.. _logging_api: Logging ####### .. contents:: :local: :depth: 2 The logging API provides a common interface to process messages issued by developers. Messages are passed through a frontend and are then processed by active backends. Custom frontend and backends can be used if needed. Summary of the logging features: - Deferred logging reduces the time needed to log a message by shifting time consuming operations to a known context instead of processing and sending the log message when called. - Multiple backends supported (up to 9 backends). - Custom frontend support. It can work together with backends. - Compile time filtering on module level. - Run time filtering independent for each backend. - Additional run time filtering on module instance level. - Timestamping with user provided function. Timestamp can have 32 or 64 bits. - Dedicated API for dumping data. - Dedicated API for handling transient strings. - Panic support - in panic mode logging switches to blocking, synchronous processing. - Printk support - printk message can be redirected to the logging. - Design ready for multi-domain/multi-processor system. - Support for logging floating point variables and long long arguments. - Built-in copying of transient strings used as arguments. Logging API is highly configurable at compile time as well as at run time. Using Kconfig options (see :ref:`logging_kconfig`) logs can be gradually removed from compilation to reduce image size and execution time when logs are not needed. During compilation logs can be filtered out on module basis and severity level. Logs can also be compiled in but filtered on run time using dedicate API. Run time filtering is independent for each backend and each source of log messages. Source of log messages can be a module or specific instance of the module. There are four severity levels available in the system: error, warning, info and debug. For each severity level the logging API (:zephyr_file:`include/zephyr/logging/log.h`) has set of dedicated macros. Logger API also has macros for logging data. For each level following set of macros are available: - ``LOG_X`` for standard printf-like messages, e.g. :c:macro:`LOG_ERR`. - ``LOG_HEXDUMP_X`` for dumping data, e.g. :c:macro:`LOG_HEXDUMP_WRN`. - ``LOG_INST_X`` for standard printf-like message associated with the particular instance, e.g. :c:macro:`LOG_INST_INF`. - ``LOG_INST_HEXDUMP_X`` for dumping data associated with the particular instance, e.g. :c:macro:`LOG_HEXDUMP_INST_DBG` There are two configuration categories: configurations per module and global configuration. When logging is enabled globally, it works for modules. However, modules can disable logging locally. Every module can specify its own logging level. The module must define the :c:macro:`LOG_LEVEL` macro before using the API. Unless a global override is set, the module logging level will be honored. The global override can only increase the logging level. It cannot be used to lower module logging levels that were previously set higher. It is also possible to globally limit logs by providing maximal severity level present in the system, where maximal means lowest severity (e.g. if maximal level in the system is set to info, it means that errors, warnings and info levels are present but debug messages are excluded). Each module which is using the logging must specify its unique name and register itself to the logging. If module consists of more than one file, registration is performed in one file but each file must define a module name. Logger's default frontend is designed to be thread safe and minimizes time needed to log the message. Time consuming operations like string formatting or access to the transport are not performed by default when logging API is called. When logging API is called a message is created and added to the list. Dedicated, configurable buffer for pool of log messages is used. There are 2 types of messages: standard and hexdump. Each message contain source ID (module or instance ID and domain ID which might be used for multiprocessor systems), timestamp and severity level. Standard message contains pointer to the string and arguments. Hexdump message contains copied data and string. .. _logging_kconfig: Global Kconfig Options ********************** These options can be found in the following path :zephyr_file:`subsys/logging/Kconfig`. :kconfig:option:`CONFIG_LOG`: Global switch, turns on/off the logging. Mode of operations: :kconfig:option:`CONFIG_LOG_MODE_DEFERRED`: Deferred mode. :kconfig:option:`CONFIG_LOG_MODE_IMMEDIATE`: Immediate (synchronous) mode. :kconfig:option:`CONFIG_LOG_MODE_MINIMAL`: Minimal footprint mode. Filtering options: :kconfig:option:`CONFIG_LOG_RUNTIME_FILTERING`: Enables runtime reconfiguration of the filtering. :kconfig:option:`CONFIG_LOG_DEFAULT_LEVEL`: Default level, sets the logging level used by modules that are not setting their own logging level. :kconfig:option:`CONFIG_LOG_OVERRIDE_LEVEL`: It overrides module logging level when it is not set or set lower than the override value. :kconfig:option:`CONFIG_LOG_MAX_LEVEL`: Maximal (lowest severity) level which is compiled in. Processing options: :kconfig:option:`CONFIG_LOG_MODE_OVERFLOW`: When new message cannot be allocated, oldest one are discarded. :kconfig:option:`CONFIG_LOG_BLOCK_IN_THREAD`: If enabled and new log message cannot be allocated thread context will block for up to :kconfig:option:`CONFIG_LOG_BLOCK_IN_THREAD_TIMEOUT_MS` or until log message is allocated. :kconfig:option:`CONFIG_LOG_PRINTK`: Redirect printk calls to the logging. :kconfig:option:`CONFIG_LOG_PROCESS_TRIGGER_THRESHOLD`: When number of buffered log messages reaches the threshold dedicated thread (see :c:func:`log_thread_set`) is waken up. If :kconfig:option:`CONFIG_LOG_PROCESS_THREAD` is enabled then this threshold is used by the internal thread. :kconfig:option:`CONFIG_LOG_PROCESS_THREAD`: When enabled, logging thread is created which handles log processing. :kconfig:option:`CONFIG_LOG_PROCESS_THREAD_STARTUP_DELAY_MS`: Delay in milliseconds after which logging thread is started. :kconfig:option:`CONFIG_LOG_BUFFER_SIZE`: Number of bytes dedicated for the circular packet buffer. :kconfig:option:`CONFIG_LOG_DOMAIN_ID`: Domain ID. Valid in multi-domain systems. :kconfig:option:`CONFIG_LOG_FRONTEND`: Direct logs to a custom frontend. :kconfig:option:`CONFIG_LOG_FRONTEND_ONLY`: No backends are used when messages goes to frontend. :kconfig:option:`CONFIG_LOG_TIMESTAMP_64BIT`: 64 bit timestamp. Formatting options: :kconfig:option:`CONFIG_LOG_FUNC_NAME_PREFIX_ERR`: Prepend standard ERROR log messages with function name. Hexdump messages are not prepended. :kconfig:option:`CONFIG_LOG_FUNC_NAME_PREFIX_WRN`: Prepend standard WARNING log messages with function name. Hexdump messages are not prepended. :kconfig:option:`CONFIG_LOG_FUNC_NAME_PREFIX_INF`: Prepend standard INFO log messages with function name. Hexdump messages are not prepended. :kconfig:option:`CONFIG_LOG_FUNC_NAME_PREFIX_DBG`: Prepend standard DEBUG log messages with function name. Hexdump messages are not prepended. :kconfig:option:`CONFIG_LOG_BACKEND_SHOW_COLOR`: Enables coloring of errors (red) and warnings (yellow). :kconfig:option:`CONFIG_LOG_BACKEND_FORMAT_TIMESTAMP`: If enabled timestamp is formatted to *hh:mm:ss:mmm,uuu*. Otherwise is printed in raw format. Backend options: :kconfig:option:`CONFIG_LOG_BACKEND_UART`: Enabled built-in UART backend. .. _log_usage: Usage ***** Logging in a module =================== In order to use logging in the module, a unique name of a module must be specified and module must be registered using :c:macro:`LOG_MODULE_REGISTER`. Optionally, a compile time log level for the module can be specified as the second parameter. Default log level (:kconfig:option:`CONFIG_LOG_DEFAULT_LEVEL`) is used if custom log level is not provided. .. code-block:: c #include LOG_MODULE_REGISTER(foo, CONFIG_FOO_LOG_LEVEL); If the module consists of multiple files, then ``LOG_MODULE_REGISTER()`` should appear in exactly one of them. Each other file should use :c:macro:`LOG_MODULE_DECLARE` to declare its membership in the module. Optionally, a compile time log level for the module can be specified as the second parameter. Default log level (:kconfig:option:`CONFIG_LOG_DEFAULT_LEVEL`) is used if custom log level is not provided. .. code-block:: c #include /* In all files comprising the module but one */ LOG_MODULE_DECLARE(foo, CONFIG_FOO_LOG_LEVEL); In order to use logging API in a function implemented in a header file :c:macro:`LOG_MODULE_DECLARE` macro must be used in the function body before logging API is called. Optionally, a compile time log level for the module can be specified as the second parameter. Default log level (:kconfig:option:`CONFIG_LOG_DEFAULT_LEVEL`) is used if custom log level is not provided. .. code-block:: c #include static inline void foo(void) { LOG_MODULE_DECLARE(foo, CONFIG_FOO_LOG_LEVEL); LOG_INF("foo"); } Dedicated Kconfig template (:zephyr_file:`subsys/logging/Kconfig.template.log_config`) can be used to create local log level configuration. Example below presents usage of the template. As a result CONFIG_FOO_LOG_LEVEL will be generated: .. code-block:: none module = FOO module-str = foo source "subsys/logging/Kconfig.template.log_config" Logging in a module instance ============================ In case of modules which are multi-instance and instances are widely used across the system enabling logs will lead to flooding. Logger provide the tools which can be used to provide filtering on instance level rather than module level. In that case logging can be enabled for particular instance. In order to use instance level filtering following steps must be performed: - a pointer to specific logging structure is declared in instance structure. :c:macro:`LOG_INSTANCE_PTR_DECLARE` is used for that. .. code-block:: c #include struct foo_object { LOG_INSTANCE_PTR_DECLARE(log); uint32_t id; } - module must provide macro for instantiation. In that macro, logging instance is registered and log instance pointer is initialized in the object structure. .. code-block:: c #define FOO_OBJECT_DEFINE(_name) \ LOG_INSTANCE_REGISTER(foo, _name, CONFIG_FOO_LOG_LEVEL) \ struct foo_object _name = { \ LOG_INSTANCE_PTR_INIT(log, foo, _name) \ } Note that when logging is disabled logging instance and pointer to that instance are not created. In order to use the instance logging API in a source file, a compile-time log level must be set using :c:macro:`LOG_LEVEL_SET`. .. code-block:: c LOG_LEVEL_SET(CONFIG_FOO_LOG_LEVEL); void foo_init(foo_object *f) { LOG_INST_INF(f->log, "Initialized."); } In order to use the instance logging API in a header file, a compile-time log level must be set using :c:macro:`LOG_LEVEL_SET`. .. code-block:: c static inline void foo_init(foo_object *f) { LOG_LEVEL_SET(CONFIG_FOO_LOG_LEVEL); LOG_INST_INF(f->log, "Initialized."); } Controlling the logging ======================= Logging can be controlled using API defined in :zephyr_file:`include/zephyr/logging/log_ctrl.h`. Logger must be initialized before it can be used. Optionally, user can provide function which returns timestamp value. If not provided, :c:macro:`k_cycle_get_32` is used for timestamping. :c:func:`log_process` function is used to trigger processing of one log message (if pending). Function returns true if there is more messages pending. Following snippet shows how logging can be processed in simple forever loop. .. code-block:: c #include void main(void) { log_init(); while (1) { if (log_process() == false) { /* sleep */ } } } If logs are processed from a thread then it is possible to enable a feature which will wake up processing thread when certain amount of log messages are buffered (see :kconfig:option:`CONFIG_LOG_PROCESS_TRIGGER_THRESHOLD`). It is also possible to enable internal logging thread (see :kconfig:option:`CONFIG_LOG_PROCESS_THREAD`). In that case, logging thread is initialized and log messages are processed implicitly. .. _logging_panic: Logging panic ************* In case of error condition system usually can no longer rely on scheduler or interrupts. In that situation deferred log message processing is not an option. Logger controlling API provides a function for entering into panic mode (:c:func:`log_panic`) which should be called in such situation. When :c:func:`log_panic` is called, _panic_ notification is sent to all active backends. Once all backends are notified, all buffered messages are flushed. Since that moment all logs are processed in a blocking way. .. _log_architecture: Architecture ************ Logging consists of 3 main parts: - Frontend - Core - Backends Log message is generated by a source of logging which can be a module or instance of a module. Default Frontend ================ Default frontend is engaged when logging API is called in a source of logging (e.g. :c:macro:`LOG_INF`) and is responsible for filtering a message (compile and run time), allocating buffer for the message, creating the message and committing that message. Since logging API can be called in an interrupt, frontend is optimized to log the message as fast as possible. Log message ----------- Log message contains message descriptor (source, domain and level), timestamp, formatted string details (see :ref:`cbprintf_packaging`) and optional data. Log messages are stored in a continuous block of memory. Memory is allocated from a circular packet buffer (:ref:`mpsc_pbuf`). It has few consequences: * Each message is self-contained, continuous block of memory thus it is suited for copying the message (e.g. for offline processing). * Messages must be sequentially freed. Backend processing is synchronous. Backend can make a copy for deferred processing. Log message has following format: +------------------+----------------------------------------------------+ | Message Header | 2 bits: MPSC packet buffer header | | +----------------------------------------------------+ | | 1 bit: Trace/Log message flag | | +----------------------------------------------------+ | | 3 bits: Domain ID | | +----------------------------------------------------+ | | 3 bits: Level | | +----------------------------------------------------+ | | 10 bits: Cbprintf Package Length | | +----------------------------------------------------+ | | 12 bits: Data length | | +----------------------------------------------------+ | | 1 bit: Reserved | | +----------------------------------------------------+ | | pointer: Pointer to the source descriptor [#l0]_ | | +----------------------------------------------------+ | | 32 or 64 bits: Timestamp [#l0]_ | | +----------------------------------------------------+ | | Optional padding [#l1]_ | +------------------+----------------------------------------------------+ | Cbprintf | Header | | +----------------------------------------------------+ | | package | Arguments | | | (optional) +----------------------------------------------------+ | | Appended strings | +------------------+----------------------------------------------------+ | Hexdump data (optional) | +------------------+----------------------------------------------------+ | Alignment padding (optional) | +------------------+----------------------------------------------------+ .. rubric:: Footnotes .. [#l0] Depending on the platform and the timestamp size fields may be swapped. .. [#l1] It may be required for cbprintf package alignment Log message allocation ---------------------- It may happen that frontend cannot allocate a message. It happens if system is generating more log messages than it can process in certain time frame. There are two strategies to handle that case: - No overflow - new log is dropped if space for a message cannot be allocated. - Overflow - oldest pending messages are freed, until new message can be allocated. Enabled by :kconfig:option:`CONFIG_LOG_MODE_OVERFLOW`. Note that it degrades performance thus it is recommended to adjust buffer size and amount of enabled logs to limit dropping. .. _logging_runtime_filtering: Run-time filtering ------------------ If run-time filtering is enabled, then for each source of logging a filter structure in RAM is declared. Such filter is using 32 bits divided into ten 3 bit slots. Except *slot 0*, each slot stores current filter for one backend in the system. *Slot 0* (bits 0-2) is used to aggregate maximal filter setting for given source of logging. Aggregate slot determines if log message is created for given entry since it indicates if there is at least one backend expecting that log entry. Backend slots are examined when message is processed by the core to determine if message is accepted by the given backend. Contrary to compile time filtering, binary footprint is increased because logs are compiled in. In the example below backend 1 is set to receive errors (*slot 1*) and backend 2 up to info level (*slot 2*). Slots 3-9 are not used. Aggregated filter (*slot 0*) is set to info level and up to this level message from that particular source will be buffered. +------+------+------+------+-----+------+ |slot 0|slot 1|slot 2|slot 3| ... |slot 9| +------+------+------+------+-----+------+ | INF | ERR | INF | OFF | ... | OFF | +------+------+------+------+-----+------+ Custom Frontend =============== Custom frontend is enabled using :kconfig:option:`CONFIG_LOG_FRONTEND`. Logs are directed to functions declared in :zephyr_file:`include/zephyr/logging/log_frontend.h`. If option :kconfig:option:`CONFIG_LOG_FRONTEND_ONLY` is enabled then log message is not created and no backend is handled. Otherwise, custom frontend can coexist with backends. .. _logging_strings: Logging strings =============== String arguments are handled by :ref:`cbprintf_packaging` thus no special action is required. Strings which are in read write memory are appended to the log message. Logging backends ================ Logging backends are registered using :c:macro:`LOG_BACKEND_DEFINE`. The macro creates an instance in the dedicated memory section. Backends can be dynamically enabled (:c:func:`log_backend_enable`) and disabled. When :ref:`logging_runtime_filtering` is enabled, :c:func:`log_filter_set` can be used to dynamically change filtering of a module logs for given backend. Module is identified by source ID and domain ID. Source ID can be retrieved if source name is known by iterating through all registered sources. Logging supports up to 9 concurrent backends. Log message is passed to the each backend in processing phase. Additionally, backend is notified when logging enter panic mode with :c:func:`log_backend_panic`. On that call backend should switch to synchronous, interrupt-less operation or shut down itself if that is not supported. Occasionally, logging may inform backend about number of dropped messages with :c:func:`log_backend_dropped`. Message processing API is version specific. :c:func:`log_backend_msg2_process` is used for processing message. It is common for standard and hexdump messages because log message hold string with arguments and data. It is also common for deferred and immediate logging. Message formatting ------------------ Logging provides set of function that can be used by the backend to format a message. Helper functions are available in :zephyr_file:`include/zephyr/logging/log_output.h`. Example message formatted using :c:func:`log_output_msg2_process`. .. code-block:: console [00:00:00.000,274] sample_instance.inst1: logging message .. _logging_guide_dictionary: Dictionary-based Logging ======================== Dictionary-based logging, instead of human readable texts, outputs the log messages in binary format. This binary format encodes arguments to formatted strings in their native storage formats which can be more compact than their text equivalents. For statically defined strings (including the format strings and any string arguments), references to the ELF file are encoded instead of the whole strings. A dictionary created at build time contains the mappings between these references and the actual strings. This allows the offline parser to obtain the strings from the dictionary to parse the log messages. This binary format allows a more compact representation of log messages in certain scenarios. However, this requires the use of an offline parser and is not as intuitive to use as text-based log messages. Note that ``long double`` is not supported by Python's ``struct`` module. Therefore, log messages with ``long double`` will not display the correct values. Configuration ------------- Here are kconfig options related to dictionary-based logging: - :kconfig:option:`CONFIG_LOG_DICTIONARY_SUPPORT` enables dictionary-based logging support. This should be selected by the backends which require it. - The UART backend can be used for dictionary-based logging. These are additional config for the UART backend: - :kconfig:option:`CONFIG_LOG_BACKEND_UART_OUTPUT_DICTIONARY_HEX` tells the UART backend to output hexadecimal characters for dictionary based logging. This is useful when the log data needs to be captured manually via terminals and consoles. - :kconfig:option:`CONFIG_LOG_BACKEND_UART_OUTPUT_DICTIONARY_BIN` tells the UART backend to output binary data. Usage ----- When dictionary-based logging is enabled via enabling related logging backends, a JSON database file, named :file:`log_dictionary.json`, will be created in the build directory. This database file contains information for the parser to correctly parse the log data. Note that this database file only works with the same build, and cannot be used for any other builds. To use the log parser: .. code-block:: console ./scripts/logging/dictionary/log_parser.py /log_dictionary.json The parser takes two required arguments, where the first one is the full path to the JSON database file, and the second part is the file containing log data. Add an optional argument ``--hex`` to the end if the log data file contains hexadecimal characters (e.g. when ``CONFIG_LOG_BACKEND_UART_OUTPUT_DICTIONARY_HEX=y``). This tells the parser to convert the hexadecimal characters to binary before parsing. Please refer to :ref:`logging_dictionary_sample` on how to use the log parser. Recommendations *************** The are following recommendations: * Enable :kconfig:option:`CONFIG_LOG_SPEED` to slightly speed up deferred logging at the cost of slight increase in memory footprint. * Compiler with C11 ``_Generic`` keyword support is recommended. Logging performance is significantly degraded without it. See :ref:`cbprintf_packaging`. * When C11 ``_Generic`` is used, it is recommended to cast pointer to ``const char *`` when it is used with ``%s`` format specifier and it points to a constant string. * When C11 ``_Generic`` is used, it is recommended to cast pointer to ``char *`` when it is used with ``%s`` format specifier and it points to a transient string. * When C11 ``_Generic`` is used, it is recommended to cast character pointer to non character pointer (e.g., ``void *``) when it is used with ``%p`` format specifier. .. code-block:: c LOG_WRN("%s", str); LOG_WRN("%p", (void *)str); Benchmark ********* Benchmark numbers from :zephyr_file:`tests/subsys/logging/log_benchmark` performed on ``qemu_x86``. It is a rough comparison to give a general overview. +--------------------------------------------+------------------+ | Feature | | +============================================+==================+ | Kernel logging | 7us [#f0]_/11us | | | | +--------------------------------------------+------------------+ | User logging | 13us | | | | +--------------------------------------------+------------------+ | kernel logging with overwrite | 10us [#f0]_/15us | +--------------------------------------------+------------------+ | Logging transient string | 42us | +--------------------------------------------+------------------+ | Logging transient string from user | 50us | +--------------------------------------------+------------------+ | Memory utilization [#f1]_ | 518 | | | | +--------------------------------------------+------------------+ | Memory footprint (test) [#f2]_ | 2k | +--------------------------------------------+------------------+ | Memory footprint (application) [#f3]_ | 3.5k | +--------------------------------------------+------------------+ | Message footprint [#f4]_ | 47 [#f0]_/32 | | | bytes | +--------------------------------------------+------------------+ .. rubric:: Benchmark details .. [#f0] :kconfig:option:`CONFIG_LOG_SPEED` enabled. .. [#f1] Number of log messages with various number of arguments that fits in 2048 bytes dedicated for logging. .. [#f2] Logging subsystem memory footprint in :zephyr_file:`tests/subsys/logging/log_benchmark` where filtering and formatting features are not used. .. [#f3] Logging subsystem memory footprint in :zephyr_file:`samples/subsys/logging/logger`. .. [#f4] Average size of a log message (excluding string) with 2 arguments on ``Cortex M3`` Stack usage *********** When logging is enabled it impacts stack usage of the context that uses logging API. If stack is optimized it may lead to stack overflow. Stack usage depends on mode and optimization. It also significantly varies between platforms. In general, when :kconfig:option:`CONFIG_LOG_MODE_DEFERRED` is used stack usage is smaller since logging is limited to creating and storing log message. When :kconfig:option:`CONFIG_LOG_MODE_IMMEDIATE` is used then log message is processed by the backend which includes string formatting. In case of that mode, stack usage will depend on which backends are used. :zephyr_file:`tests/subsys/logging/log_stack` test is used to characterize stack usage depending on mode, optimization and platform used. Test is using only the default backend. Some of the platforms characterization for log message with two ``integer`` arguments listed below: +---------------+----------+----------------------------+-----------+-----------------------------+ | Platform | Deferred | Deferred (no optimization) | Immediate | Immediate (no optimization) | +===============+==========+============================+===========+=============================+ | ARM Cortex-M3 | 40 | 152 | 412 | 783 | +---------------+----------+----------------------------+-----------+-----------------------------+ | x86 | 12 | 224 | 388 | 796 | +---------------+----------+----------------------------+-----------+-----------------------------+ | riscv32 | 24 | 208 | 456 | 844 | +---------------+----------+----------------------------+-----------+-----------------------------+ | xtensa | 72 | 336 | 504 | 944 | +---------------+----------+----------------------------+-----------+-----------------------------+ | x86_64 | 32 | 528 | 1088 | 1440 | +---------------+----------+----------------------------+-----------+-----------------------------+ API Reference ************* Logger API ========== .. doxygengroup:: log_api Logger control ============== .. doxygengroup:: log_ctrl Log message =========== .. doxygengroup:: log_msg Logger backend interface ======================== .. doxygengroup:: log_backend Logger output formatting ======================== .. doxygengroup:: log_output