linux-kernel - Re: [PATCH v2 2/2] tracing: kdb: Allow ftdump to skip all but the last few lines

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Date:   Tue, 12 Mar 2019 23:25:28 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Douglas Anderson <dianders@...omium.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Jason Wessel <jason.wessel@...driver.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        kgdb-bugreport@...ts.sourceforge.net,
        Brian Norris <briannorris@...omium.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/2] tracing: kdb: Allow ftdump to skip all but the
 last few lines

On Fri,  8 Mar 2019 11:32:05 -0800
Douglas Anderson <dianders@...omium.org> wrote:

> The 'ftdump' command in kdb is currently a bit of a last resort, at
> least if you have lots of traces turned on.  It's going to print a
> whole boatload of lines out your serial port which is probably running
> at 115200.  This could easily take many, many minutes.
> 
> Usually you're most interested in what's at the _end_ of the ftrace
> buffer, AKA what happened most recently.  That means you've got to
> wait the full time for the dump.  The 'ftdump' command does attempt to
> help you a little bit by allowing you to skip a fixed number of lines.
> Unfortunately it provides no way for you to know how many lines you
> should skip.
> 
> Let's do similar to python and allow you to use a negative number to
> indicate that you want to skip all lines except the last few.  This
> allows you to quickly see what you want.

Why not just read how many entries are in the ring buffer and return that?

	cnt = 0;
	for_each_cpu(cpu, tr->tracing_cpumask)
		cnt += ring_buffer_entries_cpu(tr->trace_buffer, cpu);
	return cnt;

The output will print out one entry per line.

-- Steve


> 
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> ---
>