Hi Beau,
On Tue, 21 Feb 2023 13:11:41 -0800
Beau Belgrave <beaub@xxxxxxxxxxxxxxxxxxx> wrote:
> The ABI for user_events has changed from mmap() based to remote writes.
> Update the documentation to reflect these changes, add new section for
> unregistering events since lifetime is now tied to tasks instead of
> files.
Thanks for updating the document. This is good. And I would like to
ask you to update one more thing in the document.
In "the Writing Data" Section,
-----
For example, if I have a struct like this::
struct payload {
int src;
int dst;
int flags;
};
-----
This payload has to be packed too.
This may be a split patch as a fix because it is not related to this
series.
Thank you,
>
> Signed-off-by: Beau Belgrave <beaub@xxxxxxxxxxxxxxxxxxx>
> ---
> Documentation/trace/user_events.rst | 177 ++++++++++++++++------------
> 1 file changed, 102 insertions(+), 75 deletions(-)
>
> diff --git a/Documentation/trace/user_events.rst b/Documentation/trace/user_events.rst
> index 9f181f342a70..0180714f10e3 100644
> --- a/Documentation/trace/user_events.rst
> +++ b/Documentation/trace/user_events.rst
> @@ -11,20 +11,19 @@ that can be viewed via existing tools, such as ftrace and perf.
> 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.
> +/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/debug/tracing/dynamic_events to register and
> +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.
>
> 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 bit. This describes which bit in little-endian format in the
> -/sys/kernel/debug/tracing/user_events_status file represents this event. The
> -second int is the write index which describes the data when a write() or
> -writev() is called on the /sys/kernel/debug/tracing/user_events_data file.
> +process tells the kernel which address and bit to reflect if any tool has
> +enabled the event and data should be written. The registration will give back
> +a write index which describes the data when a write() or writev() is called
> +on the /sys/kernel/tracing/user_events_data file.
>
> The structures referenced in this document are contained within the
> /include/uapi/linux/user_events.h file in the source tree.
> @@ -35,29 +34,70 @@ 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
> +/sys/kernel/tracing/user_events_data file. The command to issue is
> DIAG_IOCSREG.
>
> This command takes a packed struct user_reg as an argument::
>
> struct user_reg {
> - u32 size;
> - u64 name_args;
> - u32 status_bit;
> - u32 write_index;
> - };
> + /* Input: Size of the user_reg structure being used */
> + __u32 size;
> +
> + /* Input: Bit in enable address to use */
> + __u8 enable_bit;
> +
> + /* Input: Enable size in bytes at address */
> + __u8 enable_size;
> +
> + /* Input: Flags for future use, set to 0 */
> + __u16 flags;
> +
> + /* Input: Address to update when enabled */
> + __u64 enable_addr;
> +
> + /* Input: Pointer to string with event name, description and flags */
> + __u64 name_args;
> +
> + /* Output: Index of the event to use when writing data */
> + __u32 write_index;
> + } __attribute__((__packed__));
> +
> +The struct user_reg requires all the above inputs to be set appropriately.
>
> -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 bit
> -and the write index.
> ++ size: This must be set to sizeof(struct user_reg).
> +
> ++ enable_bit: The bit to reflect the event status at the address specified by
> + enable_addr.
> +
> ++ enable_size: The size of the value specified by enable_addr.
> + 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.
> + 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.
> +
> ++ enable_addr: The address of the value to use to reflect event status. This
> + must be naturally aligned and write accessible within the user program.
> +
> ++ name_args: The name and arguments to describe the event, see command format
> + for details.
> +
> +Upon successful registration the following is set.
> +
> ++ write_index: The index to use for this file descriptor that represents this
> + event when writing out data. The index is unique to this instance of the file
> + descriptor that was used for the registration. See writing data for details.
>
> 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
> +events need to use /sys/kernel/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*
> +**NOTE:** The event subsystem name by default is "user_events". Callers should
> +not assume it will always be "user_events". Operators reserve the right in the
> +future to change the subsystem name per-process to accomodate event isolation.
>
> Command Format
> ^^^^^^^^^^^^^^
> @@ -94,9 +134,9 @@ 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
> +/sys/kernel/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
> @@ -104,92 +144,79 @@ 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 bit to check in the
> -file is given back after the register ioctl() via user_reg.status_bit. The bit
> -is always in little-endian format. Programs can check if the bit is set either
> -using a byte-wise index with a mask or a long-wise index with a little-endian
> -mask.
> -
> -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.
> +Unregistering
> +-------------
> +If after registering an event it is no longer wanted to be updated then it can
> +be disabled via ioctl() out to the /sys/kernel/tracing/user_events_data file.
> +The command to issue is DIAG_IOCSUNREG. This is different than deleting, where
> +deleting actually removes the event from the system. Unregistering simply tells
> +the kernel your process is no longer interested in updates to the event.
>
> -For example, if the register ioctl() gives back a status_bit of 3 you would
> -check byte 0 (3 / 8) of the returned mmap data and then AND the result with 8
> -(1 << (3 % 8)) to see if anything is attached to that event.
> +This command takes a packed struct user_unreg as an argument::
>
> -A byte-wise index check is performed as follows::
> + struct user_unreg {
> + /* Input: Size of the user_unreg structure being used */
> + __u32 size;
>
> - int index, mask;
> - char *status_page;
> + /* Input: Bit to unregister */
> + __u8 disable_bit;
>
> - index = status_bit / 8;
> - mask = 1 << (status_bit % 8);
> + /* Input: Reserved, set to 0 */
> + __u8 __reserved;
>
> - ...
> + /* Input: Reserved, set to 0 */
> + __u16 __reserved2;
>
> - if (status_page[index] & mask) {
> - /* Enabled */
> - }
> + /* Input: Address to unregister */
> + __u64 disable_addr;
> + } __attribute__((__packed__));
>
> -A long-wise index check is performed as follows::
> +The struct user_unreg requires all the above inputs to be set appropriately.
>
> - #include <asm/bitsperlong.h>
> - #include <endian.h>
> ++ size: This must be set to sizeof(struct user_unreg).
>
> - #if __BITS_PER_LONG == 64
> - #define endian_swap(x) htole64(x)
> - #else
> - #define endian_swap(x) htole32(x)
> - #endif
> ++ disable_bit: This must be set to the bit to disable (same bit that was
> + previously registered via enable_bit).
>
> - long index, mask, *status_page;
> ++ disable_addr: This must be set to the address to disable (same address that was
> + previously registered via enable_addr).
>
> - index = status_bit / __BITS_PER_LONG;
> - mask = 1L << (status_bit % __BITS_PER_LONG);
> - mask = endian_swap(mask);
> +**NOTE:** Events are automatically unregistered when execve() is invoked. During
> +fork() the registered events will be retained and must be unregistered manually
> +in each process if wanted.
>
> - ...
> +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.
>
> - if (status_page[index] & mask) {
> - /* Enabled */
> - }
> +The kernel will update the specified bit that was registered for the event as
> +tools attach/detach from the event. User programs simply check if the bit is set
> +to see if something is attached or not.
>
> 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]
> + 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
> + test
>
> Active: 1
> Busy: 0
> - Max: 32768
>
> If a user enables the user event via ftrace, the output would change to this::
>
> - 1:test # Used by ftrace
> + test # Used by ftrace
>
> Active: 1
> Busy: 1
> - Max: 32768
> -
> -**NOTE:** *A status bit of 0 will never be returned. This allows user programs
> -to have a bit that can be used on error cases.*
>
> Writing Data
> ------------
> --
> 2.25.1
>
--
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>
