From 2c6d0950f65e537ddec96548e2f73c00e758f87e Mon Sep 17 00:00:00 2001 From: Beau Belgrave Date: Tue, 12 Sep 2023 18:07:04 +0000 Subject: tracing/user_events: Document persist event flags Users need to know how to make events persist now that we allow for that. We also now allow the dynamic_events file to create events by utilizing the persist flag during event register. Add back in to documentation how /sys/kernel/tracing/dynamic_events can be used to create persistent user_events. Add a section under registering for the currently supported flags (USER_EVENT_REG_PERSIST) and the required permissions. Add a note under deleting that deleting a persistent event also requires sufficient permission. Link: https://lkml.kernel.org/r/20230912180704.1284-4-beaub@linux.microsoft.com Signed-off-by: Beau Belgrave Signed-off-by: Steven Rostedt (Google) --- Documentation/trace/user_events.rst | 21 ++++++++++++++++++--- 1 file changed, 18 insertions(+), 3 deletions(-) (limited to 'Documentation/trace') diff --git a/Documentation/trace/user_events.rst b/Documentation/trace/user_events.rst index f9530d0ac5d3..d8f12442aaa6 100644 --- a/Documentation/trace/user_events.rst +++ b/Documentation/trace/user_events.rst @@ -14,6 +14,11 @@ Programs can view status of the events via /sys/kernel/tracing/user_events_status and can both register and write data out via /sys/kernel/tracing/user_events_data. +Programs can also use /sys/kernel/tracing/dynamic_events to register and +delete user based events via the u: prefix. The format of the command to +dynamic_events is the same as the ioctl with the u: prefix applied. This +requires CAP_PERFMON due to the event persisting, otherwise -EPERM is returned. + Typically programs will register a set of events that they wish to expose to tools that can read trace_events (such as ftrace and perf). The registration process tells the kernel which address and bit to reflect if any tool has @@ -45,7 +50,7 @@ This command takes a packed struct user_reg as an argument:: /* Input: Enable size in bytes at address */ __u8 enable_size; - /* Input: Flags for future use, set to 0 */ + /* Input: Flags to use, if any */ __u16 flags; /* Input: Address to update when enabled */ @@ -69,7 +74,7 @@ The struct user_reg requires all the above inputs to be set appropriately. This must be 4 (32-bit) or 8 (64-bit). 64-bit values are only allowed to be used on 64-bit kernels, however, 32-bit can be used on all kernels. -+ flags: The flags to use, if any. For the initial version this must be 0. ++ flags: The flags to use, if any. Callers should first attempt to use flags and retry without flags to ensure support for lower versions of the kernel. If a flag is not supported -EINVAL is returned. @@ -80,6 +85,13 @@ The struct user_reg requires all the above inputs to be set appropriately. + name_args: The name and arguments to describe the event, see command format for details. +The following flags are currently supported. + ++ USER_EVENT_REG_PERSIST: The event will not delete upon the last reference + closing. Callers may use this if an event should exist even after the + process closes or unregisters the event. Requires CAP_PERFMON otherwise + -EPERM is returned. + Upon successful registration the following is set. + write_index: The index to use for this file descriptor that represents this @@ -141,7 +153,10 @@ event (in both user and kernel space). User programs should use a separate file to request deletes than the one used for registration due to this. **NOTE:** By default events will auto-delete when there are no references left -to the event. Flags in the future may change this logic. +to the event. If programs do not want auto-delete, they must use the +USER_EVENT_REG_PERSIST flag when registering the event. Once that flag is used +the event exists until DIAG_IOCSDEL is invoked. Both register and delete of an +event that persists requires CAP_PERFMON, otherwise -EPERM is returned. Unregistering ------------- -- cgit