doc/developer/tracing.rst

   1 .. _tracing:
   2
   3 Tracing
   4 =======
   5
   6 FRR has a small but growing number of static tracepoints available for use with
   7 various tracing systems. These tracepoints can assist with debugging,
   8 performance analysis and to help understand program flow. They can also be used
   9 for monitoring.
  10
  11 Developers are encouraged to write new static tracepoints where sensible. They
  12 are not compiled in by default, and even when they are, they have no overhead
  13 unless enabled by a tracer, so it is okay to be liberal with them.
  14
  15
  16 Supported tracers
  17 -----------------
  18
  19 Presently two types of tracepoints are supported:
  20
  21 - `LTTng tracepoints <https://lttng.org/>`_
  22 - `USDT probes <http://dtrace.org/guide/chp-usdt.html>`_
  23
  24 LTTng is a tracing framework for Linux only. It offers extremely low overhead
  25 and very rich tracing capabilities. FRR supports LTTng-UST, which is the
  26 userspace implementation. LTTng tracepoints are very rich in detail. No kernel
  27 modules are needed. Besides only being available for Linux, the primary
  28 downside of LTTng is the need to link to ``lttng-ust``.
  29
  30 USDT probes originate from Solaris, where they were invented for use with
  31 dtrace. They are a kernel feature. At least Linux and FreeBSD support them. No
  32 library is needed; support is compiled in via a system header
  33 (``<sys/sdt.h>``). USDT probes are much slower than LTTng tracepoints and offer
  34 less flexibility in what information can be gleaned from them.
  35
  36 LTTng is capable of tracing USDT probes but has limited support for them.
  37 SystemTap and dtrace both work only with USDT probes.
  38
  39
  40 Usage
  41 -----
  42
  43 To compile with tracepoints, use one of the following configure flags:
  44
  45 .. program:: configure.ac
  46
  47 .. option:: --enable-lttng=yes
  48
  49    Generate LTTng tracepoints
  50
  51 .. option:: --enable-usdt=yes
  52
  53    Generate USDT probes
  54
  55 To trace with LTTng, compile with either one (prefer :option:`--enable-lttng`
  56 run the target in non-forking mode (no ``-d``) and use LTTng as usual (refer to
  57 LTTng user manual). When using USDT probes with LTTng, follow the example in
  58 `this article
  59 <https://lttng.org/blog/2019/10/15/new-dynamic-user-space-tracing-in-lttng/>`_.
  60 To trace with dtrace or SystemTap, compile with :option:`--enable-usdt=yes` and
  61 use your tracer as usual.
  62
  63 To see available USDT probes::
  64
  65   readelf -n /usr/lib/frr/bgpd
  66
  67 Example::
  68
  69   root@host ~> readelf -n /usr/lib/frr/bgpd
  70
  71   Displaying notes found in: .note.ABI-tag
  72     Owner                 Data size     Description
  73     GNU                  0x00000010     NT_GNU_ABI_TAG (ABI version tag)
  74       OS: Linux, ABI: 3.2.0
  75
  76   Displaying notes found in: .note.gnu.build-id
  77     Owner                 Data size     Description
  78     GNU                  0x00000014     NT_GNU_BUILD_ID (unique build ID bitstring)
  79       Build ID: 4f42933a69dcb42a519bc459b2105177c8adf55d
  80
  81   Displaying notes found in: .note.stapsdt
  82     Owner                 Data size     Description
  83     stapsdt              0x00000045     NT_STAPSDT (SystemTap probe descriptors)
  84       Provider: frr_bgp
  85       Name: packet_read
  86       Location: 0x000000000045ee48, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000
  87       Arguments: 8@-96(%rbp) 8@-104(%rbp)
  88     stapsdt              0x00000047     NT_STAPSDT (SystemTap probe descriptors)
  89       Provider: frr_bgp
  90       Name: open_process
  91       Location: 0x000000000047c43b, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000
  92       Arguments: 8@-224(%rbp) 2@-226(%rbp)
  93     stapsdt              0x00000049     NT_STAPSDT (SystemTap probe descriptors)
  94       Provider: frr_bgp
  95       Name: update_process
  96       Location: 0x000000000047c4bf, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000
  97       Arguments: 8@-208(%rbp) 2@-210(%rbp)
  98     stapsdt              0x0000004f     NT_STAPSDT (SystemTap probe descriptors)
  99       Provider: frr_bgp
 100       Name: notification_process
 101       Location: 0x000000000047c557, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000
 102       Arguments: 8@-192(%rbp) 2@-194(%rbp)
 103     stapsdt              0x0000004c     NT_STAPSDT (SystemTap probe descriptors)
 104       Provider: frr_bgp
 105       Name: keepalive_process
 106       Location: 0x000000000047c5db, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000
 107       Arguments: 8@-176(%rbp) 2@-178(%rbp)
 108     stapsdt              0x0000004a     NT_STAPSDT (SystemTap probe descriptors)
 109       Provider: frr_bgp
 110       Name: refresh_process
 111       Location: 0x000000000047c673, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000
 112       Arguments: 8@-160(%rbp) 2@-162(%rbp)
 113     stapsdt              0x0000004d     NT_STAPSDT (SystemTap probe descriptors)
 114       Provider: frr_bgp
 115       Name: capability_process
 116       Location: 0x000000000047c6f7, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000
 117       Arguments: 8@-144(%rbp) 2@-146(%rbp)
 118     stapsdt              0x0000006f     NT_STAPSDT (SystemTap probe descriptors)
 119       Provider: frr_bgp
 120       Name: output_filter
 121       Location: 0x000000000048e33a, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000
 122       Arguments: 8@-144(%rbp) 8@-152(%rbp) 4@-156(%rbp) 4@-160(%rbp) 8@-168(%rbp)
 123     stapsdt              0x0000007d     NT_STAPSDT (SystemTap probe descriptors)
 124       Provider: frr_bgp
 125       Name: process_update
 126       Location: 0x0000000000491f10, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000
 127       Arguments: 8@-800(%rbp) 8@-808(%rbp) 4@-812(%rbp) 4@-816(%rbp) 4@-820(%rbp) 8@-832(%rbp)
 128     stapsdt              0x0000006e     NT_STAPSDT (SystemTap probe descriptors)
 129       Provider: frr_bgp
 130       Name: input_filter
 131       Location: 0x00000000004940ed, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000
 132       Arguments: 8@-144(%rbp) 8@-152(%rbp) 4@-156(%rbp) 4@-160(%rbp) 8@-168(%rbp)
 133
 134
 135 To see available LTTng probes, run the target, create a session and then::
 136
 137   lttng list --userspace | grep frr
 138
 139 Example::
 140
 141   root@host ~> lttng list --userspace | grep frr
 142   PID: 11157 - Name: /usr/lib/frr/bgpd
 143         frr_libfrr:route_node_get (loglevel: TRACE_DEBUG_LINE (13)) (type: tracepoint)
 144         frr_libfrr:list_sort (loglevel: TRACE_DEBUG_LINE (13)) (type: tracepoint)
 145         frr_libfrr:list_delete_node (loglevel: TRACE_DEBUG_LINE (13)) (type: tracepoint)
 146         frr_libfrr:list_remove (loglevel: TRACE_DEBUG_LINE (13)) (type: tracepoint)
 147         frr_libfrr:list_add (loglevel: TRACE_DEBUG_LINE (13)) (type: tracepoint)
 148         frr_libfrr:memfree (loglevel: TRACE_DEBUG_LINE (13)) (type: tracepoint)
 149         frr_libfrr:memalloc (loglevel: TRACE_DEBUG_LINE (13)) (type: tracepoint)
 150         frr_libfrr:frr_pthread_stop (loglevel: TRACE_DEBUG_LINE (13)) (type: tracepoint)
 151         frr_libfrr:frr_pthread_run (loglevel: TRACE_DEBUG_LINE (13)) (type: tracepoint)
 152         frr_libfrr:thread_call (loglevel: TRACE_INFO (6)) (type: tracepoint)
 153         frr_libfrr:thread_cancel_async (loglevel: TRACE_INFO (6)) (type: tracepoint)
 154         frr_libfrr:thread_cancel (loglevel: TRACE_INFO (6)) (type: tracepoint)
 155         frr_libfrr:schedule_write (loglevel: TRACE_INFO (6)) (type: tracepoint)
 156         frr_libfrr:schedule_read (loglevel: TRACE_INFO (6)) (type: tracepoint)
 157         frr_libfrr:schedule_event (loglevel: TRACE_INFO (6)) (type: tracepoint)
 158         frr_libfrr:schedule_timer (loglevel: TRACE_INFO (6)) (type: tracepoint)
 159         frr_libfrr:hash_release (loglevel: TRACE_INFO (6)) (type: tracepoint)
 160         frr_libfrr:hash_insert (loglevel: TRACE_INFO (6)) (type: tracepoint)
 161         frr_libfrr:hash_get (loglevel: TRACE_INFO (6)) (type: tracepoint)
 162         frr_bgp:output_filter (loglevel: TRACE_INFO (6)) (type: tracepoint)
 163         frr_bgp:input_filter (loglevel: TRACE_INFO (6)) (type: tracepoint)
 164         frr_bgp:process_update (loglevel: TRACE_INFO (6)) (type: tracepoint)
 165         frr_bgp:packet_read (loglevel: TRACE_INFO (6)) (type: tracepoint)
 166         frr_bgp:refresh_process (loglevel: TRACE_INFO (6)) (type: tracepoint)
 167         frr_bgp:capability_process (loglevel: TRACE_INFO (6)) (type: tracepoint)
 168         frr_bgp:notification_process (loglevel: TRACE_INFO (6)) (type: tracepoint)
 169         frr_bgp:update_process (loglevel: TRACE_INFO (6)) (type: tracepoint)
 170         frr_bgp:keepalive_process (loglevel: TRACE_INFO (6)) (type: tracepoint)
 171         frr_bgp:open_process (loglevel: TRACE_INFO (6)) (type: tracepoint)
 172
 173 When using LTTng, you can also get zlogs as trace events by enabling
 174 the ``lttng_ust_tracelog:*`` event class.
 175
 176 Concepts
 177 --------
 178
 179 Tracepoints are statically defined points in code where a developer has
 180 determined that outside observers might gain something from knowing what is
 181 going on at that point. It's like logging but with the ability to dump large
 182 amounts of internal data with much higher performance. LTTng has a good summary
 183 `here <https://lttng.org/docs/#doc-what-is-tracing>`_.
 184
 185 Each tracepoint has a "provider" and name. The provider is basically a
 186 namespace; for example, ``bgpd`` uses the provider name ``frr_bgp``. The name
 187 is arbitrary, but because providers share a global namespace on the user's
 188 system, all providers from FRR should be prefixed by ``frr_``. The tracepoint
 189 name is just the name of the event. Events are globally named by their provider
 190 and name. For example, the event when BGP reads a packet from a peer is
 191 ``frr_bgp:packet_read``.
 192
 193 To do tracing, the tracing tool of choice is told which events to listen to.
 194 For example, to listen to all events from FRR's BGP implementation, you would
 195 enable the events ``frr_bgp:*``. In the same tracing session you could also
 196 choose to record all memory allocations by enabling the ``malloc`` tracepoints
 197 in ``libc`` as well as all kernel skb operations using the various in-kernel
 198 tracepoints. This allows you to build as complete a view as desired of what the
 199 system is doing during the tracing window (subject to what tracepoints are
 200 available).
 201
 202 Of particular use are the tracepoints for FRR's internal event scheduler;
 203 tracing these allows you to see all events executed by all event loops for the
 204 target(s) in question. Here's a couple events selected from a trace of BGP
 205 during startup::
 206
 207    ...
 208
 209    [18:41:35.750131763] (+0.000048901) host frr_libfrr:thread_call: { cpu_id =
 210    1 }, { threadmaster_name = "default", function_name = "zclient_connect",
 211    scheduled_from = "lib/zclient.c", scheduled_on_line = 3877, thread_addr =
 212    0x0, file_descriptor = 0, event_value = 0, argument_ptr = 0xA37F70, timer =
 213    0 }
 214
 215    [18:41:35.750175124] (+0.000020001) host frr_libfrr:thread_call: { cpu_id =
 216    1 }, { threadmaster_name = "default", function_name = "frr_config_read_in",
 217    scheduled_from = "lib/libfrr.c", scheduled_on_line = 934, thread_addr = 0x0,
 218    file_descriptor = 0, event_value = 0, argument_ptr = 0x0, timer = 0 }
 219
 220    [18:41:35.753341264] (+0.000010532) host frr_libfrr:thread_call: { cpu_id =
 221    1 }, { threadmaster_name = "default", function_name = "bgp_event",
 222    scheduled_from = "bgpd/bgpd.c", scheduled_on_line = 142, thread_addr = 0x0,
 223    file_descriptor = 2, event_value = 2, argument_ptr = 0xE4D780, timer = 2 }
 224
 225    [18:41:35.753404186] (+0.000004910) host frr_libfrr:thread_call: { cpu_id =
 226    1 }, { threadmaster_name = "default", function_name = "zclient_read",
 227    scheduled_from = "lib/zclient.c", scheduled_on_line = 3891, thread_addr =
 228    0x0, file_descriptor = 40, event_value = 40, argument_ptr = 0xA37F70, timer
 229    = 40 }
 230
 231    ...
 232
 233
 234 Very useful for getting a time-ordered look into what the process is doing.
 235
 236
 237 Adding Tracepoints
 238 ------------------
 239
 240 Adding new tracepoints is a two step process:
 241
 242 1. Define the tracepoint
 243 2. Use the tracepoint
 244
 245 Tracepoint definitions state the "provider" and name of the tracepoint, along
 246 with any values it will produce, and how to format them. This is done with
 247 macros provided by LTTng. USDT probes do not use definitions and are inserted
 248 at the trace site with a single macro. However, to maintain support for both
 249 platforms, you must define an LTTng tracepoint when adding a new one.
 250 ``frrtrace()`` will expand to the appropriate ``DTRACE_PROBEn`` macro when USDT
 251 is in use.
 252
 253 If you are adding new tracepoints to a daemon that has no tracepoints, that
 254 daemon's ``subdir.am`` must be updated to conditionally link ``lttng-ust``.
 255 Look at ``bgpd/subdir.am`` for an example of how to do this; grep for
 256 ``UST_LIBS``. Create new files named ``<daemon>_trace.[ch]``. Use
 257 ``bgpd/bgp_trace.[h]`` as boilerplate. If you are adding tracepoints to a
 258 daemon that already has them, look for the ``<daemon>_trace.h`` file;
 259 tracepoints are written here.
 260
 261 Refer to the `LTTng developer docs
 262 <https://lttng.org/docs/#doc-c-application>`_ for details on how to define
 263 tracepoints.
 264
 265 To use them, simply add a call to ``frrtrace()`` at the point you'd like the
 266 event to be emitted, like so:
 267
 268 .. code-block:: c
 269
 270    ...
 271
 272    switch (type) {
 273    case BGP_MSG_OPEN:
 274            frrtrace(2, frr_bgp, open_process, peer, size); /* tracepoint */
 275            atomic_fetch_add_explicit(&peer->open_in, 1,
 276                                      memory_order_relaxed);
 277            mprc = bgp_open_receive(peer, size);
 278
 279    ...
 280
 281 After recompiling this tracepoint will now be available, either as a USDT probe
 282 or LTTng tracepoint, depending on your compilation choice.
 283
 284
 285 trace.h
 286 ^^^^^^^
 287
 288 Because FRR supports multiple types of tracepoints, the code for creating them
 289 abstracts away the underlying system being used. This abstraction code is in
 290 ``lib/trace.h``. There are 2 function-like macros that are used for working
 291 with tracepoints.
 292
 293 - ``frrtrace()`` defines tracepoints
 294 - ``frrtrace_enabled()`` checks whether a tracepoint is enabled
 295
 296 There is also ``frrtracelog()``, which is used in zlog core code to make zlog
 297 messages available as trace events to LTTng. This should not be used elsewhere.
 298
 299 There is additional documentation in the header. The key thing to note is that
 300 you should never include ``trace.h`` in source where you plan to put
 301 tracepoints; include the tracepoint definition header instead (e.g.
 302 :file:`bgp_trace.h`).
 303
 304
 305 Limitations
 306 -----------
 307
 308 Tracers do not like ``fork()`` or ``dlopen()``. LTTng has some workarounds for
 309 this involving interceptor libraries using ``LD_PRELOAD``.
 310
 311 USDT tracepoints are relatively high overhead and probably shouldn't be used
 312 for "flight recorder" functionality, i.e. enabling and passively recording all
 313 events for monitoring purposes. It's generally okay to use LTTng like this,
 314 though.