[SNIP]
I'm confused here: Atm ->moving isn't in resv_obj, there's only one
exclusive fence. And yes you need to set that every time you do a move
(because a move needs to be pretty exclusive access). But I'm not seeing a
separate not_quite_exclusive fence slot for moves.
Yeah, but shouldn't that be sufficient? I mean why does somebody else
than the exporter needs to know when a BO is moving?
I think for buffer sharing there's not much use for this, but it
sounded like you want to use ->move_notify also more internally. And
in that case, for vk, you want to be able to ignore the implicit
fences as much as possible. But you can't ignore the buffer moves ofc.
Hence tracking those separate could be useful.
Yeah, but for this case I can still rely on using ttm_bo->moving. So no
need to actually change that.
amdgpu seems to be solving this internally by never attaching an
exclusive fence for implicit stuff, or something like that, except
when it's shared. But in general you need to assume a funky mix of
implicit and explicit sync'ed workloads, and for those tracking the
moves separately would be good.
Actually we have an "owner" for each fence which is basically a "void*"
pointer.
If we see that a command submission is coming from the same "owner" we
just avoid synchronization at all.
For buffer moves the owner is simply NULL (or some other special value),
and so we always sync to those.
[SNIP]
- You sound like you want to use this a lot more, even internally in
amdgpu. For that I do think the sepearate dma_fence just to make sure
the buffer is accessible will be needed in resv_obj.
- Once we have ->moving I think there's some good chances to extract a bit
of the eviction/pipeline bo move boilerplate from ttm, and maybe use it
in other drivers. i915 could already make use of this in upstream, since
we already pipeline get_pages and clflush of buffers. Ofc once we have
vram support, even more useful.
I actually indeed wanted to add more stuff to the reservation object
implementation, like finally cleaning up the distinction of readers/writers.
Hm, more details? Not ringing a bell ...
I'm not yet sure about the details either, so please just wait until I
solved that all up for me first.
Ah is this about amdgpu doing something else for implicit sync than
what's supposed to be done, and a bit a mismatch when you deal with
shared buffers?
Yes, exactly.
And cleaning up the fence removal hack we have in the KFD for freed up BOs.
That would also allow for getting rid of this in the long term.
Hm, what's that for?
When the KFD frees up memory it removes their eviction fence from the
reservation object instead of setting it as signaled and adding a new
one to all other used reservation objects.
Oh so just a fast-path for destryoing memory that's in-flight in some move?
Yes exactly that again.
Christian.
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