linux-kernel - Re: [PATCH v10 08/10] dyndbg: add print-to-tracefs, selftest with it

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Date:   Fri, 12 Nov 2021 12:32:23 -0500
From:   Jason Baron <jbaron@...mai.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Vincent Whitchurch <vincent.whitchurch@...s.com>,
        Jim Cromie <jim.cromie@...il.com>, gregkh@...uxfoundation.org,
        robdclark@...il.com, sean@...rly.run, daniel.vetter@...ll.ch,
        seanpaul@...omium.org, lyude@...hat.com,
        linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
        dri-devel@...ts.freedesktop.org, amd-gfx@...ts.freedesktop.org,
        intel-gvt-dev@...ts.freedesktop.org,
        intel-gfx@...ts.freedesktop.org, quic_saipraka@...cinc.com,
        will@...nel.org, catalin.marinas@....com,
        quic_psodagud@...cinc.com, maz@...nel.org, arnd@...db.de,
        linux-arm-kernel@...ts.infradead.org,
        linux-arm-msm@...r.kernel.org, mingo@...hat.com
Subject: Re: [PATCH v10 08/10] dyndbg: add print-to-tracefs, selftest with it
 - RFC



On 11/12/21 12:07 PM, Steven Rostedt wrote:
> On Fri, 12 Nov 2021 10:08:41 -0500
> Jason Baron <jbaron@...mai.com> wrote:
> 
>>> A key difference between that patchset and this patch (besides that
>>> small fact that I used +x instead of +T) was that my patchset allowed
>>> the dyndbg trace to be emitted to the main buffer and did not force them
>>> to be in an instance-specific buffer.  
>>
>> Yes, I agree I'd prefer that we print here to the 'main' buffer - it seems to keep things simpler and easier to combine the output from different
>> sources as you mentioned.
> 
> I do not want anything to print to the "main buffer" that can not be
> filtered or turned off by the tracing infrastructure itself (aka tracefs
> file system).
> 
> Once we allow that, then the trace file will become useless because
> everything will write to the main buffer and fill it with noise.
> 
> Events that can be enabled and disabled from tracefs are fine, as they can
> be limited. This is why I added that nasty warning if people leave around
> trace_printk(), because it does exactly this (write to the main buffer).
> It's fine for debugging a custom kernel, but should never be enabled in
> something that is shipped, or part of mainline.
> 
> -- Steve
> 

Ok, it looks like Vincent's patch defines a dyndbg event and then uses
'trace_dyndbg()' to output to the 'main' log. So all dynamic output to
the 'main' ftrace buffer goes through that event if I understand it
correctly. Here's a pointer to it for reference:

https://lore.kernel.org/lkml/20200825153338.17061-3-vincent.whitchurch@axis.com/

Would you be ok with that approach?

Thanks,

-Jason