Re: [PATCH v9 3/3] tracing/Documentation: Add a section about backup instance

[Steven Rostedt <rostedt@goodmis.org>]

On Wed,  1 Apr 2026 01:32:41 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> Add a section about backup instance to the debugging.rst.
> 
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> ---
>   Changes in v6:
>    - Fix typos.
> ---
>  Documentation/trace/debugging.rst |   19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/Documentation/trace/debugging.rst b/Documentation/trace/debugging.rst
> index 4d88c346fc38..15857951b506 100644
> --- a/Documentation/trace/debugging.rst
> +++ b/Documentation/trace/debugging.rst
> @@ -159,3 +159,22 @@ If setting it from the kernel command line, it is recommended to also
>  disable tracing with the "traceoff" flag, and enable tracing after boot up.
>  Otherwise the trace from the most recent boot will be mixed with the trace
>  from the previous boot, and may make it confusing to read.
> +
> +Using a backup instance for keeping previous boot data
> +------------------------------------------------------
> +
> +It is also possible to record trace data at system boot time by specifying
> +events with the persistent ring buffer, but in this case the data before the
> +reboot will be lost before it can be read. This problem can be solved by a
> +backup instance. From the kernel command line::
> +
> +  reserve_mem=12M:4096:trace trace_instance=boot_map@trace,sched,irq trace_instance=backup=boot_map

The above didn't actually work well without my patch to enable events on
the persistent ring buffer with the backup. But keep it, as it will work
with my patch ;-)

-- Steve


> +
> +On boot up, the previous data in the "boot_map" is copied to the "backup"
> +instance, and the "sched:*" and "irq:*" events for the current boot are traced
> +in the "boot_map". Thus the user can read the previous boot data from the "backup"
> +instance without stopping the trace.
> +
> +Note that this "backup" instance is readonly, and will be removed automatically
> +if you clear the trace data or read out all trace data from the "trace_pipe"
> +or the "trace_pipe_raw" files.
> \ No newline at end of file