On Wed, Apr 1, 2020 at 8:27 PM Thomas Monjalon <tho...@monjalon.net> wrote: > > Hi, > > I did not review the code. > I want to understand the design first,
I don't intend to write this as a low-level design document. I would suggest please look into API documentation in the first trace patch. http://patches.dpdk.org/patch/67322/ > and I start with the user documentation. OK. > > John, Marko, who is going to review this new documentation? > I think we need some help for syntax and phrasing here. Thanks > > > > +Trace Library > > +============= > > + > > +DPDK provides a tracing library that gives the ability to add tracepoints > > +in application to get runtime trace/debug information for control and fast > > +APIs with minimum impact on fast path performance. Typical trace overhead > > is > > +~20 cycles and instrumentation overhead is 1 cycle. > > Please could you start this doc with explaining what is a tracepoint? OK. I will pull of the what is tracepoint from the wiki. > > How is it different of a log? OK. > > My main question is to understand why it is not implemented as a CTF output > of rte_log. Reminder we have rte_openlog_stream(FILE *). > Is there a way to adapt/extend this API to specify using CTF formatter? > What are the other gaps? Log and trace quite different. Trace will have a directory, which contains the trace files for all the cores, quite different from the log. See rte_trace_save() API. Another difference is how fast trace can be written. log is snprint() style stuff (pretty slow), Where trace deals with fixed primitives like rte_trace_ctf_* See lttng library API to understand it better. We have similar API like lttng( based on community feedback) > > > + > > +Library mainly caters below mentioned use cases: > > + > > +- The DPDK provider will not have access to the DPDK customer applications. > > There is no provider/customer. > DPDK is available on dpdk.org and users are not paying. I will reword. > > > + Inbuilt tracer support will us enable to debug/analyze the slow path and > > + fast path DPDK API usage. > > + > > +- Provides a low overhead fast path multi-core PMD driver's > > debugging/analysis > > + infrastructure to fix the functional and performance issue(s). > > + > > +- Post trace analysis tools can provide various status across the system > > such > > + as cpu_idle() using the timestamp added in the trace. > > + > > +Below sections will provide detailed information about: > > + > > + - Trace a user application > > + - View and analyze the recorded events > > + > > +Trace a user application > > +------------------------ > > + > > +This section steps you through a simple example to trace an application. > > +A trace can be achieved using below mentioned steps: > > + > > +Define and register a tracepoint > > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > +The application can define and register tracepoints either existing C file > > or > > +create a new file (say xyz_app_trace_point.c). Also, all the tracepoints > > must be > > +resolved before rte_eal_init i.e. tracepoints must be registered as > > constructor > > +using RTE_INIT interface. > > I think you want to explain the design before, > so usage constraints will be better understood. I think, first usage and then design. Not all care about internal design.(In the context of DPDK programming guide) > > > + > > +Following are the MACRO definition exposed by the trace Library to define > > and > > +register a tracepoint. > > + > > +.. code-block:: c > > + > > + #define RTE_TRACE_POINT_DEFINE(tp)\ > > + uint64_t __attribute__((section("__rte_trace_point"))) __##tp > > + > > + #define RTE_TRACE_POINT_REGISTER(trace, name, level)\ > > + __rte_trace_point_register(&__##trace, RTE_STR(name), RTE_LOG_ ## > > level,\ > > + (void (*)(void)) trace) > > Showing the macro definitions does not help to understand > neither the concept of define and register, nor the usage. Ack. I will remove. > > > +Example tracepoint definition and registration > > + > > +.. code-block:: c > > + > > + RTE_TRACE_POINT_DEFINE(rte_trace_lib_eal_generic_str); /* Definition */ > > What is a definition for? > Please explain a very simple use case by stating the goal and the output > of the example clearly. OK. > > > + > > + RTE_INIT(eal_trace_init) > > + { > > + /* Registration */ > > + RTE_TRACE_POINT_REGISTER(rte_trace_lib_eal_generic_str, > > + lib.eal.generic.str, INFO); > > + } > > Here we just see some function calls without understanding what it does. I think, one needs to go through code for that. I am NOT adding low-level design here. > > > + > > +For more details refer trace API documentation. > > Yes the details must be in the API documentation. > This guide should explain the design and goals. > > > +Defined tracepoint must be exported into corresponding .map file. > > + > > +.. Note:: > > + > > + A tracepoint is defined like __##tp i.e. > > __rte_trace_lib_eal_generic_str > > + for above example. Same must be updated into corresponding .map file. > > We don't know what is __##tp. That's so cryptic :-) I will fix it. > > > + > > +Define trace function to write events > > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > +After a successful tracepoint registration, the application must define a > > +trace function which solves three purposes: > > + > > + - Calculates the size of the event. > > + - Generate CTF metadata field string for the event. > > The format of the trace is the responsibility of the application? No. > > > + - Emit the event to trace memory. > > + > > +A tracepoint can be classified as either a data path or a slow path > > tracepoint. > > +So based on that, the application must define tracepoint function using > > one of > > +the mentioned MACRO > > + > > +.. code-block:: c > > + > > + /* Define tracepoint function for slow path */ > > + #define RTE_TRACE_POINT(tp, args, ...)\ > > + __RTE_TRACE_POINT(generic, tp, args, __VA_ARGS__) > > + > > + /* Define tracepoint function for data path */ > > + #define RTE_TRACE_POINT_DP(tp, args, ...)\ > > + __RTE_TRACE_POINT(dp, tp, args, __VA_ARGS__) > > No need to copy/paste the code (which will be outdated at some point). > I think naming the macros is enough. OK > > > +RTE_TRACE_POINT_DP is compiled out by default and can be enabled using > > +CONFIG_RTE_ENABLE_TRACE_DP configuration parameter. Also application can > > use > > +``rte_trace_is_dp_enabled`` to get current status of RTE_TRACE_POINT_DP. > > +For more details, refer DPDK Trace API documentation. > > + > > +Example tracepoint function definition > > + > > +.. code-block:: c > > + > > + /* Slow path tracepoint */ > > + RTE_TRACE_POINT( > > + rte_trace_lib_eal_generic_str, > > + RTE_TRACE_POINT_ARGS(const char *str), > > + rte_trace_ctf_string(str); > > + ) > > + > > + /* Data path tracepoint */ > > + RTE_TRACE_POINT_DP( > > + rte_trace_lib_eal_generic_str, > > + RTE_TRACE_POINT_ARGS(const char *str), > > + rte_trace_ctf_string(str); > > + ) > > + > > +Emit events to trace memory > > +~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > +After trace function definition is ready to emit tracepoints. > > +To emit the event application needs to invoke tracepoint function, as > > defined > > +in the above steps, at the desired location. > > + > > +Below examples emit tracepoints in ``rte_eth_dev_configure`` to print a > > test > > +string: > > + > > +.. code-block:: c > > + > > + int > > + rte_eth_dev_configure(uint16_t port_id, uint16_t nb_rx_q, uint16_t > > nb_tx_q, > > + const struct rte_eth_conf *dev_conf) > > + { > > + struct rte_eth_dev *dev; > > + struct rte_eth_dev_info dev_info; > > + struct rte_eth_conf orig_conf; > > + int diag; > > + int ret; > > + > > + RTE_ETH_VALID_PORTID_OR_ERR_RET(port_id, -EINVAL); > > + > > + dev = &rte_eth_devices[port_id]; > > + > > + RTE_FUNC_PTR_OR_ERR_RET(*dev->dev_ops->dev_configure, -ENOTSUP); > > + > > + ... > > + > > + rte_trace_lib_eal_generic_str("tp_test_string"); > > + return ret; > > + } > > Please do not copy real code. Hello-world style is enough. > We do not want to be disturbed by the context of the code in such guide. OK. Ack. > > > + > > +Generate CTF formatted metadata > > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > +As of now emitted events just specify the debug information written by the > > +application but to view/analyze these events must be formatted into Common > > Trace > > +Format(CTF) so that any CTF compliant trace analysis tool can view those > > traces. > > + > > +Trace library exposes below API to write events to CTF formatted metadata > > file. > > + > > +.. code-block:: c > > + > > + int rte_trace_save(void); > > + > > +Currently library invokes this API implicitly during tear down and > > metadata file > > +is generated at either > > ``/root/dpdk-traces/rte-yyyy-mm-dd-[AP]M-hh-mm-ss/`` or > > +at location if user has passed during command line(``say /tmp``) then > > +``/tmp/rte-yyyy-mm-dd-[AP]M-hh-mm-ss/`` > > The trace is written in a file when the application is closing? rte_trace_save() invoked in rte_eal_cleanup() > How is it working if application crashes? Segfault should call rte_eal_cleanup() and rte_trace_save() will be called internally. > How can we debug the application in real-time? The application can call rte_trace_save() any point in time to save the trace buffers to the filesystem, > > > + > > +For more information, refer :doc:`../linux_gsg/linux_eal_parameters` for > > trace. > > + > > +View and analyze the recorded events > > +------------------------------------ > > +Once ``Trace a user application`` is completed, the user can view/inspect > > the > > +recorded events. > > + > > +There are many tools you can use to read DPDK traces: > > + > > + - ``babeltrace`` is a command-line utility that converts trace formats; it > > + supports the format that DPDK trace library produces, CTF, as well as a > > + basic text output that can be grep ed. The babeltrace command is part > > of the > > + opensource ``Babeltrace`` project. > > + > > + - ``Trace Compass`` is a graphical user interface for viewing and > > analyzing any > > + type of logs or traces, including DPDK traces. > > + > > +.. Note:: > > + > > + This section assumes that the trace library saved the traces, it > > recorded > > + during the previous tutorials, to their specified location. > > + > > + > > +Use the babeltrace command-line tool > > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > +The simplest way to list all the recorded events of a trace is to pass its > > path > > +to babeltrace with no options:: > > + > > + babeltrace </path-to-trace-events/rte-yyyy-mm-dd-[AP]M-hh-mm-ss/> > > + > > +``babeltrace`` finds all traces recursively within the given path and > > prints all > > +their events, merging them in chronological order. > > + > > +You can pipe the output of the babeltrace into a tool like grep(1) for > > further > > +filtering. Below example grep the events for ``ethdev`` only:: > > + > > + babeltrace /tmp/my-dpdk-trace | grep ethdev > > + > > +You can pipe the output of babeltrace into a tool like wc(1) to count the > > +recorded events. Below example count the number of ``ethdev`` events:: > > + > > + babeltrace /tmp/my-dpdk-trace | grep ethdev | wc --lines > > + > > +Use the tracecompass GUI tool > > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > +``Tracecompass`` is another tool to view/analyze the DPDK traces which > > gives > > +a graphical view of events. Like ``babeltrace``, tracecompass also provides > > +an interface to search for a particular event. To use ``tracecompass``, > > following are > > +the minimum required steps: > > + > > + - Install ``tracecompass`` to the localhost. Variants are available for > > Linux, > > + Windows, and OS-X. > > + - Launch ``tracecompass`` which will open a graphical window with trace > > + management interfaces. > > + - Open a trace using ``File->Open Trace`` option and select metadata file > > + which is to be viewed/analyzed. > > + > > +For more details, refer `Trace Compass > > <https://www.eclipse.org/tracecompass/>`_ > > I don't think we need to explain external tools here. They can evolve. > Giving their names and URL is enough. It takes time for someone to understand the tool usage model. So added minimum information such as " - Open a trace using ``File->Open Trace`` option and select metadata file which is to be viewed/analyzed. " > Maybe just with a short description (text vs graphical). > > > + > > +Core Concepts > > +------------- > > I think the concepts must be explained at the beginning. > > > +As DPDK trace library is designed to generate traces that uses Common Trace > > +Format(CTF). CTF specification consist of following units to create a > > trace. > > + > > + - ``Stream`` Sequence of packets. > > + - ``Packet`` Header and one or more events. > > + - ``Event`` Header and payload. > > + > > +For detailed information, refer `Common Trace Format > > <https://diamon.org/ctf/>`_ > > + > > +Channel and trace memory > > +~~~~~~~~~~~~~~~~~~~~~~~~ > > +A channel is an object which is responsible for holding the trace memory. > > +The trace library creates the trace memory per thread to enable the > > lock-less > > +scheme to emit the event. When a DPDK tracer emits an event, it will be > > recorded > > +to the trace buffers that associated with that thread. > > + > > +Event record mode > > +~~~~~~~~~~~~~~~~~ > > +Event record mode is an attribute of trace buffers. Trace library exposes > > two > > +modes: > > + > > + - ``Overwrite`` This mode enables trace buffers to wrap around when trace > > buffer memory is full. > > + - ``Discard`` This mode enables trace buffers to discard when trace > > buffer memory is full. > > + > > +This mode can be enabled/disabled either using eal command line parameters > > or > > +DPDK trace library API to configure the mode. > > +Refer :doc:`../linux_gsg/linux_eal_parameters` and trace API documentation > > more > > +details. > > + > > +Metadata > > +~~~~~~~~ > > +Metadata defines the layout of event records so that trace analysis tool > > can > > +read the streams and show into the relevant format. > > I don't understand metadata from the explanation above. > Is it useful to understand when using this library? Some understanding of CTF will be required to consume the output of tracer. More details about CTF metadata is here: https://diamon.org/ctf/ > What is the impact for the user? The last section has the implementation details. The format followed in general in the doc: - API usage model - Implementation details( in case some would like to contribute to the trace subsystem) > > > +For more details, refer `Common Trace Format <https://diamon.org/ctf/>`_. > > > Thanks for the work. > Please let's improve the doc while discussing the design. Sure. > >
