.. _tracing: Tracing ======= FRR has a small but growing number of static tracepoints available for use with various tracing systems. These tracepoints can assist with debugging, performance analysis and to help understand program flow. They can also be used for monitoring. Developers are encouraged to write new static tracepoints where sensible. They are not compiled in by default, and even when they are, they have no overhead unless enabled by a tracer, so it is okay to be liberal with them. Supported tracers ----------------- Presently two types of tracepoints are supported: - `LTTng tracepoints `_ - `USDT probes `_ LTTng is a tracing framework for Linux only. It offers extremely low overhead and very rich tracing capabilities. FRR supports LTTng-UST, which is the userspace implementation. LTTng tracepoints are very rich in detail. No kernel modules are needed. Besides only being available for Linux, the primary downside of LTTng is the need to link to ``lttng-ust``. USDT probes originate from Solaris, where they were invented for use with dtrace. They are a kernel feature. At least Linux and FreeBSD support them. No library is needed; support is compiled in via a system header (````). USDT probes are much slower than LTTng tracepoints and offer less flexibility in what information can be gleaned from them. LTTng is capable of tracing USDT probes but has limited support for them. SystemTap and dtrace both work only with USDT probes. Usage ----- To compile with tracepoints, use one of the following configure flags: .. program:: configure.ac .. option:: --enable-lttng=yes Generate LTTng tracepoints .. option:: --enable-usdt=yes Generate USDT probes To trace with LTTng, compile with either one (prefer :option:`--enable-lttng` run the target in non-forking mode (no ``-d``) and use LTTng as usual (refer to LTTng user manual). When using USDT probes with LTTng, follow the example in `this article `_. To trace with dtrace or SystemTap, compile with `--enable-usdt=yes` and use your tracer as usual. To see available USDT probes:: readelf -n /usr/lib/frr/bgpd Example:: root@host ~> readelf -n /usr/lib/frr/bgpd Displaying notes found in: .note.ABI-tag Owner Data size Description GNU 0x00000010 NT_GNU_ABI_TAG (ABI version tag) OS: Linux, ABI: 3.2.0 Displaying notes found in: .note.gnu.build-id Owner Data size Description GNU 0x00000014 NT_GNU_BUILD_ID (unique build ID bitstring) Build ID: 4f42933a69dcb42a519bc459b2105177c8adf55d Displaying notes found in: .note.stapsdt Owner Data size Description stapsdt 0x00000045 NT_STAPSDT (SystemTap probe descriptors) Provider: frr_bgp Name: packet_read Location: 0x000000000045ee48, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000 Arguments: 8@-96(%rbp) 8@-104(%rbp) stapsdt 0x00000047 NT_STAPSDT (SystemTap probe descriptors) Provider: frr_bgp Name: open_process Location: 0x000000000047c43b, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000 Arguments: 8@-224(%rbp) 2@-226(%rbp) stapsdt 0x00000049 NT_STAPSDT (SystemTap probe descriptors) Provider: frr_bgp Name: update_process Location: 0x000000000047c4bf, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000 Arguments: 8@-208(%rbp) 2@-210(%rbp) stapsdt 0x0000004f NT_STAPSDT (SystemTap probe descriptors) Provider: frr_bgp Name: notification_process Location: 0x000000000047c557, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000 Arguments: 8@-192(%rbp) 2@-194(%rbp) stapsdt 0x0000004c NT_STAPSDT (SystemTap probe descriptors) Provider: frr_bgp Name: keepalive_process Location: 0x000000000047c5db, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000 Arguments: 8@-176(%rbp) 2@-178(%rbp) stapsdt 0x0000004a NT_STAPSDT (SystemTap probe descriptors) Provider: frr_bgp Name: refresh_process Location: 0x000000000047c673, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000 Arguments: 8@-160(%rbp) 2@-162(%rbp) stapsdt 0x0000004d NT_STAPSDT (SystemTap probe descriptors) Provider: frr_bgp Name: capability_process Location: 0x000000000047c6f7, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000 Arguments: 8@-144(%rbp) 2@-146(%rbp) stapsdt 0x0000006f NT_STAPSDT (SystemTap probe descriptors) Provider: frr_bgp Name: output_filter Location: 0x000000000048e33a, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000 Arguments: 8@-144(%rbp) 8@-152(%rbp) 4@-156(%rbp) 4@-160(%rbp) 8@-168(%rbp) stapsdt 0x0000007d NT_STAPSDT (SystemTap probe descriptors) Provider: frr_bgp Name: process_update Location: 0x0000000000491f10, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000 Arguments: 8@-800(%rbp) 8@-808(%rbp) 4@-812(%rbp) 4@-816(%rbp) 4@-820(%rbp) 8@-832(%rbp) stapsdt 0x0000006e NT_STAPSDT (SystemTap probe descriptors) Provider: frr_bgp Name: input_filter Location: 0x00000000004940ed, Base: 0x00000000005a09d2, Semaphore: 0x0000000000000000 Arguments: 8@-144(%rbp) 8@-152(%rbp) 4@-156(%rbp) 4@-160(%rbp) 8@-168(%rbp) To see available LTTng probes, run the target, create a session and then:: lttng list --userspace | grep frr Example:: root@host ~> lttng list --userspace | grep frr PID: 11157 - Name: /usr/lib/frr/bgpd frr_libfrr:route_node_get (loglevel: TRACE_DEBUG_LINE (13)) (type: tracepoint) frr_libfrr:list_sort (loglevel: TRACE_DEBUG_LINE (13)) (type: tracepoint) frr_libfrr:list_delete_node (loglevel: TRACE_DEBUG_LINE (13)) (type: tracepoint) frr_libfrr:list_remove (loglevel: TRACE_DEBUG_LINE (13)) (type: tracepoint) frr_libfrr:list_add (loglevel: TRACE_DEBUG_LINE (13)) (type: tracepoint) frr_libfrr:memfree (loglevel: TRACE_DEBUG_LINE (13)) (type: tracepoint) frr_libfrr:memalloc (loglevel: TRACE_DEBUG_LINE (13)) (type: tracepoint) frr_libfrr:frr_pthread_stop (loglevel: TRACE_DEBUG_LINE (13)) (type: tracepoint) frr_libfrr:frr_pthread_run (loglevel: TRACE_DEBUG_LINE (13)) (type: tracepoint) frr_libfrr:thread_call (loglevel: TRACE_INFO (6)) (type: tracepoint) frr_libfrr:event_cancel_async (loglevel: TRACE_INFO (6)) (type: tracepoint) frr_libfrr:event_cancel (loglevel: TRACE_INFO (6)) (type: tracepoint) frr_libfrr:schedule_write (loglevel: TRACE_INFO (6)) (type: tracepoint) frr_libfrr:schedule_read (loglevel: TRACE_INFO (6)) (type: tracepoint) frr_libfrr:schedule_event (loglevel: TRACE_INFO (6)) (type: tracepoint) frr_libfrr:schedule_timer (loglevel: TRACE_INFO (6)) (type: tracepoint) frr_libfrr:hash_release (loglevel: TRACE_INFO (6)) (type: tracepoint) frr_libfrr:hash_insert (loglevel: TRACE_INFO (6)) (type: tracepoint) frr_libfrr:hash_get (loglevel: TRACE_INFO (6)) (type: tracepoint) frr_bgp:output_filter (loglevel: TRACE_INFO (6)) (type: tracepoint) frr_bgp:input_filter (loglevel: TRACE_INFO (6)) (type: tracepoint) frr_bgp:process_update (loglevel: TRACE_INFO (6)) (type: tracepoint) frr_bgp:packet_read (loglevel: TRACE_INFO (6)) (type: tracepoint) frr_bgp:refresh_process (loglevel: TRACE_INFO (6)) (type: tracepoint) frr_bgp:capability_process (loglevel: TRACE_INFO (6)) (type: tracepoint) frr_bgp:notification_process (loglevel: TRACE_INFO (6)) (type: tracepoint) frr_bgp:update_process (loglevel: TRACE_INFO (6)) (type: tracepoint) frr_bgp:keepalive_process (loglevel: TRACE_INFO (6)) (type: tracepoint) frr_bgp:open_process (loglevel: TRACE_INFO (6)) (type: tracepoint) When using LTTng, you can also get zlogs as trace events by enabling the ``lttng_ust_tracelog:*`` event class. To see available SystemTap USDT probes, run:: stap -L 'process("/usr/lib/frr/bgpd").mark("*")' Example:: root@host ~> stap -L 'process("/usr/lib/frr/bgpd").mark("*")' process("/usr/lib/frr/bgpd").mark("capability_process") $arg1:long $arg2:long process("/usr/lib/frr/bgpd").mark("input_filter") $arg1:long $arg2:long $arg3:long $arg4:long $arg5:long process("/usr/lib/frr/bgpd").mark("keepalive_process") $arg1:long $arg2:long process("/usr/lib/frr/bgpd").mark("notification_process") $arg1:long $arg2:long process("/usr/lib/frr/bgpd").mark("open_process") $arg1:long $arg2:long process("/usr/lib/frr/bgpd").mark("output_filter") $arg1:long $arg2:long $arg3:long $arg4:long $arg5:long process("/usr/lib/frr/bgpd").mark("packet_read") $arg1:long $arg2:long process("/usr/lib/frr/bgpd").mark("process_update") $arg1:long $arg2:long $arg3:long $arg4:long $arg5:long $arg6:long process("/usr/lib/frr/bgpd").mark("refresh_process") $arg1:long $arg2:long process("/usr/lib/frr/bgpd").mark("update_process") $arg1:long $arg2:long When using SystemTap, you can also easily attach to an existing function:: stap -L 'process("/usr/lib/frr/bgpd").function("bgp_update_receive")' Example:: root@host ~> stap -L 'process("/usr/lib/frr/bgpd").function("bgp_update_receive")' process("/usr/lib/frr/bgpd").function("bgp_update_receive@bgpd/bgp_packet.c:1531") $peer:struct peer* $size:bgp_size_t $attr:struct attr $restart:_Bool $nlris:struct bgp_nlri[] $__func__:char const[] const Complete ``bgp.stp`` example using SystemTap to show BGP peer, prefix and aspath using ``process_update`` USDT:: global pkt_size; probe begin { ansi_clear_screen(); println("Starting..."); } probe process("/usr/lib/frr/bgpd").function("bgp_update_receive") { pkt_size <<< $size; } probe process("/usr/lib/frr/bgpd").mark("process_update") { aspath = @cast($arg6, "attr")->aspath; printf("> %s via %s (%s)\n", user_string($arg2), user_string(@cast($arg1, "peer")->host), user_string(@cast(aspath, "aspath")->str)); } probe end { if (@count(pkt_size)) print(@hist_linear(pkt_size, 0, 20, 2)); } Output:: Starting... > 192.168.0.0/24 via 192.168.0.1 (65534) > 192.168.100.1/32 via 192.168.0.1 (65534) > 172.16.16.1/32 via 192.168.0.1 (65534 65030) ^Cvalue |-------------------------------------------------- count 0 | 0 2 | 0 4 |@ 1 6 | 0 8 | 0 ~ 18 | 0 20 | 0 >20 |@@@@@ 5 Concepts -------- Tracepoints are statically defined points in code where a developer has determined that outside observers might gain something from knowing what is going on at that point. It's like logging but with the ability to dump large amounts of internal data with much higher performance. LTTng has a good summary `here `_. Each tracepoint has a "provider" and name. The provider is basically a namespace; for example, ``bgpd`` uses the provider name ``frr_bgp``. The name is arbitrary, but because providers share a global namespace on the user's system, all providers from FRR should be prefixed by ``frr_``. The tracepoint name is just the name of the event. Events are globally named by their provider and name. For example, the event when BGP reads a packet from a peer is ``frr_bgp:packet_read``. To do tracing, the tracing tool of choice is told which events to listen to. For example, to listen to all events from FRR's BGP implementation, you would enable the events ``frr_bgp:*``. In the same tracing session you could also choose to record all memory allocations by enabling the ``malloc`` tracepoints in ``libc`` as well as all kernel skb operations using the various in-kernel tracepoints. This allows you to build as complete a view as desired of what the system is doing during the tracing window (subject to what tracepoints are available). Of particular use are the tracepoints for FRR's internal event scheduler; tracing these allows you to see all events executed by all event loops for the target(s) in question. Here's a couple events selected from a trace of BGP during startup:: ... [18:41:35.750131763] (+0.000048901) host frr_libfrr:thread_call: { cpu_id = 1 }, { threadmaster_name = "default", function_name = "zclient_connect", scheduled_from = "lib/zclient.c", scheduled_on_line = 3877, thread_addr = 0x0, file_descriptor = 0, event_value = 0, argument_ptr = 0xA37F70, timer = 0 } [18:41:35.750175124] (+0.000020001) host frr_libfrr:thread_call: { cpu_id = 1 }, { threadmaster_name = "default", function_name = "frr_config_read_in", scheduled_from = "lib/libfrr.c", scheduled_on_line = 934, thread_addr = 0x0, file_descriptor = 0, event_value = 0, argument_ptr = 0x0, timer = 0 } [18:41:35.753341264] (+0.000010532) host frr_libfrr:thread_call: { cpu_id = 1 }, { threadmaster_name = "default", function_name = "bgp_event", scheduled_from = "bgpd/bgpd.c", scheduled_on_line = 142, thread_addr = 0x0, file_descriptor = 2, event_value = 2, argument_ptr = 0xE4D780, timer = 2 } [18:41:35.753404186] (+0.000004910) host frr_libfrr:thread_call: { cpu_id = 1 }, { threadmaster_name = "default", function_name = "zclient_read", scheduled_from = "lib/zclient.c", scheduled_on_line = 3891, thread_addr = 0x0, file_descriptor = 40, event_value = 40, argument_ptr = 0xA37F70, timer = 40 } ... Very useful for getting a time-ordered look into what the process is doing. Adding Tracepoints ------------------ Adding new tracepoints is a two step process: 1. Define the tracepoint 2. Use the tracepoint Tracepoint definitions state the "provider" and name of the tracepoint, along with any values it will produce, and how to format them. This is done with macros provided by LTTng. USDT probes do not use definitions and are inserted at the trace site with a single macro. However, to maintain support for both platforms, you must define an LTTng tracepoint when adding a new one. ``frrtrace()`` will expand to the appropriate ``DTRACE_PROBEn`` macro when USDT is in use. If you are adding new tracepoints to a daemon that has no tracepoints, that daemon's ``subdir.am`` must be updated to conditionally link ``lttng-ust``. Look at ``bgpd/subdir.am`` for an example of how to do this; grep for ``UST_LIBS``. Create new files named ``_trace.[ch]``. Use ``bgpd/bgp_trace.[h]`` as boilerplate. If you are adding tracepoints to a daemon that already has them, look for the ``_trace.h`` file; tracepoints are written here. Refer to the `LTTng developer docs `_ for details on how to define tracepoints. To use them, simply add a call to ``frrtrace()`` at the point you'd like the event to be emitted, like so: .. code-block:: c ... switch (type) { case BGP_MSG_OPEN: frrtrace(2, frr_bgp, open_process, peer, size); /* tracepoint */ atomic_fetch_add_explicit(&peer->open_in, 1, memory_order_relaxed); mprc = bgp_open_receive(peer, size); ... After recompiling this tracepoint will now be available, either as a USDT probe or LTTng tracepoint, depending on your compilation choice. trace.h ^^^^^^^ Because FRR supports multiple types of tracepoints, the code for creating them abstracts away the underlying system being used. This abstraction code is in ``lib/trace.h``. There are 2 function-like macros that are used for working with tracepoints. - ``frrtrace()`` defines tracepoints - ``frrtrace_enabled()`` checks whether a tracepoint is enabled There is also ``frrtracelog()``, which is used in zlog core code to make zlog messages available as trace events to LTTng. This should not be used elsewhere. There is additional documentation in the header. The key thing to note is that you should never include ``trace.h`` in source where you plan to put tracepoints; include the tracepoint definition header instead (e.g. :file:`bgp_trace.h`). Limitations ----------- Tracers do not like ``fork()`` or ``dlopen()``. LTTng has some workarounds for this involving interceptor libraries using ``LD_PRELOAD``. If you're running FRR in a typical daemonizing way (``-d`` to the daemons) you'll need to run the daemons like so: .. code-block:: shell LD_PRELOAD=liblttng-ust-fork.so If you're using systemd this you can accomplish this for all daemons by modifying ``frr.service`` like so: .. code-block:: diff --- a/frr.service +++ b/frr.service @@ -7,6 +7,7 @@ Before=network.target OnFailure=heartbeat-failed@%n [Service] +Environment="LD_PRELOAD=liblttng-ust-fork.so" Nice=-5 Type=forking NotifyAccess=all USDT tracepoints are relatively high overhead and probably shouldn't be used for "flight recorder" functionality, i.e. enabling and passively recording all events for monitoring purposes. It's generally okay to use LTTng like this, though.