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authorLinus Torvalds <torvalds@linux-foundation.org>2022-03-23 21:40:25 +0300
committerLinus Torvalds <torvalds@linux-foundation.org>2022-03-23 21:40:25 +0300
commit1bc191051dca28fa6d20fd1dc34a1903e7d4fb62 (patch)
tree788fc5896c343ee3d84af7a2e3c6d78f5d55bca6 /Documentation
parent20f463fb38686dd3fe7e6903cab56bdbbf756238 (diff)
parent795301d3c28996219d555023ac6863401b6076bc (diff)
downloadlinux-1bc191051dca28fa6d20fd1dc34a1903e7d4fb62.tar.xz
Merge tag 'trace-v5.18' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace
Pull tracing updates from Steven Rostedt: - New user_events interface. User space can register an event with the kernel describing the format of the event. Then it will receive a byte in a page mapping that it can check against. A privileged task can then enable that event like any other event, which will change the mapped byte to true, telling the user space application to start writing the event to the tracing buffer. - Add new "ftrace_boot_snapshot" kernel command line parameter. When set, the tracing buffer will be saved in the snapshot buffer at boot up when the kernel hands things over to user space. This will keep the traces that happened at boot up available even if user space boot up has tracing as well. - Have TRACE_EVENT_ENUM() also update trace event field type descriptions. Thus if a static array defines its size with an enum, the user space trace event parsers can still know how to parse that array. - Add new TRACE_CUSTOM_EVENT() macro. This acts the same as the TRACE_EVENT() macro, but will attach to an existing tracepoint. This will make one tracepoint be able to trace different content and not be stuck at only what the original TRACE_EVENT() macro exports. - Fixes to tracing error logging. - Better saving of cmdlines to PIDs when tracing (use the wakeup events for mapping). * tag 'trace-v5.18' of git://git.kernel.org/pub/scm/linux/kernel/git/rostedt/linux-trace: (30 commits) tracing: Have type enum modifications copy the strings user_events: Add trace event call as root for low permission cases tracing/user_events: Use alloc_pages instead of kzalloc() for register pages tracing: Add snapshot at end of kernel boot up tracing: Have TRACE_DEFINE_ENUM affect trace event types as well tracing: Fix strncpy warning in trace_events_synth.c user_events: Prevent dyn_event delete racing with ioctl add/delete tracing: Add TRACE_CUSTOM_EVENT() macro tracing: Move the defines to create TRACE_EVENTS into their own files tracing: Add sample code for custom trace events tracing: Allow custom events to be added to the tracefs directory tracing: Fix last_cmd_set() string management in histogram code user_events: Fix potential uninitialized pointer while parsing field tracing: Fix allocation of last_cmd in last_cmd_set() user_events: Add documentation file user_events: Add sample code for typical usage user_events: Add self-test for validator boundaries user_events: Add self-test for perf_event integration user_events: Add self-test for dynamic_events integration user_events: Add self-test for ftrace integration ...
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/admin-guide/kernel-parameters.txt8
-rw-r--r--Documentation/trace/index.rst1
-rw-r--r--Documentation/trace/user_events.rst216
3 files changed, 225 insertions, 0 deletions
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 1a9f89a835b3..c658e5d2d52c 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1465,6 +1465,14 @@
as early as possible in order to facilitate early
boot debugging.
+ ftrace_boot_snapshot
+ [FTRACE] On boot up, a snapshot will be taken of the
+ ftrace ring buffer that can be read at:
+ /sys/kernel/tracing/snapshot.
+ This is useful if you need tracing information from kernel
+ boot up that is likely to be overridden by user space
+ start up functionality.
+
ftrace_dump_on_oops[=orig_cpu]
[FTRACE] will dump the trace buffers on oops.
If no parameter is passed, ftrace will dump
diff --git a/Documentation/trace/index.rst b/Documentation/trace/index.rst
index 3769b9b7aed8..3a47aa8341c6 100644
--- a/Documentation/trace/index.rst
+++ b/Documentation/trace/index.rst
@@ -30,3 +30,4 @@ Linux Tracing Technologies
stm
sys-t
coresight/index
+ user_events
diff --git a/Documentation/trace/user_events.rst b/Documentation/trace/user_events.rst
new file mode 100644
index 000000000000..bddedabaca80
--- /dev/null
+++ b/Documentation/trace/user_events.rst
@@ -0,0 +1,216 @@
+=========================================
+user_events: User-based Event Tracing
+=========================================
+
+:Author: Beau Belgrave
+
+Overview
+--------
+User based trace events allow user processes to create events and trace data
+that can be viewed via existing tools, such as ftrace, perf and eBPF.
+To enable this feature, build your kernel with CONFIG_USER_EVENTS=y.
+
+Programs can view status of the events via
+/sys/kernel/debug/tracing/user_events_status and can both register and write
+data out via /sys/kernel/debug/tracing/user_events_data.
+
+Programs can also use /sys/kernel/debug/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.
+
+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 gives back two ints to the program for each event. The first int is the
+status index. This index describes which byte in the
+/sys/kernel/debug/tracing/user_events_status file represents this event. The
+second int is the write index. This index describes the data when a write() or
+writev() is called on the /sys/kernel/debug/tracing/user_events_data file.
+
+The structures referenced in this document are contained with the
+/include/uap/linux/user_events.h file in the source tree.
+
+**NOTE:** *Both user_events_status and user_events_data are under the tracefs
+filesystem and may be mounted at different paths than above.*
+
+Registering
+-----------
+Registering within a user process is done via ioctl() out to the
+/sys/kernel/debug/tracing/user_events_data file. The command to issue is
+DIAG_IOCSREG.
+
+This command takes a struct user_reg as an argument::
+
+ struct user_reg {
+ u32 size;
+ u64 name_args;
+ u32 status_index;
+ u32 write_index;
+ };
+
+The struct user_reg requires two inputs, the first is the size of the structure
+to ensure forward and backward compatibility. The second is the command string
+to issue for registering. Upon success two outputs are set, the status index
+and the write index.
+
+User based events show up under tracefs like any other event under the
+subsystem named "user_events". This means tools that wish to attach to the
+events need to use /sys/kernel/debug/tracing/events/user_events/[name]/enable
+or perf record -e user_events:[name] when attaching/recording.
+
+**NOTE:** *The write_index returned is only valid for the FD that was used*
+
+Command Format
+^^^^^^^^^^^^^^
+The command string format is as follows::
+
+ name[:FLAG1[,FLAG2...]] [Field1[;Field2...]]
+
+Supported Flags
+^^^^^^^^^^^^^^^
+**BPF_ITER** - EBPF programs attached to this event will get the raw iovec
+struct instead of any data copies for max performance.
+
+Field Format
+^^^^^^^^^^^^
+::
+
+ type name [size]
+
+Basic types are supported (__data_loc, u32, u64, int, char, char[20], etc).
+User programs are encouraged to use clearly sized types like u32.
+
+**NOTE:** *Long is not supported since size can vary between user and kernel.*
+
+The size is only valid for types that start with a struct prefix.
+This allows user programs to describe custom structs out to tools, if required.
+
+For example, a struct in C that looks like this::
+
+ struct mytype {
+ char data[20];
+ };
+
+Would be represented by the following field::
+
+ struct mytype myname 20
+
+Deleting
+-----------
+Deleting an event from within a user process is done via ioctl() out to the
+/sys/kernel/debug/tracing/user_events_data file. The command to issue is
+DIAG_IOCSDEL.
+
+This command only requires a single string specifying the event to delete by
+its name. Delete will only succeed if there are no references left to the
+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.
+
+Status
+------
+When tools attach/record user based events the status of the event is updated
+in realtime. This allows user programs to only incur the cost of the write() or
+writev() calls when something is actively attached to the event.
+
+User programs call mmap() on /sys/kernel/debug/tracing/user_events_status to
+check the status for each event that is registered. The byte to check in the
+file is given back after the register ioctl() via user_reg.status_index.
+Currently the size of user_events_status is a single page, however, custom
+kernel configurations can change this size to allow more user based events. In
+all cases the size of the file is a multiple of a page size.
+
+For example, if the register ioctl() gives back a status_index of 3 you would
+check byte 3 of the returned mmap data to see if anything is attached to that
+event.
+
+Administrators can easily check the status of all registered events by reading
+the user_events_status file directly via a terminal. The output is as follows::
+
+ Byte:Name [# Comments]
+ ...
+
+ Active: ActiveCount
+ Busy: BusyCount
+ Max: MaxCount
+
+For example, on a system that has a single event the output looks like this::
+
+ 1:test
+
+ Active: 1
+ Busy: 0
+ Max: 4096
+
+If a user enables the user event via ftrace, the output would change to this::
+
+ 1:test # Used by ftrace
+
+ Active: 1
+ Busy: 1
+ Max: 4096
+
+**NOTE:** *A status index of 0 will never be returned. This allows user
+programs to have an index that can be used on error cases.*
+
+Status Bits
+^^^^^^^^^^^
+The byte being checked will be non-zero if anything is attached. Programs can
+check specific bits in the byte to see what mechanism has been attached.
+
+The following values are defined to aid in checking what has been attached:
+
+**EVENT_STATUS_FTRACE** - Bit set if ftrace has been attached (Bit 0).
+
+**EVENT_STATUS_PERF** - Bit set if perf/eBPF has been attached (Bit 1).
+
+Writing Data
+------------
+After registering an event the same fd that was used to register can be used
+to write an entry for that event. The write_index returned must be at the start
+of the data, then the remaining data is treated as the payload of the event.
+
+For example, if write_index returned was 1 and I wanted to write out an int
+payload of the event. Then the data would have to be 8 bytes (2 ints) in size,
+with the first 4 bytes being equal to 1 and the last 4 bytes being equal to the
+value I want as the payload.
+
+In memory this would look like this::
+
+ int index;
+ int payload;
+
+User programs might have well known structs that they wish to use to emit out
+as payloads. In those cases writev() can be used, with the first vector being
+the index and the following vector(s) being the actual event payload.
+
+For example, if I have a struct like this::
+
+ struct payload {
+ int src;
+ int dst;
+ int flags;
+ };
+
+It's advised for user programs to do the following::
+
+ struct iovec io[2];
+ struct payload e;
+
+ io[0].iov_base = &write_index;
+ io[0].iov_len = sizeof(write_index);
+ io[1].iov_base = &e;
+ io[1].iov_len = sizeof(e);
+
+ writev(fd, (const struct iovec*)io, 2);
+
+**NOTE:** *The write_index is not emitted out into the trace being recorded.*
+
+EBPF
+----
+EBPF programs that attach to a user-based event tracepoint are given a pointer
+to a struct user_bpf_context. The bpf context contains the data type (which can
+be a user or kernel buffer, or can be a pointer to the iovec) and the data
+length that was emitted (minus the write_index).
+
+Example Code
+------------
+See sample code in samples/user_events.