On 8/31/2023 3:59 PM, Felix Kuehling wrote:
On 2023-08-31 16:33, Chen, Xiaogang wrote:
That said, I'm not actually sure why we're freeing the DMA
address array after migration to RAM at all. I think we still
need it even when we're using VRAM. We call svm_range_dma_map in
svm_range_validate_and_map regardless of whether the range is in
VRAM or system memory. So it will just allocate a new array the
next time the range is validated anyway. VRAM pages use a
special address encoding to indicate VRAM pages to the GPUVM code.
I think we do not need free DMA address array as you said, it is
another thing though.
We need unmap dma address(dma_unmap_page) after migrate from ram
to vram because we always do dma_map_page at
svm_range_validate_and_map. If not we would have multiple dma
maps for same sys ram page.
svm_range_dma_map_dev calls dma_unmap_page before overwriting an
existing valid entry in the dma_addr array. Anyway, dma unmapping
the old pages in bulk may still be cleaner. And it avoids delays
in cleaning up DMA mappings after migrations.
Regards,
Felix
then we may not need do dma_unmap after migrate from ram to vram
since svm_range_dma_map_dev always do dma_unmap_page if the address
is valid dma address for sys ram, and after migrate from ram to
vram we always do gpu mapping?
I think with XNACK enabled, the DMA mapping may be delayed until a
page fault. For example on a multi-GPU system, GPU1 page faults and
migrates data from system memory to its VRAM. Immediately
afterwards, the page fault handler should use svm_validate_and_map
to update GPU1 page tables. But GPU2 page tables are not updated
immediately. So the now stale DMA mappings for GPU2 would continue
to exist until the next page fault on GPU2.
Regards,
Felix
If I understand correctly: when user call svm_range_set_attr, if
p->xnack_enabled is true, we can skip call
svm_range_validate_and_map. We postpone the buffer validating and gpu
mapping until page fault or the time the buffer really got used by a
GPU, and only dma map and gpu map for this GPU.
The current implementation of svm_range_set_attr skips the validation
after migration if XNACK is off, because it is handled by
svm_range_restore_work that gets scheduled by the MMU notifier
triggered by the migration.
With XNACK on, svm_range_set_attr currently validates and maps after
migration assuming that the data will be used by the GPU(s) soon. That
is something we could change and let page faults take care of the
mappings as needed.
Yes, with xnack on, my understanding is we can skip
svm_range_validate_and_map at svm_range_set_attr after migration, then
page fault handler will do dma and gpu mapping. That would save the
first time dma and gpu mapping which apply to all GPUs that user ask for
access. Then current gpu page fault handler just does dma and gpu
mapping for the GPU that triggered page fault. Is that ok?
Regards
Xiaogang
Regards,
Felix
Regards
Xiaogang
