linux-kernel - Re: [PATCH v1] arch/x86: port I/O tracing on x86

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Message-ID: <20230919211214.GE424@noisy.programming.kicks-ass.net>
Date:   Tue, 19 Sep 2023 23:12:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dan Raymond <draymond@...valley.net>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com
Subject: Re: [PATCH v1] arch/x86: port I/O tracing on x86

On Tue, Sep 19, 2023 at 01:56:15PM -0600, Dan Raymond wrote:
> > No, very much no.
> > 
> > This sticks tracing in the very rawest of raw output paths.
> 
> That's the point.  Tracing is a low level debug tool that can be
> used to debug the kernel itself.  If you don't trace all port I/O
> then you are bound to miss something important while debugging.
> 
> > This means I can no longer use early_console->write() to print to my
> > early_serial_console.
> 
> Why not?  Did you try it?

I have tried debugging the kernel for the last 15+ years. The only
reliable way to get something out of the machine is outb on the serial
port. Anything else is a waste of time..

Adding tracing to it (which relies on RCU, which might not be alive at
this point) which might itself be the problem, is a total no-go.

You do not wreck early_serial_console.

> > That is the one and only fully reliably output path we have. You're not
> > sticking tracing in it.
> 
> Keep in mind that tracing is optional.  It can be turned off using
> CONFIG_TRACEPOINTS.

Nobody ever does that. Also, tracepoints in general are useful.