Adds documentation about the safe_wtd and safe_wtd_nwo RV monitors, and their usage via a safety application. Cc: Wim Van Sebroeck <wim@xxxxxxxxxxxxxxxxxx> Cc: Guenter Roeck <linux@xxxxxxxxxxxx> Cc: Jonathan Corbet <corbet@xxxxxxx> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx> Cc: Ingo Molnar <mingo@xxxxxxxxxx> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: Will Deacon <will@xxxxxxxxxx> Cc: Catalin Marinas <catalin.marinas@xxxxxxx> Cc: Marco Elver <elver@xxxxxxxxxx> Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx> Cc: Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx> Cc: Gabriele Paoloni <gpaoloni@xxxxxxxxxx> Cc: Juri Lelli <juri.lelli@xxxxxxxxxx> Cc: Clark Williams <williams@xxxxxxxxxx> Cc: linux-doc@xxxxxxxxxxxxxxx Cc: linux-kernel@xxxxxxxxxxxxxxx Cc: linux-trace-devel@xxxxxxxxxxxxxxx Signed-off-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx> --- Documentation/trace/rv/watchdog-monitor.rst | 307 ++++++++++++++++++++ 1 file changed, 307 insertions(+) create mode 100644 Documentation/trace/rv/watchdog-monitor.rst diff --git a/Documentation/trace/rv/watchdog-monitor.rst b/Documentation/trace/rv/watchdog-monitor.rst new file mode 100644 index 000000000000..98d5bdd0ed85 --- /dev/null +++ b/Documentation/trace/rv/watchdog-monitor.rst @@ -0,0 +1,307 @@ +Watchdog monitor +---------------- + +The watchdog is an essential building block for the usage of Linux in +safety-critical systems because it allows the system to be monitored from +an external element - the watchdog hardware, acting as a safety-monitor. + +A user-space application controls the watchdog device via the watchdog +interface. This application, hereafter safety_app, enables the watchdog +and periodically pets the watchdog upon correct completion of the safety +related processing. + +If the safety_app, for any reason, stops pinging the watchdog, +the watchdog hardware can set the system in a fail-safe state. For +example, shutting the system down. + +Given the importance of the safety_app / watchdog hardware couple, +the interaction between these software pieces also needs some +sort of monitoring. In other words, "who monitors the monitor?" + +The safe watchdog (safe_wtd) RV monitor monitors the interaction between +the safety_app and the watchdog device, enforcing the correct sequence of +events that leads the system to a safe state. + +Furthermore, the safety_app can monitor the RV monitor by collecting the +events generated by the RV monitor itself via tracing interface. In this way, +closing the monitoring loop with the safety_app. + +A diagram of the components and their interactions is:: + + user-space: + +--------------------------------+ + | safety_app |-----------+ + +--------------------------------+ | + | ^ | + | Configure | Enable and | + | | check data | + ===================+====================+=============== | + kernel-space: | | | + v v | + +----------+ instr. +-------------+ | + | watchdog | ----------->| RV Monitor |----+ | + | device | +-------------+ | | + +----------+ | | + | | | + | | | + ================+====================================== | | + hardware: | | | + v | +-> Bring the system + +--------------------+ +----> to a safe state, + | watchdog hardware |---------------------------> e.g., halt. + +--------------------+ + +Sample safety_app +----------------- + +The user-space safety_app sample code in ``tools/tracing/rv/safety_app/`` +serves to illustrate the usage of the RV monitors for this use-case, as +well as the starting point to the development of a user-specific safety_app. + +Watchdog events +--------------- + +The RV monitor observes the watchdog by using instrumentation to +process the events generated by the interaction between the +safety_app and the watchdog device layer in kernel. + +The monitored events are: + + - watchdog:watchdog_open: open the watchdog device; + - watchdog:watchdog_close: close the watchdog device; + - watchdog:watchdog_start: start the watchdog; + - watchdog:watchdog_stop: stop the watchdog; + - watchdog:watchdog_set_timeout: set the watchdog timeout; + - watchdog:watchdog_ping: reprogram the watchdog with the previously set + timeout; + - watchdog:watchdog_nowayout: prevents the watchdog from stopping; + - watchdog:watchdog_set_keep_alive: set an intermediary ping to overcome + the limitation of a hardware watchdog maximum timeout being shorter than + the timeout set by the user-space tool; + - watchdog:watchdog_keep_alive: the execution of the function that runs the + intermediary keep alive ping; + +RV monitor events +----------------- + +The RV monitor monitors the relevant events as an outside observer, +interpreting all the components (the hardware; the watchdog device +interface; and the safety monitor) as an integrated component. + +The events selected for the monitor are: + + - other_threads: an event generated by any thread other than the + one that set nowayout or open the watchdog the last time. + - open: a thread opens the watchdog to manipulate it; + - close: a thread closes the watchdog; + - start: starts the watchdog countdown; + - stop: stops the watchdog; + - set_safe_timeout: configures the watchdog with a given timeout; + - ping: resets the watchdog countdown with the previously configured timeout; + - nowayout: prevents the watchdog to be stopped until the system's shutdown; + - sched_keep_alive: schedules a kernel worker to ping the watchdog if the + timeout is longer than the watchdog hardware can handle. + - keep_alive: executes the previously scheduled watchdog ping; + +Noting that the events that does not appear in the automata models are +considered blocked events, and their execution will always cause the +RV monitor to react to an unexpected event. + +RV monitor specification +------------------------ + +The monitor's goal is to assess a set of specifications that conducts the +system to a safe state. + +These specifications are: + + - 1: Once open, only one process manipulates the watchdog; + - 2: Following 1, the keep-alive mechanisms will not be used; + - 3: If required, nowayout will be set before opening the watchdog; + - 4: A safe timeout must be set; + - 5: At least one ping must be made before entering the safe/safe_nwo states + - 6: The The RV monitor does not react if the watchdog is closed without stopping. + But the hardware watchdog is expected to react. + +Deterministic automata monitors +------------------------------- + +Following the specifications, two deterministic automata monitors +were developed. The are the safe_wtd and the safe_wtd_nwo monitors. +The monitors are modeled as Deterministic Automata models. + +The difference between the safe_wtd and the safe_wtd_nwo is that +the latter enforces the usage of the watchdog's nowayout option +to reach the safe state. + +The deterministic automata model for safe_wtd is:: + + #==================================# other_threads + H H ----------------+ + -----------> H init H | + H H <---------------+ + #==================================# + | | ^ + | | | close + | | +----------------------------------------------------+ + | | | + | | open | + | +------------------------------------------------------+ | + | | | + | nowayout | | + v | | + nowayout +-------------------+ | | + other_threads | | nowayout | | + +---------------- | nwo |<-------------------------------------+ | | + | | | | | | + +---------------> | | <+ | | | + +-------------------+ | | | | + | | | | | + | open | close | | | + v | | | | + +-------------------+ | | | | + | opened_nwo | -+ | | | + +-------------------+ | | | + | | | | + | start | | | + v | | | + +-------------------+ | | | + +---------------> | started_nwo | -+ | | | + | +-------------------+ | | | | + | | | | | | + | open | set_safe_timeout | | | | + | v | | | | + | +-------------------+ | | | | + | | set_nwo | | | | | + | +-------------------+ | | | | + | | | | | | + | +-------------+ | ping | | | | + | | | | | | | | + | | ping v v | | | | + | | +-------------------+ | | | | + | +-----------| safe_nwo | | | | | + | +-------------------+ | | | | + | | | | | | + | | close | close | | | + | v v | | | + | +----------------------------------+ nowayout | | | + | | | other_threads | | | + | | closed_running_nwo | ----------------+ | | | + | | | | | | | + +---------------- | | <---------------+ | | | + +----------------------------------+ | | | + | nowayout ^ | | | + +-----------------------------+ | | | + | | | + | | | + +-------------------+ +--------+ | | | + | | | |------+---+ | + | started | start | opened | | | + +---------------- | | <-------- | |>-----+-------+ + | +-------------------+ +--------+ | ^ + | | | | + | | set_safe_timeout +-------------+-------+ + | v | | + | +-------------------+ | | + | | | | | + | | set | | | + +----------+---------------> | | | | + | | +-------------------+ | | + | | | | | + | | | ping | | + | | v | | + | | +-------------------+ ping | | + | | | | -------+ | | + | | +---- | safe | | | | + | | | | | <------+ | | + | | | +-------------------+ | | + | | | | | | + | | stop | | stop | | + | | | v | | + | | | +-------------------+ close | | + | +-----------+---> | stopped |-------------+ | + | | +-------------------+ | + | +---+ | + | | close | + | v | + | other_threads +----------------------------------------+ | + | +--------------> | | | + | | | closed_running | | + | +--------------- | |--------------+ + | +----------------------------------------+ + | | ^ + | open | | close + | v | + | set_safe_timeout +-------------------+ + +-------------------------> | reopened | + +-------------------+ + +The deterministic automata model for safe_wtd_nwo is:: + + | + | + v + #===================# + H init H + #===================# + | + | nowayout + v + nowayout +-------------------+ + other_threads | | + +---------------- | nwo | + | | | + +---------------> | | <---+ + +-------------------+ | + | | + | open | close + v | + +-------------------+ | + | opened | ----+ + +-------------------+ + | + | start + v + +-------------------+ + +---------------> | started | ---+ + | +-------------------+ | + | | | + | open | set_safe_timeout | + | v | + | +-------------------+ | + | | set | | + | +-------------------+ | + | | | + | +------------+ | ping | + | | v v | + | | ping +-------------------+ | + | +---------| safe | | + | +-------------------+ | + | | | + | | close | close + | v | + | +-------------------+ | + +---------------- | closed_running | <--+ + +-------------------+ + ^ | + | nowayout | + | other_threads | + +---------------+ + +It is important to note that the events sched_keep_alive and keep_alive +are not allowed on any of the present monitors (they are said to be blocked +events). The execution of any blocked events leads the RV monitor to react. + +Additional options +------------------ + +The RV monitor also has a set of options enabled via kernel command +line/module options. They are: + + - watchdog_id: the device id to monitor (default 0); + - dont_stop: once enabled, do not allow the RV monitor to be stopped (default off); + - safe_timeout: define a maximum safe value that a user-space application can + set as the watchdog timeout (default unlimited); + - check_timeout: After every ping, check if the time left in the watchdog is less + than or equal to the last timeout set for the watchdog. It only works for watchdog + devices that provide the get_timeleft() function (default off); -- 2.33.1
