[Intel-gfx] [PATCH] dma-buf: Enhance dma-fence tracing

[Chris Wilson]

Quoting Koenig, Christian (2019-01-22 08:49:30)
> Am 22.01.19 um 00:20 schrieb Chris Wilson:
> > Rather than every backend and GPU driver reinventing the same wheel for
> > user level debugging of HW execution, the common dma-fence framework
> > should include the tracing infrastructure required for most client API
> > level flow visualisation.
> >
> > With these common dma-fence level tracepoints, the userspace tools can
> > establish a detailed view of the client <-> HW flow across different
> > kernels. There is a strong ask to have this available, so that the
> > userspace developer can effectively assess if they're doing a good job
> > about feeding the beast of a GPU hardware.
> >
> > In the case of needing to look into more fine-grained details of how
> > kernel internals work towards the goal of feeding the beast, the tools
> > may optionally amend the dma-fence tracing information with the driver
> > implementation specific. But for such cases, the tools should have a
> > graceful degradation in case the expected extra tracepoints have
> > changed or their format differs from the expected, as the kernel
> > implementation internals are not expected to stay the same.
> >
> > It is important to distinguish between tracing for the purpose of client
> > flow visualisation and tracing for the purpose of low-level kernel
> > debugging. The latter is highly implementation specific, tied to
> > a particular HW and driver, whereas the former addresses a common goal
> > of user level tracing and likely a common set of userspace tools.
> > Having made the distinction that these tracepoints will be consumed for
> > client API tooling, we raise the spectre of tracepoint ABI stability. It
> > is hoped that by defining a common set of dma-fence tracepoints, we avoid
> > the pitfall of exposing low level details and so restrict ourselves only
> > to the high level flow that is applicable to all drivers and hardware.
> > Thus the reserved guarantee that this set of tracepoints will be stable
> > (with the emphasis on depicting client <-> HW flow as opposed to
> > driver <-> HW).
> >
> > In terms of specific changes to the dma-fence tracing, we remove the
> > emission of the strings for every tracepoint (reserving them for
> > dma_fence_init for cases where they have unique dma_fence_ops, and
> > preferring to have descriptors for the whole fence context). strings do
> > not pack as well into the ftrace ringbuffer and we would prefer to
> > reduce the amount of indirect callbacks required for frequent tracepoint
> > emission.
> >
> > Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Joonas Lahtinen <joonas.lahtinen at linux.intel.com>
> > Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> > Cc: Alex Deucher <alexdeucher at gmail.com>
> > Cc: "Christian König" <christian.koenig at amd.com>
> > Cc: Eric Anholt <eric at anholt.net>
> > Cc: Pierre-Loup Griffais <pgriffais at valvesoftware.com>
> > Cc: Michael Sartain <mikesart at fastmail.com>
> > Cc: Steven Rostedt <rostedt at goodmis.org>
> 
> In general yes please! If possible please separate out the changes to 
> the common dma_fence infrastructure from the i915 changes.

Sure, I was just stressing the impact: remove some randomly placed
internal debugging tracepoints, try to define useful ones instead :)

On the list of things to do was to convert at least 2 other drivers
(I was thinking nouveau/msm for simplicity, vc4 for a simpler
introduction to drm_sched than amdgpu) over to be sure we have the right
tracepoints.
 
> One thing I'm wondering is why the enable_signaling trace point doesn't 
> need to be exported any more. Is that only used internally in the common 
> infrastructure?

Right. Only used inside the core, and I don't see much call for making
it easy for drivers to fiddle around bypassing the core
enable_signaling/signal. (I'm not sure it's useful for client flow
either, it feels more like dma-fence debugging, but they can just
not listen to that tracepoint.)
-Chris