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

["Marek Olšák" <maraeo@gmail.com>]

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Would it be better to set the VM_ALWAYS_VALID flag to have a greater
guarantee that the best placement will be chosen?

See, the main feature is getting the best placement, not being discardable.
The best placement is a hw design requirement due to using memory for uses
that are expected to have performance similar to onchip SRAMs. We need to
make sure the best placement is guaranteed if it's VRAM.

Marek

On Thu., May 12, 2022, 03:26 Christian König, <
ckoenig.leichtzumerken@gmail.com> wrote:

> 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@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@gmail.com> wrote:
>>
>>> OK that sounds good.
>>>
>>> Marek
>>>
>>> On Wed, May 11, 2022 at 2:04 AM Christian König <
>>> ckoenig.leichtzumerken@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@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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