[Intel-gfx] [RFC PATCH 0/4] perf pmu driver for OA counters

[Robert Bragg]

This is still a work in progress, but as I've started to get some useful
results with some simple igt tools and integration into Mesa too, others
could be interested in this proof-of-concept perf pmu driver to forward
Haswell Observation Architecture counters to userspace...

This has some similarities to the pmu driver Chris wrote back in
September last year, except that in this case the driver is configuring
a unit of the gpu to write out periodic counter snapshots to a circular
buffer that then get forwarded to userspace as perf samples, instead of
using hrtimer based sampling. I think if there is general agreement on
leveraging perf pmus for gpu metrics that we'll likely also want some
separate hrtimer based pmus later too.

As well as this RFC, I also recently sent an RFC to the LKML since I
wanted to see if the core perf maintainers would be happy with us
exposing device metrics via perf pmus (currently perf is very cpu
centric).  Initial feedback seems positive and receptive to the idea
which is reassuring:

  https://lkml.org/lkml/2014/10/22/462

When I started looking at this, it wasn't entirely clear that perf would
be a good fit for covering the tooling and Mesa use cases we have and in
particular there was a big question mark over how the permission model
would work considering that GL apps shouldn't need to run as root to use
INTEL_performance_query. Having got something working though I do feel
much more confident now that it can work for us to build on perf
infrastructure here.

For the permission issue, the current driver depends on a small change
to kernel/events/core so it can flag that its pmu relates to a device,
as opposed to the cpu which lets the driver also handle its own
authentication. In this case a drm file descriptor and context id can be
passed as part of the config when opening an event so it's only possible
to profile a specific context that you have a corresponding open drm
file descriptor for. You have to be running as root to profile across
all contexts.

For reference the PRM for these counters can be found here:

  https://01.org/linuxgraphics/sites/default/files/documentation/
  observability_performance_counters_haswell.pdf

To test the driver out I have two igt tools; intel_oacounter_top_pmu, 
which is something like intel_gpu_top but based on our observability
counters, and intel_gpu_trace_pmu which gives me a way to log all of the
pmu sample data in detail in a json format that can be visualised using
the chrome://tracing ui. These tools can be found here:

  https://github.com/rib/intel-gpu-tools/tree/wip/rib/intel-i915-oa-pmu

I've also been experimenting with using the driver from Mesa, and
that can be seen here:

  https://github.com/rib/mesa/tree/wip/rib/i915_oa_perf

Currently the mesa driver is limited to only using the i915_oa driver
as a way to configure the OA unit and still needs updating to also
collect periodic counter snapshots for detecting counter wrapping.

For anyone interested in an introduction to how the lower level perf
interface works this may be a useful reference:

  http://web.eece.maine.edu/~vweaver/projects/perf_events/
  perf_event_open.html

For convenience these patches can also be pulled from here:

  https://github.com/rib/linux/tree/wip/rib/i915_oa_perf

Regards,
- Robert

Robert Bragg (4):
  perf: export perf_event_overflow
  perf: Add PERF_PMU_CAP_IS_DEVICE flag
  drm/i915: add api to pin/unpin context state
  drm/i915: Expose PMU for Observation Architecture

 drivers/gpu/drm/i915/Makefile           |   1 +
 drivers/gpu/drm/i915/i915_dma.c         |   2 +
 drivers/gpu/drm/i915/i915_drv.h         |  37 ++
 drivers/gpu/drm/i915/i915_gem_context.c |  30 +-
 drivers/gpu/drm/i915/i915_oa_perf.c     | 649 ++++++++++++++++++++++++++++++++
 drivers/gpu/drm/i915/i915_reg.h         |  68 ++++
 include/linux/perf_event.h              |   1 +
 include/uapi/drm/i915_drm.h             |  21 ++
 kernel/events/core.c                    |  40 +-
 9 files changed, 834 insertions(+), 15 deletions(-)
 create mode 100644 drivers/gpu/drm/i915/i915_oa_perf.c

-- 
2.1.3