From: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxxx> 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@xxxxxxx> Cc: Friedrich Vock <friedrich.vock@xxxxxx> 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(-) -- 2.44.0
