Re: [PATCH 1/2] drm/ttm: Don't evict SG BOs

[Felix Kuehling <felix.kuehling@amd.com>]

Am 2021-04-28 um 3:04 a.m. schrieb Christian König:
> Am 28.04.21 um 07:33 schrieb Felix Kuehling:
>> SG BOs do not occupy space that is managed by TTM. So do not evict them.
>>
>> This fixes unexpected evictions of KFD's userptr BOs. KFD only expects
>> userptr "evictions" in the form of MMU notifiers.
>
> NAK, SG BOs also account for the memory the GPU can currently access. 
>
> We can ignore them for the allocated memory, but not for the GTT domain.
Hmm, the only reason I found this problem is, that I am now testing with
IOMMU enabled. Evicting the userptr BO destroys the DMA mapping. Without
IOMMU-enforced device isolation I was blissfully unaware that the
userptr BOs were being evicted. The GPUVM mappings were unaffected and
just worked without problems. Having to evict these BOs is crippling
KFD's ability to map system memory for GPU access, once again.

I think this affects not only userptr BOs but also DMABuf imports for
BOs shared between multiple GPUs.

The GTT size limitation is entirely artificial. And the only reason I
know of for keeping it limited to the VRAM size is to work around some
OOM issues with GTT BOs. Applying this to userptrs and DMABuf imports
makes no sense. But I understand that the way TTM manages the GTT domain
there is no easy fix for this. Maybe we'd have to create a new domain
for validating SG BOs that's separate from GTT, so that TTM would not
try to allocate GTT space for them.

Failing that, I'd probably have to abandon userptr BOs altogether and
switch system memory mappings over to using the new SVM API on systems
where it is avaliable.

Regards,
  Felix


>
> Christian.
>
>>
>> Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
>> ---
>>   drivers/gpu/drm/ttm/ttm_bo.c | 4 ++++
>>   1 file changed, 4 insertions(+)
>>
>> diff --git a/drivers/gpu/drm/ttm/ttm_bo.c b/drivers/gpu/drm/ttm/ttm_bo.c
>> index de1ec838cf8b..0b953654fdbf 100644
>> --- a/drivers/gpu/drm/ttm/ttm_bo.c
>> +++ b/drivers/gpu/drm/ttm/ttm_bo.c
>> @@ -655,6 +655,10 @@ int ttm_mem_evict_first(struct ttm_device *bdev,
>>           list_for_each_entry(bo, &man->lru[i], lru) {
>>               bool busy;
>>   +            /* Don't evict SG BOs */
>> +            if (bo->ttm && bo->ttm->sg)
>> +                continue;
>> +
>>               if (!ttm_bo_evict_swapout_allowable(bo, ctx, &locked,
>>                                   &busy)) {
>>                   if (busy && !busy_bo && ticket !=
>
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