[RFC v2 0/2] Discussion around eviction improvements

[Alex Deucher]

On Thu, May 16, 2024 at 8:18 AM Tvrtko Ursulin <tursulin at igalia.com> wrote:
>
> From: Tvrtko Ursulin <tvrtko.ursulin at igalia.com>
>
> Reduced re-spin of my previous series after Christian corrected a few
> misconceptions that I had. So lets see if what remains makes sense or is still
> misguided.
>
> To summarise, the series address the following two issues:
>
>  * Migration rate limiting does not 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. When
>    both masks are identical fiddling with them achieves nothing. Even when they
>    are not identical allowed has a fallback GTT placement which means that when
>    over the migration budget ttm_bo_validate with bo->allowed_domains can cause
>    migration from GTT to VRAM.
>
>  * 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.

For APUs at least, we should never migrate because GTT and VRAM are
both system memory so are effectively equal performance-wise.  Maybe
this regressed when Christian reworked ttm to better handle migrating
buffers back to VRAM after suspend on dGPUs?

Alex

>
> These two patches appear to have a positive result for a memory intensive game
> like Assassin's Creed Valhalla. On an APU like Steam Deck the game has a working
> set around 5 GiB, while the VRAM is configured to 1 GiB. Correctly respecting
> the migration budget appears to keep buffer blits at bay and improves the
> minimum frame rate, ie. makes things smoother.
>
> From the game's built-in benchmark, average of three runs each:
>
>                                                 FPS
>                 migrated KiB    min     avg     max     min-1%  min-0.1%
>   because          20784781     10.00  37.00   89.67    22.00    12.33
>   patched           4227688     13.67  37.00   81.33    23.33    15.00
>
> Disclaimers that I have is that more runs would be needed to be more confident
> about the results. And more games. And APU versus discrete.
>
> Cc: Christian König <christian.koenig at amd.com>
> Cc: Friedrich Vock <friedrich.vock at gmx.de>
>
> Tvrtko Ursulin (2):
>   drm/amdgpu: Re-validate evicted buffers
>   drm/amdgpu: Actually respect buffer migration budget
>
>  drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c | 112 +++++++++++++++++++------
>  drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c |  21 ++++-
>  2 files changed, 103 insertions(+), 30 deletions(-)
>
> --
> 2.44.0
>