Received-SPF: pass (google.com: domain of linux-kernel+bounces-48107-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:40f1:3f00::1 as permitted sender) client-ip=2604:1380:40f1:3f00::1;
Message-ID: <d33e5271-219d-4b8e-be5a-8903219d7fd6@pengutronix.de>
Date: Thu, 1 Feb 2024 13:21:37 +0100
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Content-Language: en-US
From: Ahmad Fatoum <a.fatoum@pengutronix.de>
To: linux-trace-kernel@vger.kernel.org, Masami Hiramatsu <mhiramat@kernel.org>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
 Pengutronix Kernel Team <kernel@pengutronix.de>
Subject: Boot-time dumping of ftrace fuctiongraph buffer
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit

Hello,

I semi-regularly debug probe failures. For drivers that use dev_err_probe
rigorously, this is a quick matter: The probe function records a deferral reason
and if the deferral persists, deferred_probe_timeout_work_func() will print
the collected reasons, even if PID 1 is never started.

For drivers that don't call dev_err_probe, I find myself sometimes doing printf
debugging inside the probe function.

I would like to replace this with the function graph tracer:

  - record the probe function, configured over kernel command line
    (The device indefinitely deferring probe is printed to the console,
     so I know what I am looking for on the next boot)

  - Dump the function graph trace

  - See if the last call before (non-devm) cleanup is getting a clock, a GPIO,
    a regulator or w/e.

For this to be maximally useful, I need to configure this not only at boot-time,
but also dump the ftrace buffer at boot time. Probe deferral can hinder the kernel from
calling init and providing a shell, where I could read /sys/kernel/tracing/trace.

I found following two mechanisms that looked relevant, but seem not to
do exactly what I want:

  - tp_printk: seems to be related to trace points only and not usable
    for the function graph output

  - dump_on_oops: I don't get an Oops if probe deferral times out, but maybe
    one could patch the kernel to check a oops_on_probe_deferral or dump_on_probe_deferral
    kernel command line parameter in deferred_probe_timeout_work_func()?


Is there existing support that I am missing? Any input on whether this
would be a welcome feature to have?

Thanks!

Cheers,
Ahmad
    
-- 
Pengutronix e.K.                           |                             |
Steuerwalder Str. 21                       | http://www.pengutronix.de/  |
31137 Hildesheim, Germany                  | Phone: +49-5121-206917-0    |
Amtsgericht Hildesheim, HRA 2686           | Fax:   +49-5121-206917-5555 |