Subject: Re: [PATCHv3 3/3] dynamic_debug: Add a flag for dynamic event tracing
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Sai Prakash Ranjan <quic_saipraka@quicinc.com>,
        Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        quic_psodagud@quicinc.com, Marc Zyngier <maz@kernel.org>,
        gregkh@linuxfoundation.org, arnd@arndb.de,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-arm-msm@vger.kernel.org, mingo@redhat.com,
        jim.cromie@gmail.com, seanpaul@chromium.org,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
References: <cover.1636452784.git.quic_saipraka@quicinc.com>
 <3706af20bc64a320ff8f3ff8950738b988f4bdf5.1636452784.git.quic_saipraka@quicinc.com>
 <20211109104941.2d50eafc@gandalf.local.home>
 <f7c665b9-dc17-5a7f-de80-9fa0605721fc@quicinc.com>
 <20211109115951.1c2b5228@gandalf.local.home>
 <264b77dd-5509-60f9-248c-a93135b01aa9@quicinc.com>
 <20211109124046.2a772bcb@gandalf.local.home>
 <c5715db5-965b-c1f5-3e99-04caec3d4f2c@quicinc.com>
 <e037f449-9784-c78e-431d-43f035a9f49f@akamai.com>
 <20211109165104.176b4cf9@gandalf.local.home>
 <55a9fe7b-5573-0f80-e075-758b377a6c47@akamai.com>
 <20211109172848.304b1c19@gandalf.local.home>
From:   Jason Baron <jbaron@akamai.com>
Message-ID: <43eeb71b-7a06-1dcb-3ae4-1eca31883df2@akamai.com>
Date:   Wed, 10 Nov 2021 15:03:22 -0500
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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@akamai.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
>