linux-kernel - Re: [PATCHv3 3/3] dynamic_debug: Add a flag for dynamic event tracing

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Date:   Wed, 10 Nov 2021 15:03:22 -0500
From:   Jason Baron <jbaron@...mai.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Sai Prakash Ranjan <quic_saipraka@...cinc.com>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        quic_psodagud@...cinc.com, Marc Zyngier <maz@...nel.org>,
        gregkh@...uxfoundation.org, arnd@...db.de,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org, mingo@...hat.com,
        jim.cromie@...il.com, seanpaul@...omium.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCHv3 3/3] dynamic_debug: Add a flag for dynamic event tracing



On 11/9/21 5:28 PM, Steven Rostedt wrote:
> [ Hmm, should add Mathieu in on this discussion ]
> 
> On Tue, 9 Nov 2021 17:13:13 -0500
> Jason Baron <jbaron@...mai.com> wrote:
> 
>>> What we are looking at there is to pass the dynamic debug descriptor to the
>>> trace event filtering logic, where you could filter on information passed
>>> to it. For example, on a specific file if a trace event is called by
>>> several different files or modules.
>>>
>>> -- Steve  
>>
>> Ok, Could this be done at the dynamic debug level as it can already match
>> on specific files and line numbers currently?
> 
> Not sure what you mean by that.

I was just trying to say that via dynamic debug one could enable all debugs in a kernel source directory as in your example below via:
# echo "file drivers/soc/qcom/* +p" > /sys/kernel/debug/dynamic_debug/control

So if we are just looking for a printk here, one could just use pr_debug(). Or if we want that print to go the tracing ring buffer we have been
discussing how to add that as a 'backend' for dynamic debug as well.

If we want to use tracepionts, then yeah I think it's going to require adding the file, line, module data to each tracepoint site. I think this is
probably done via the tracing macros and then that could be filtered on, but yes that's going to add bloat.

The proposal here seems to be a mix of things - use the file control that dynamic debug already has to match on file name and then enable a tracepoint
that is behind it. That seems overly complex?

Thanks,

-Jason

> 
> The idea was that this would only be enabled if dynamic debug is enabled
> and that the DEFINE_DYNAMIC_DEBUG_METADATA() could be used at the
> tracepoint function location (trace_foo()) by the tracepoint macros. And
> then if one of the callbacks registered for the tracepoint had a
> "dynamic_debug" flag set, it would be passed the descriptor in as a pointer.
> 
> And then, for example, the filtering logic of ftrace could then reference
> the information of the event, if the user passed in something special.
> 
>  # echo 'DEBUG_FILE ~ "drivers/soc/qcom/*"' > events/rwmmio/rwmmio_write/filter
>  # echo 1 > events/rwmmio/rwmmio_write/enable
> 
> And then only the rwmmio_write events that came from the qcom directory
> would be printed.
> 
> We would create special event fields like "DEBUG_FILE", "DEBUG_FUNC",
> "DEBUG_MOD", "DEBUG_LINE", etc, that could be used if dyndebug is enabled
> in the kernel.
> 
> Of course this is going to bloat the kernel as it will create a dynamic
> debug descriptor at every tracepoint location.
> 
> -- Steve
>