[RFC 0/5] Discussion around eviction improvements

[Tvrtko Ursulin]

From: Tvrtko Ursulin <tvrtko.ursulin at igalia.com>

Last few days I was looking at the situation with VRAM over subscription, what
happens versus what perhaps should happen. Browsing through the driver and
running some simple experiments.

I ended up with this patch series which, as a disclaimer, may be completely
wrong but as I found some suspicious things, to me at least, I thought it was a
good point to stop and request some comments.

To perhaps summarise what are the main issues I think I found:

 * Migration rate limiting does not bother knowing if actual migration happened
   and so can over-account and unfairly penalise.

 * Migration rate limiting does not even work, at least not for the common case
   where userspace configures VRAM+GTT. It thinks it can stop migration attempts
   by playing with bo->allowed_domains vs bo->preferred domains but, both from
   the code, and from empirical experiments, I see that not working at all. Both
   masks are identical so fiddling with them achieves nothing.

 * Idea of the fallback placement only works when VRAM has free space. As soon
   as it does not, ttm_resource_compatible is happy to leave the buffers in the
   secondary placement forever.

 * Driver thinks it will be re-validating evicted buffers on the next submission
   but it does not for the very common case of VRAM+GTT because it only checks
   if current placement is *none* of the preferred placements.

All those problems are addressed in individual patches.

End result of this series appears to be driver which will try harder to move
buffers back into VRAM, but will be (more) correctly throttled in doing so by
the existing rate limiting logic.

I have run a quick benchmark of Cyberpunk 2077 and cannot say that I saw a
change but that could be a good thing too. At least I did not break anything,
perhaps.. On one occassion I did see the rate limiting logic get confused while
for a period of few minutes it went to a mode where it was constantly giving a
high migration budget. But that recovered itself when I switched clients and did
not come back so I don't know. If there is something wrong there I don't think
it would be caused by any patches in this series.

Series is probably rough but should be good enough for dicsussion. I am curious
to hear if I identified at least something correctly as a real problem.

It would also be good to hear what are the suggested games to check and see
whether there is any improvement.

Cc: Christian König <christian.koenig at amd.com>
Cc: Friedrich Vock <friedrich.vock at gmx.de>

Tvrtko Ursulin (5):
  drm/amdgpu: Fix migration rate limiting accounting
  drm/amdgpu: Actually respect buffer migration budget
  drm/ttm: Add preferred placement flag
  drm/amdgpu: Use preferred placement for VRAM+GTT
  drm/amdgpu: Re-validate evicted buffers

 drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c     | 38 +++++++++++++++++-----
 drivers/gpu/drm/amd/amdgpu/amdgpu_object.c |  8 +++--
 drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c     | 21 ++++++++++--
 drivers/gpu/drm/ttm/ttm_resource.c         | 13 +++++---
 include/drm/ttm/ttm_placement.h            |  3 ++
 5 files changed, 65 insertions(+), 18 deletions(-)

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2.44.0