Reverted another change to fix buffer move hangs (was Re: [PATCH] drm/ttm: partial revert "cleanup ttm_tt_(unbind|destroy)" v2)

[Felix Kuehling]

Patch against current amd-staging-4.6 is attached.

Regards,
  Felix


On 16-08-13 05:25 AM, Christian König wrote:
> Am 13.08.2016 um 01:22 schrieb Felix Kuehling:
>> [CC Kent FYI]
>>
>> On 16-08-11 04:31 PM, Deucher, Alexander wrote:
>>>> -----Original Message-----
>>>> From: amd-gfx [mailto:amd-gfx-bounces at lists.freedesktop.org] On Behalf
>>>> Of Felix Kuehling
>>>> Sent: Thursday, August 11, 2016 3:52 PM
>>>> To: Michel Dänzer; Christian König
>>>> Cc: amd-gfx at lists.freedesktop.org
>>>> Subject: Reverted another change to fix buffer move hangs (was Re:
>>>> [PATCH] drm/ttm: partial revert "cleanup ttm_tt_(unbind|destroy)" v2)
>>>>
>>>> We had to revert another change on the KFD branch to fix a buffer move
>>>> problem: 8b6b79f43801f00ddcdc10a4d5719eba4b2e32aa (drm/amdgpu:
>>>> group BOs
>>>> by log2 of the size on the LRU v2
>>> That makes sense.  I think you may want a different LRU scheme for
>>> KFD or at least special handling for KFD buffers.
>> [FK] But I think the patch shouldn't cause hangs, regardless.
>>
>> I eventually found what the problem was. The "group BOs by log2 of the
>> size on the LRU v2" patch exposed a latent bug related to the GART size.
>> On our KFD branch, we calculate the GART size differently, and it can
>> easily go above 4GB. I think on amd-staging-4.6 the GART size can also
>> go above 4GB on cards with lots of VRAM.
>>
>> However, the offset parameter in amdgpu_gart_bind and unbind is only
>> 32-bit. With the patch our test ended up using GART offsets beyond 4GB
>> for the first time. Changing the offset parameter to uint64_t fixes the
>> problem.
>
> Nice catch, please provide a patch to fix this.
>
>> Our test also demonstrates a potential flaw in the log2 grouping patch:
>> When a buffer of a previously unused size is added to the LRU, it gets
>> added to the front of the list, rather than the tail. So an application
>> that allocates a very large buffer after a bunch of smaller buffers, is
>> very likely to have that buffer evicted over and over again before any
>> smaller buffers are considered for eviction. I believe, this can result
>> in thrashing of large buffers.
>>
>> Some other observations: When the last BO of a given size is removed
>> from the LRU list, the LRU tail for that size is left "floating" in the
>> middle of the LRU list. So the next BO of that size that is added, will
>> be added at an arbitrary position in the list. It may even end up in the
>> middle of a block of pages of a different size. So a log2 grouping may
>> end up being split.
>
> Yeah, those are more or less known issues.
>
> Keep in mind that we only added the grouping by log2 of the size to
> have a justification to push the TTM changes upstream for the coming
> KFD fences.
>
> E.g. so that we are able to have this upstream before we try to push
> on the fence code.
>
> I will take a look at fixing those issues when I have time, shouldn't
> be to complicated to set the entries to zero when they aren't used or
> adjust other entries as well when some are added.
>
> Regards,
> Christian.
>
>>
>> Regards,
>>    Felix
>>
>> _______________________________________________
>> amd-gfx mailing list
>> amd-gfx at lists.freedesktop.org
>> https://lists.freedesktop.org/mailman/listinfo/amd-gfx
>
>

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