[RFC 09/11] coresight: etm-perf: Disable the path before capturing the trace data
mathieu.poirier at linaro.org
Fri Dec 11 15:31:26 EST 2020
On Fri, Nov 27, 2020 at 10:32:28AM +0000, Suzuki K Poulose wrote:
> On 11/10/20 12:45 PM, Anshuman Khandual wrote:
> > perf handle structure needs to be shared with the TRBE IRQ handler for
> > capturing trace data and restarting the handle. There is a probability
> > of an undefined reference based crash when etm event is being stopped
> > while a TRBE IRQ also getting processed. This happens due the release
> > of perf handle via perf_aux_output_end(). This stops the sinks via the
> > link before releasing the handle, which will ensure that a simultaneous
> > TRBE IRQ could not happen.
> Or in other words :
> We now have :
> This is problematic due to various reasons :
> 1) The semantics of update_buffer() is not clear. i.e, whether it
> should leave the "sink" "stopped" or "disabled" or "active"
I'm a little confused by the above as the modes that apply here are
CS_MODE_DISABLED and CS_MODE_PERF, so I'll go with those. Let me know if you
meant something else.
So far ->update_buffer() doesn't touch drvdata->mode and as such it is still set
to CS_MODE_PERF when the update has completed.
> 2) This breaks the recommended trace collection sequence of
> "flush" and "stop" from source to the sink for trace collection.
> i.e, we stop the source now. But don't flush the components
> from source to sink, rather we stop and flush from the sink.
> And we flush and stop the path after we have collected the
> trace data at sink, which is pointless.
The above assesment is correct. Fixing it though has far reaching ramifications
that go far beyond the scope of this patch.
> 3) For a sink with IRQ handler, if we don't stop the sink with
> update_buffer(), we could have a situation :
> perf_aux_outpuf_end(handle) # handle is invalid now
> -----------------> IRQ -> irq_handler()
> perf_aux_output_end(handle) # Wrong !
That's the picture of the issue I had in my head when looking at the code -
I'm glad we came to the same conclusion.
> The sysfs mode is fine, as we defer the trace collection to disable_path().
> The proposed patch is still racy, as we could still hit the problem.
> So, to avoid all of these situations, I think we should defer the the
> update_buffer() to sink_ops->disable(), when we have flushed and stopped
> the all the components upstream and avoid any races with the IRQ
> disable_path(handle); // similar to the enable_path
> sink_ops->disable(csdev, handle)
> /* flush & stop */
> /* collect trace */
> perf_aux_output_end(handle, size);
That is one solution. The advantage here is that it takes care of the
flusing problem you described above. On the flip side it is moving a lot of
code around, something that is better to do in another set.
Another solution is to disable the TRBE IRQ in ->udpate_buffer(). The ETR does
the same kind of thing with tmc_flush_and_stop(). I don't know how feasible
that is but it would be a simple solution for this set. Properly flushing the
pipeline could be done later. I'm fine with either approach.
> Kind regards
> > Signed-off-by: Anshuman Khandual <anshuman.khandual at arm.com>
> > ---
> > This might cause problem with traditional sink devices which can be
> > operated in both sysfs and perf mode. This needs to be addressed
> > correctly. One option would be to move the update_buffer callback
> > into the respective sink devices. e.g, disable().
> > drivers/hwtracing/coresight/coresight-etm-perf.c | 2 ++
> > 1 file changed, 2 insertions(+)
> > diff --git a/drivers/hwtracing/coresight/coresight-etm-perf.c b/drivers/hwtracing/coresight/coresight-etm-perf.c
> > index 534e205..1a37991 100644
> > --- a/drivers/hwtracing/coresight/coresight-etm-perf.c
> > +++ b/drivers/hwtracing/coresight/coresight-etm-perf.c
> > @@ -429,7 +429,9 @@ static void etm_event_stop(struct perf_event *event, int mode)
> > size = sink_ops(sink)->update_buffer(sink, handle,
> > event_data->snk_config);
> > + coresight_disable_path(path);
> > perf_aux_output_end(handle, size);
> > + return;
> > }
> > /* Disabling the path make its elements available to other sessions */
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