From: Philippe Proulx Date: Mon, 31 Jul 2017 22:11:48 +0000 (-0400) Subject: 2.10: document the notification and trigger API, --monitor-timer option X-Git-Url: https://git.lttng.org./?a=commitdiff_plain;h=90c4e38a967f3ad7f22c404eab9b6e6b0c7d6df0;p=lttng-docs.git 2.10: document the notification and trigger API, --monitor-timer option Signed-off-by: Philippe Proulx --- diff --git a/2.10/lttng-docs-2.10.txt b/2.10/lttng-docs-2.10.txt index 12da1e3..6c54939 100644 --- a/2.10/lttng-docs-2.10.txt +++ b/2.10/lttng-docs-2.10.txt @@ -1,7 +1,7 @@ The LTTng Documentation ======================= Philippe Proulx -v2.10, 25 July 2017 +v2.10, 31 July 2017 include::../common/copyright.txt[] @@ -111,7 +111,9 @@ $ lttng enable-event --userspace '*_my_org:*msg*' <> buffer usage conditions are available. Documentation is available in the https://github.com/lttng/lttng-tools/tree/stable-{revision}/include/lttng[`liblttng-ctl` - header files]. + header files] and in + <>. ** You can now embed the whole textual LTTng-tools man pages into the executables at build time with the `--enable-embedded-help` @@ -6736,6 +6738,252 @@ $ lttng-crash --extract=/path/to/trace /path/to/shm -- +[role="since-2.10"] +[[notif-trigger-api]] +=== Get notified when a channel's buffer usage is too high or too low + +With LTTng's $$C/C++$$ notification and trigger API, your user +application can get notified when the buffer usage of one or more +<> becomes too low or too high. You can use this API +and enable or disable <> during tracing to avoid +<>. + +.Have a user application get notified when an LTTng channel's buffer usage is too high. +==== +In this example, we create and build an application which gets notified +when the buffer usage of a specific LTTng channel is higher than +75{nbsp}%. We only print that it is the case in the example, but we +could as well use the API of <> to +disable event rules when this happens. + +. Create the application's C source file: ++ +-- +[source,c] +.path:{notif-app.c} +---- +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include + +int main(int argc, char *argv[]) +{ + int exit_status = 0; + struct lttng_notification_channel *notification_channel; + struct lttng_condition *condition; + struct lttng_action *action; + struct lttng_trigger *trigger; + const char *tracing_session_name; + const char *channel_name; + + assert(argc >= 3); + tracing_session_name = argv[1]; + channel_name = argv[2]; + + /* + * Create a notification channel. A notification channel + * connects the user application to the LTTng session daemon. + * This notification channel can be used to listen for various + * types of notifications. + */ + notification_channel = lttng_notification_channel_create( + lttng_session_daemon_notification_endpoint); + + /* + * Create a "high buffer usage" condition. In this case, the + * condition is reached when the buffer usage is greater than or + * equal to 75 %. We create the condition for a specific session + * name, channel name, and for the user space tracing domain. + * + * The "low buffer usage" condition type also exists. + */ + condition = lttng_condition_buffer_usage_high_create(); + lttng_condition_buffer_usage_set_threshold_ratio(condition, .75); + lttng_condition_buffer_usage_set_session_name( + condition, tracing_session_name); + lttng_condition_buffer_usage_set_channel_name(condition, + channel_name); + lttng_condition_buffer_usage_set_domain_type(condition, + LTTNG_DOMAIN_UST); + + /* + * Create an action (get a notification) to take when the + * condition created above is reached. + */ + action = lttng_action_notify_create(); + + /* + * Create a trigger. A trigger associates a condition to an + * action: the action is executed when the condition is reached. + */ + trigger = lttng_trigger_create(condition, action); + + /* Register the trigger to LTTng. */ + lttng_register_trigger(trigger); + + /* + * Now that we have registered a trigger, a notification will be + * emitted everytime its condition is met. To receive this + * notification, we must subscribe to notifications that match + * the same condition. + */ + lttng_notification_channel_subscribe(notification_channel, condition); + + /* + * Notification loop. This can be in a dedicated thread to avoid + * blocking the main thread. + */ + for (;;) { + struct lttng_notification *notification; + enum lttng_notification_channel_status status; + const struct lttng_evaluation *notification_evaluation; + const struct lttng_condition *notification_condition; + double buffer_usage; + + /* Receive the next notification. */ + status = lttng_notification_channel_get_next_notification( + notification_channel, + ¬ification); + + switch (status) { + case LTTNG_NOTIFICATION_CHANNEL_STATUS_OK: + break; + case LTTNG_NOTIFICATION_CHANNEL_STATUS_NOTIFICATIONS_DROPPED: + /* + * The session daemon can drop notifications if + * a monitoring application is not consuming the + * notifications fast enough. + */ + continue; + case LTTNG_NOTIFICATION_CHANNEL_STATUS_CLOSED: + /* + * The notification channel has been closed by the + * session daemon. This is typically caused by a session + * daemon shutting down (cleanly or because of a crash). + */ + goto end; + default: + /* Unhandled conditions or errors. */ + exit_status = 1; + goto end; + } + + /* + * A notification provides, amongst other things: + * + * * The condition that caused this notification to be + * emitted. + * * The condition evaluation, which provides more + * specific information on the evaluation of the + * condition. + * + * The condition evaluation provides the buffer usage + * value at the moment the condition was met. + */ + notification_condition = lttng_notification_get_condition( + notification); + notification_evaluation = lttng_notification_get_evaluation( + notification); + + /* We're subscribed to only one condition. */ + assert(lttng_condition_get_type(notification_condition) == + LTTNG_CONDITION_TYPE_BUFFER_USAGE_HIGH); + + /* + * Get the exact sampled buffer usage from the + * condition evaluation. + */ + lttng_evaluation_buffer_usage_get_usage_ratio( + notification_evaluation, &buffer_usage); + + /* + * At this point, instead of printing a message, we + * could do something to reduce the channel's buffer + * usage, like disable specific events. + */ + printf("Buffer usage is %f %% in tracing session \"%s\", " + "user space channel \"%s\".\n", + buffer_usage * 100, tracing_session_name, + channel_name); + lttng_notification_destroy(notification); + } + +end: + lttng_action_destroy(action); + lttng_condition_destroy(condition); + lttng_trigger_destroy(trigger); + lttng_notification_channel_destroy(notification_channel); + return exit_status; +} +---- +-- + +. Build the `notif-app` application, linking it to `liblttng-ctl`: ++ +-- +[role="term"] +---- +$ gcc -o notif-app notif-app.c -llttng-ctl +---- +-- + +. <>, + <> matching all the + user space tracepoints, and + <>: ++ +-- +[role="term"] +---- +$ lttng create my-session +$ lttng enable-event --userspace --all +$ lttng start +---- +-- ++ +If you create the channel manually with the man:lttng-enable-channel(1) +command, you can control how frequently are the current values of the +channel's properties sampled to evaluate user conditions with the +opt:lttng-enable-channel(1):--monitor-timer option. + +. Run the `notif-app` application. This program accepts the + <> name and the user space channel + name as its two first arguments. The channel which LTTng automatically + creates with the man:lttng-enable-event(1) command above is named + `channel0`: ++ +-- +[role="term"] +---- +$ ./notif-app my-session channel0 +---- +-- + +. In another terminal, run an application with a very high event + throughput so that the 75{nbsp}% buffer usage condition is reached. ++ +In the first terminal, the application should print lines like this: ++ +---- +Buffer usage is 81.45197 % in tracing session "my-session", user space +channel "channel0". +---- ++ +If you don't see anything, try modifying the condition in +path:{notif-app.c} to a lower value (0.1, for example), rebuilding it +(step 2) and running it again (step 4). +==== + + [[reference]] == Reference