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

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Tue, 31 Mar 2026 17:21:05 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> 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 ;-)

Ah, thanks!
Let me rebase on it.


> 
> -- 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
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>