Messages in this thread |  | | Subject | Re: [RFC 09/11] coresight: etm-perf: Disable the path before capturing the trace data | From | Suzuki K Poulose <> | Date | Mon, 14 Dec 2020 10:00:27 +0000 |
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On 12/11/20 8:31 PM, Mathieu Poirier wrote: > 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 : >> >> update_buffer() >> >> perf_aux_output_end(handle) >> >> ... >> disable_path() >> >> 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.
Sorry, I think it is a bit confusing.
stopped => Sink is in stopped HW state, but the software mode is not changed (i.e, could be PERF or SYSF)
disabled => Sink is in stopped hw state, the software mode is DISABLED
active => Sink is active and flushing trace, with respective mode (PERF vs SYSFS).
> > 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 : >> >> update_buffer() >> >> perf_aux_outpuf_end(handle) # handle is invalid now >> >> -----------------> IRQ -> irq_handler() >> perf_aux_output_end(handle) # Wrong ! >> >> >> disable_path() > > 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 >> handler. >> >> i.e, >> >> source_ops->stop(csdev); >> >> 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.
Agreed. I think this is reasonable forthis set. i.e, leave the hardware disabled. We could do the proper solution above as a separate series, to keep the changes incremental.
Kind regards Suzuki
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