[RFC v2 0/2] Discussion around eviction improvements

[Christian König]

Am 16.05.24 um 21:21 schrieb 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?

Yeah, that's quite likely. I'm already looking into this.

Regards,
Christian.

>
> 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
>>