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.
Signed-off-by: Beau Belgrave <be...@linux.microsoft.com>
---
Documentation/trace/user_events.rst | 21 ++++++++++++++++++---
1 file changed, 18 insertions(+), 3 deletions(-)
diff --git a/Documentation/trace/user_events.rst
b/Documentation/trace/user_events.rst
index e7b07313550a..576d2c35f22e 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
-------------
--
2.34.1
