[PATCH 1/3] drm/amdgpu: add AMDGPU_GEM_CREATE_DISCARDABLE

[Christian König]

Am 12.05.22 um 00:06 schrieb Marek Olšák:
> 3rd question: Is it worth using this on APUs?

It makes memory management somewhat easier when we are really OOM.

E.g. it should also work for GTT allocations and when the core kernel 
says "Hey please free something up or I will start the OOM-killer" it's 
something we can easily throw away.

Not sure how many of those buffers we have, but marking everything which 
is temporary with that flag is probably a good idea.

>
> Thanks,
> Marek
>
> On Wed, May 11, 2022 at 5:58 PM Marek Olšák <maraeo at gmail.com> wrote:
>
>     Will the kernel keep all discardable buffers in VRAM if VRAM is
>     not overcommitted by discardable buffers, or will other buffers
>     also affect the placement of discardable buffers?
>

Regarding the eviction pressure the buffers will be handled like any 
other buffer, but instead of preserving the content it is just discarded 
on eviction.

>
>     Do evictions deallocate the buffer, or do they keep an allocation
>     in GTT and only the copy is skipped?
>

It really deallocates the backing store of the buffer, just keeps a 
dummy page array around where all entries are NULL.

There is a patch set on the mailing list to make this a little bit more 
efficient, but even using the dummy page array should only have a few 
bytes overhead.

Regards,
Christian.

>
>     Thanks,
>     Marek
>
>     On Wed, May 11, 2022 at 3:08 AM Marek Olšák <maraeo at gmail.com> wrote:
>
>         OK that sounds good.
>
>         Marek
>
>         On Wed, May 11, 2022 at 2:04 AM Christian König
>         <ckoenig.leichtzumerken at gmail.com> wrote:
>
>             Hi Marek,
>
>             Am 10.05.22 um 22:43 schrieb Marek Olšák:
>>             A better flag name would be:
>>             AMDGPU_GEM_CREATE_BEST_PLACEMENT_OR_DISCARD
>
>             A bit long for my taste and I think the best placement is
>             just a side effect.
>
>>
>>             Marek
>>
>>             On Tue, May 10, 2022 at 4:13 PM Marek Olšák
>>             <maraeo at gmail.com> wrote:
>>
>>                 Does this really guarantee VRAM placement? The code
>>                 doesn't say anything about that.
>>
>
>             Yes, see the code here:
>
>>
>>                     diff --git
>>                     a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
>>                     b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
>>                     index 8b7ee1142d9a..1944ef37a61e 100644
>>                     --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
>>                     +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_object.c
>>                     @@ -567,6 +567,7 @@ int amdgpu_bo_create(struct
>>                     amdgpu_device *adev,
>>                                     bp->domain;
>>                             bo->allowed_domains = bo->preferred_domains;
>>                             if (bp->type != ttm_bo_type_kernel &&
>>                     +           !(bp->flags &
>>                     AMDGPU_GEM_CREATE_DISCARDABLE) &&
>>                                 bo->allowed_domains ==
>>                     AMDGPU_GEM_DOMAIN_VRAM)
>>                                     bo->allowed_domains |=
>>                     AMDGPU_GEM_DOMAIN_GTT;
>>
>
>             The only case where this could be circumvented is when you
>             try to allocate more than physically available on an APU.
>
>             E.g. you only have something like 32 MiB VRAM and request
>             64 MiB, then the GEM code will catch the error and
>             fallback to GTT (IIRC).
>
>             Regards,
>             Christian.
>
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