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tracing/user_events: Document persist event flags

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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 <beaub@linux.microsoft.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
This commit is contained in:
Beau Belgrave
2023-09-12 18:07:04 +00:00
committed by Steven Rostedt (Google)
parent cf74c59c4f
commit 2c6d0950f6
1 changed files with 18 additions and 3 deletions
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21
Documentation/trace/user_events.rst
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@ -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
-------------