[Mesa-dev] Mesa CI with trace regression testing

[Eero Tamminen]

Hi,

On 27.9.2019 4.32, Eric Anholt wrote:
> Alexandros Frantzis <alexandros.frantzis at collabora.com> writes:
>> The last couple of months we (at Collabora) have been working on a
>> prototype for a Mesa testing system based on trace replays that supports
>> correctness regression testing and, in the future, performance
>> regression testing.
>>
>> We are aware that large-scale CI systems that perform extensive checks
>> on Mesa already exist. However, our goal is not to reach that kind of
>> scale or exhaustiveness, but to produce a system that will be simple and
>> robust enough to be maintained by the community, while being useful
>> enough so that the community will want to use and maintain it. We also
>> want to be able to make it fast enough so that it will be run eventually
>> on a regular basis, ideally in pre-commit fashion.
>>
>> The current prototype focuses on the correctness aspect, replaying
>> traces and comparing images against a set of reference images on
>> multiple devices. At the moment, we run on softpipe and
>> intel/chromebook, but it's straightforward to add other devices through
>> gitlab runners.
>>
>> For the prototype we have used a simple approach for image comparison,
>> storing a separate set of reference images per device and using exact
>> image comparison, but we are also investigating alternative ways to deal
>> with this. First results indicate that the frequency of reference image
>> mismatches due to non-bug changes in Mesa is acceptable, but we will get
>> a more complete picture once we have a richer set of traces and a longer
>> CI run history.

For CI, I think discarding/ignoring too unstable / slow traces would
be perfectly acceptable. [1]


> Some missing context: I was told that over 2400 commits, in glmark2 + a
> couple of other open source traces, on intel, there was one spurious
> failure due to this diff method.  This is lower than I felt like it was
> when I did this in piglit on vc4, but then I was very actively changing
> optimization in the compiler while I was using that tool.

Few years ago when I was looking at the results from ezBench (at
the same time) bisecting Mesa commit ranges for build, run-time,
performance and rendering issues, it was very useful to have
rendering diff results in addition to performance numbers.

Rendering didn't change too often, but one needs to look at every change
screenshot directly, error metrics about them aren't enough.  Some
innocent accuracy difference due to calculation order change can cause
e.g. marginally different color on huge area on the rendered result [1],
whereas some real rendering error can be just some tiny reflection
missing from the render, which one would never see from running
benchmark (even with correct one running beside it), one sees them only
from static screenshots.

[1] Whether screenshots are affected by calculation changes, depends
a lot on the benchmark, how stable its calculations are in regards to
accuracy variations. Some benchmarks even use random in their shaders...

(If I remember correctly, good example of unstable results were some
of the GpuTest benchmarks.)


>> The current design is based on an out-of-tree approach, where the tracie
>> CI works independently from Mesa CI, fetching and building the latest
>> Mesa on its own. We did this for maximum flexibility in the prototyping
>> phase, but this has a complexity cost, and although we could continue to
>> work this way, we would like to hear people's thoughts about eventually
>> integrating with Mesa more closely, by becoming part of the upstream
>> Mesa testing pipelines.
>>
>> It's worth noting that the last few months other people, most notably
>> Eric Anholt, have made proposals to extend the scope of testing in CI.
>> We believe there is much common ground here (multiple devices,
>> deployment with gitlab runners) and room for cooperation and eventual
>> integration into upstream Mesa. In the end, the main difference between
>> all these efforts are the kind of tests (deqp, traces, performance) that
>> are being run, which all have their place and offer different
>> trade-offs.
>>
>> We have also implemented a prototype dashboard to display the results,
>> which we have deployed at:
>>
>> https://tracie.freedesktop.org
>>
>> We are working to improve the dashboard and provide more value by
>> extracting and displaying additional information, e.g., "softpipe broken
>> since commit NNN".
>>
>> The dashboard is currently specific to the trace playback results, but
>> it would be nice to eventually converge to a single MesaCI dashboard
>> covering all kinds of Mesa CI test results. We would be happy to help
>> develop in this direction if there is interest.
>>
>> You can find the CI scripts for tracie at:
>>
>> https://gitlab.freedesktop.org/gfx-ci/tracie/tracie
>>
>> Code for the dashboard is at:
>>
>> https://gitlab.freedesktop.org/gfx-ci/tracie/tracie_dashboard
>>
>> Here is an example of a failed CI job (for a purposefully broken Mesa
>> commit) and the report of the failed trace (click on the red X to
>> see the image diffs):
>>
>> https://tracie.freedesktop.org/dashboard/job/642369/
>>
>> Looking forward to your thoughts and comments.
> 
> A couple of thoughts on this:
> 
> A separate dashboard is useful if we have traces that are too slow to
> run pre-merge or are not redistributable.  For traces that are
> redistributable and cheap to run, we should run them in our CI and block
> the merge instead of having someone have to watch an external dashboard
> and report things to get patched up after regressions have already
> landed.
> 
> I'm reluctant to add "maintain a web service codebase" as one of the
> things that the Mesa project does, if there are alternatives that don't
> involve that.  I've been thinking about a perf dashboard, and for that
> I'd like to reuse existing open source projects like grafana.  If we
> start our own dashboard project, are we going to end up reimplementing
> that one?

FYI: We're tried Grafana and while it's otherwise nice and fast, we
didn't find a way to get each data point in graph to be a link to
additional data (logs, screenshots etc), which IMHO makes it much less
useful for trend tracking.

(If there actually *is* a way to add links to each data point, I would
be very much interested.)


	- Eero