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

[Alex Deucher]

On Mon, Aug 15, 2016 at 3:06 PM, Felix Kuehling <felix.kuehling at amd.com> wrote:
> Patch against current amd-staging-4.6 is attached.

Reviewed-by: Alex Deucher <alexander.deucher at amd.com>

>
> 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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