linux-kernel - Boot-time dumping of ftrace fuctiongraph buffer

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Date: Thu, 1 Feb 2024 13:21:37 +0100
From: Ahmad Fatoum <a.fatoum@...gutronix.de>
To: linux-trace-kernel@...r.kernel.org, Masami Hiramatsu <mhiramat@...nel.org>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 Pengutronix Kernel Team <kernel@...gutronix.de>
Subject: Boot-time dumping of ftrace fuctiongraph buffer

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

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