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SubjectRe: [PATCH v2] docs: ftrace: Specifies when buffers get clear
On Wed, 18 Dec 2019 14:15:53 -0500
"Frank A. Cancio Bello" <frank@generalsoftwareinc.com> wrote:

> Clarify a few places where the ring buffer and the "snapshot" buffer
> are cleared as a side effect of an operation.
>
> This will avoid users lost of tracing data because of these so far
> undocumented behavior.
>
> Signed-off-by: Frank A. Cancio Bello <frank@generalsoftwareinc.com>

Thanks for the update.

Jon, do you want to take this?

Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

-- Steve

> ---
> Changes in v2:
> - Per Steven comment correct the fact that the "snapshot" buffer is
> not touched when writing in the "trace" file.
> - Use tab instead of spaces for alignment.
>
> Documentation/trace/ftrace.rst | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/trace/ftrace.rst b/Documentation/trace/ftrace.rst
> index d2b5657ed33e..46df39300d22 100644
> --- a/Documentation/trace/ftrace.rst
> +++ b/Documentation/trace/ftrace.rst
> @@ -95,7 +95,8 @@ of ftrace. Here is a list of some of the key files:
> current_tracer:
>
> This is used to set or display the current tracer
> - that is configured.
> + that is configured. Changing the current tracer clears
> + the ring buffer content as well as the "snapshot" buffer.
>
> available_tracers:
>
> @@ -126,7 +127,8 @@ of ftrace. Here is a list of some of the key files:
> This file holds the output of the trace in a human
> readable format (described below). Note, tracing is temporarily
> disabled when the file is open for reading. Once all readers
> - are closed, tracing is re-enabled.
> + are closed, tracing is re-enabled. Opening this file for
> + writing with the O_TRUNC flag clears the ring buffer content.
>
> trace_pipe:
>
> @@ -490,6 +492,9 @@ of ftrace. Here is a list of some of the key files:
>
> # echo global > trace_clock
>
> + Setting a clock clears the ring buffer content as well as the
> + "snapshot" buffer.
> +
> trace_marker:
>
> This is a very useful file for synchronizing user space

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