Am 24.08.21 um 19:32 schrieb Jason Gunthorpe:
On Tue, Aug 24, 2021 at 10:27:23AM -0700, John Hubbard wrote:
On 8/24/21 2:32 AM, Christian König wrote:
Am 24.08.21 um 11:06 schrieb Gal Pressman:
On 23/08/2021 13:43, Christian König wrote:
Am 21.08.21 um 11:16 schrieb Gal Pressman:
On 20/08/2021 17:32, Jason Gunthorpe wrote:
On Fri, Aug 20, 2021 at 03:58:33PM +0300, Gal Pressman wrote:
...
IIUC, we're talking about three different exporter "types":
- Dynamic with move_notify (requires ODP)
- Dynamic with revoke_notify
- Static
Which changes do we need to make the third one work?
Basically none at all in the framework.
You just need to properly use the dma_buf_pin() function when you start using a
buffer (e.g. before you create an attachment) and the dma_buf_unpin() function
after you are done with the DMA-buf.
I replied to your previous mail, but I'll ask again.
Doesn't the pin operation migrate the memory to host memory?
Sorry missed your previous reply.
And yes at least for the amdgpu driver we migrate the memory to host
memory as soon as it is pinned and I would expect that other GPU drivers
do something similar.
Well...for many topologies, migrating to host memory will result in a
dramatically slower p2p setup. For that reason, some GPU drivers may
want to allow pinning of video memory in some situations.
Ideally, you've got modern ODP devices and you don't even need to pin.
But if not, and you still hope to do high performance p2p between a GPU
and a non-ODP Infiniband device, then you would need to leave the pinned
memory in vidmem.
So I think we don't want to rule out that behavior, right? Or is the
thinking more like, "you're lucky that this old non-ODP setup works at
all, and we'll make it work by routing through host/cpu memory, but it
will be slow"?
I think it depends on the user, if the user creates memory which is
permanently located on the GPU then it should be pinnable in this way
without force migration. But if the memory is inherently migratable
then it just cannot be pinned in the GPU at all as we can't
indefinately block migration from happening eg if the CPU touches it
later or something.
Yes, exactly that's the point. Especially GPUs have a great variety of
setups.
For example we have APUs where the local memory is just stolen system
memory and all buffers must be migrate-able because you might need all
of this stolen memory for scanout or page tables. In this case P2P only
makes sense to avoid the migration overhead in the first place.
Then you got dGPUs where only a fraction of the VRAM is accessible from
the PCIe BUS. Here you also absolutely don't want to pin any buffers
because that can easily crash when we need to migrate something into the
visible window for CPU access.
The only real option where you could do P2P with buffer pinning are
those compute boards where we know that everything is always accessible
to everybody and we will never need to migrate anything. But even then
you want some mechanism like cgroups to take care of limiting this.
Otherwise any runaway process can bring down your whole system.
Key question at least for me as GPU maintainer is if we are going to see
modern compute boards together with old non-ODP setups. Since those
compute boards are usually used with new hardware (like PCIe v4 for
example) the answer I think is most likely "no".
Christian.
Jason
