all; l != nil; l = l.allLink {
   705  		n++
   706  	}
   707  	if n == 0 {
   708  		printunlock()
   709  		return
   710  	}
   711  
   712  	// Prepare read state for all logs.
   713  	type readState struct {
   714  		debugLogReader
   715  		first    bool
   716  		lost     uint64
   717  		nextTick uint64
   718  	}
   719  	// Use sysAllocOS instead of sysAlloc because we want to interfere
   720  	// with the runtime as little as possible, and sysAlloc updates accounting.
   721  	state1 := sysAllocOS(unsafe.Sizeof(readState{}) * uintptr(n))
   722  	if state1 == nil {
   723  		println("failed to allocate read state for", n, "logs")
   724  		printunlock()
   725  		return
   726  	}
   727  	state := (*[1 << 20]readState)(state1)[:n]
   728  	{
   729  		l := all
   730  		for i := range state {
   731  			s := &state[i]
   732  			s.debugLogReader = l.w.r
   733  			s.first = true
   734  			s.lost = l.w.r.begin
   735  			s.nextTick = s.peek()
   736  			l = l.allLink
   737  		}
   738  	}
   739  
   740  	// Print records.
   741  	for {
   742  		// Find the next record.
   743  		var best struct {
   744  			tick uint64
   745  			i    int
   746  		}
   747  		best.tick = ^uint64(0)
   748  		for i := range state {
   749  			if state[i].nextTick < best.tick {
   750  				best.tick = state[i].nextTick
   751  				best.i = i
   752  			}
   753  		}
   754  		if best.tick == ^uint64(0) {
   755  			break
   756  		}
   757  
   758  		// Print record.
   759  		s := &state[best.i]
   760  		if s.first {
   761  			print(">> begin log ", best.i)
   762  			if s.lost != 0 {
   763  				print("; lost first ", s.lost>>10, "KB")
   764  			}
   765  			print(" <<\n")
   766  			s.first = false
   767  		}
   768  
   769  		end, _, nano, p := s.header()
   770  		oldEnd := s.end
   771  		s.end = end
   772  
   773  		print("[")
   774  		var tmpbuf [21]byte
   775  		pnano := int64(nano) - runtimeInitTime
   776  		if pnano < 0 {
   777  			// Logged before runtimeInitTime was set.
   778  			pnano = 0
   779  		}
   780  		pnanoBytes := itoaDiv(tmpbuf[:], uint64(pnano), 9)
   781  		print(slicebytetostringtmp((*byte)(noescape(unsafe.Pointer(&pnanoBytes[0]))), len(pnanoBytes)))
   782  		print(" P ", p, "] ")
   783  
   784  		for i := 0; s.begin < s.end; i++ {
   785  			if i > 0 {
   786  				print(" ")
   787  			}
   788  			if !s.printVal() {
   789  				// Abort this P log.
   790  				print("<aborting P log>")
   791  				end = oldEnd
   792  				break
   793  			}
   794  		}
   795  		println()
   796  
   797  		// Move on to the next record.
   798  		s.begin = end
   799  		s.end = oldEnd
   800  		s.nextTick = s.peek()
   801  	}
   802  
   803  	printunlock()
   804  }
   805  
   806  // printDebugLogPC prints a single symbolized PC. If returnPC is true,
   807  // pc is a return PC that must first be converted to a call PC.
   808  func printDebugLogPC(pc uintptr, returnPC bool) {
   809  	fn := findfunc(pc)
   810  	if returnPC && (!fn.valid() || pc > fn.entry()) {
   811  		// TODO(austin): Don't back up if the previous frame
   812  		// was a sigpanic.
   813  		pc--
   814  	}
   815  
   816  	print(hex(pc))
   817  	if !fn.valid() {
   818  		print(" [unknown PC]")
   819  	} else {
   820  		name := funcname(fn)
   821  		file, line := funcline(fn, pc)
   822  		print(" [", name, "+", hex(pc-fn.entry()),
   823  			" ", file, ":", line, "]")
   824  	}
   825  }
   826
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