linux-kernel - Re: [PATCH v2] docs: ftrace: Specifies when buffers get clear

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Date:   Wed, 18 Dec 2019 14:33:51 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Frank A. Cancio Bello" <frank@...eralsoftwareinc.com>
Cc:     Ingo Molnar <mingo@...hat.com>, Jonathan Corbet <corbet@....net>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        joel@...lfernandes.org, nachukannan@...il.com,
        saiprakash.ranjan@...look.com
Subject: Re: [PATCH v2] docs: ftrace: Specifies when buffers get clear

On Wed, 18 Dec 2019 14:15:53 -0500
"Frank A. Cancio Bello" <frank@...eralsoftwareinc.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@...eralsoftwareinc.com>

Thanks for the update.

Jon, do you want to take this?

Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.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