Re: [PATCH v4 00/22] perf: Add infrastructure and support for Intel PT

[Alexander Shishkin <alexander.shishkin@linux.intel.com>]

Mathieu Poirier <mathieu.poirier@linaro.org> writes:

> On 4 September 2014 02:26, Peter Zijlstra <peterz@infradead.org> wrote:
>> On Tue, Sep 02, 2014 at 02:18:16PM -0600, Mathieu Poirier wrote:
>>> Pawell, many thanks for looping me in.
>>>
>>> I am definitely not a perf-internal guru and as such won't be able to
>>> comment on the implementation.  On the flip side it is easy for me to see
>>> how the work on coresight done at Linaro can be made to tie-in what
>>> Alexander is proposing.  Albeit not at the top of the priority list at this
>>> time, integration with perf (and ftrace) is definitely on the roadmap.
>>>
>>> Powell is correct in his statement that Linaro's work in HW trace decoding
>>> is (currently) mainly focused on processor tracing but that will change
>>> when we have the basic infrastructure upstreamed.
>>>
>>> Last but not least it would be interesting to have more information on the
>>> "sideband data".  With coresight we have something called "metadata", also
>>> related to how the trace source was configured and instrumental to proper
>>> trace decoding.  I'm pretty sure we are facing the same problems.
>>
>> So we use the sideband or AUX data stream to export the 'big' data
>> stream generated by the CPU in an opaque manner. For every AUX data
>> block 'posted' we issue an event into the regular data buffer that
>> describes it.
>
> In the context of "describe it" written above, what kind of
> information one would typically find in that description?

It's like "got a chunk of AUX data in the AUX buffer, starting at offset
$X, length $Y".

>>
>> I was assuming that both ARM and MIPS would generate a single data
>> stream as well. So please do tell more about your meta-data; is that a
>> one time thing or a second continuous stream of data, albeit smaller
>> than the main stream?
>
> Coresight does indeed generate a single steam of compressed data.
> Depending on the tracing specifics that were requested by the use case
> (trace engine configuration) the format of the packets in the
> compressed stream will change.  Since the compressed stream itself
> doesn't carry clues about the formatting information, knowledge of how
> the trace engine was configured is mandatory for the proper decoding
> of the trace stream.

Ok, in perf the trace configuration would be part of 'session'
information, so the way the tracing was configured by userspace will be
saved to the resulting trace file (perf.data) by the userspace.
We have that with Intel PT as well.

> Metadata refer to exactly that - the configuration of the trace
> engine.  It has to be somehow lumped with the trace stream for off
> target analysis.

What we call sideband data in our code is more like runtime metadata,
such as executable mappings (so that you know what is mapped to which
addresses, iirc ETM/PTM also deals in virtual addresses, so you'll need
this information to make sense of the trace, right?) and context
switches.

One of the great things about perf here is that it provides all this
information practically for free.

>>
>> The way I read your explanation it is a one time blob generated once you
>> setup the hardware.
>
> Correct.
>
>> I suppose we could either dump it once into the
>> normal data stream or maybe dump it once every time we generate an AUX
>> buffer event into the normal data stream -- if its not too big.
>
> Right, there is a set of meta-data to be generated with each trace
> run.  With the current implementation a "trace run" pertains to all
> the information collected between the beginning and end of a trace
> scenario.  Future work involve triggering a DMA transfer of the full
> coresight buffer to a kernel memory area, something that is probably
> close to the "buffer event" you are referring to.

Again correct me if I'm wrong, but the TMC(?) controller can be
configured to direct ETM/PTM output right into system memory by means of
a scatter-gather table. This is what we call AUX area, it's basically a
circular buffer with trace data. Trace output is sent to the system
memory, which is also mmap()ed to the userspace tracing tool (perf), so
that it can capture it in real time. Well, that's one of the scenarios.

Regards,
--
Alex