New KFD ioctls: taking the skeletons out of the closet

[Dave Airlie]

On 7 March 2018 at 08:44, Felix Kuehling <felix.kuehling at amd.com> wrote:
> Hi all,
>
> Christian raised two potential issues in a recent KFD upstreaming code
> review that are related to the KFD ioctl APIs:
>
>  1. behaviour of -ERESTARTSYS
>  2. transactional nature of KFD ioctl definitions, or lack thereof
>
> I appreciate constructive feedback, but I also want to encourage an
> open-minded rather than a dogmatic approach to API definitions. So let
> me take all the skeletons out of my closet and get these APIs reviewed
> in the appropriate forum before we commit to them upstream. See the end
> of this email for reference.
>
> The controversial part at this point is kfd_ioctl_map_memory_to_gpu. If
> any of the other APIs raise concerns or questions, please ask.
>
> Because of the HSA programming model, KFD memory management APIs are
> synchronous. There is no pipelining. Command submission to GPUs through
> user mode queues does not involve KFD. This means KFD doesn't know what
> memory is used by the GPUs and when it's used. That means, when the
> map_memory_to_gpu ioctl returns to user mode, all memory mapping
> operations are complete and the memory can be used by the CPUs or GPUs
> immediately.

I've got a few opinions, but first up I still dislike user-mode queues
and everything
they entail. I still feel they are solving a Windows problem and not a
Linux problem,
and it would be nice if we had some Linux numbers on what they gain us over
a dispatch ioctl, because they sure bring a lot of memory management issues.

That said amdkfd is here.

The first question you should ask (which you haven't asked here at all) is
what should userspace do with the ioctl result.

>
> HSA also uses a shared virtual memory model, so typically memory gets
> mapped on multiple GPUs and CPUs at the same virtual address.
>
> The point of contention seems to be the ability to map memory to
> multiple GPUs in a single ioctl and the behaviour in failure cases. I'll
> discuss two main failure cases:
>
> 1: Failure after all mappings have been dispatched via SDMA, but a
> signal interrupts the wait for completion and we return -ERESTARTSYS.
> Documentation/kernel-hacking/hacking.rst only says "[...] you should be
> prepared to process the restart, e.g. if you're in the middle of
> manipulating some data structure." I think we do that by ensuring that
> memory that's already mapped won't be mapped again. So the restart will
> become a no-op and just end up waiting for all the previous mappings to
> complete.

-ERESTARTSYS at that late stage points to a badly synchronous API,
I'd have said you should have two ioctls, one that returns a fence after
starting the processes, and one that waits for the fence separately.

The overhead of ioctls isn't your enemy until you've measured it and
proven it's a problem.

>
> Christian has a stricter requirement, and I'd like to know where that
> comes from: "An interrupted IOCTL should never have a visible effect."

Christian might be taking things a bit further but synchronous gpu access
APIs are bad, but I don't think undoing a bunch of work is a good plan either
just because you got ERESTARTSYS. If you get ERESTARTSYS can you
handle it, if I've fired off 5 SDMAs and wait for them will I fire off 5 more?
will I wait for the original SDMAs if I reenter?

>
> 2: Failure to map on some but not all GPUs. This comes down to the
> question, do all ioctl APIs or system calls in general need to be
> transactional? As a counter example I'd give incomplete read or write
> system calls that return how much was actually read or written. Our
> current implementation of map_memory_to_gpu doesn't do this, but it
> could be modified to return to user mode how many of the mappings, or
> which mappings specifically failed or succeeded.

What should userspace do? if it only get mappings on 3 of the gpus instead
of say 4? Is there a sane resolution other than calling the ioctl again with
the single GPU? Would it drop the GPU from the working set from that point on?

Need more info to do what can come out of the API doing incomplete
operations.

> The alternative would be to break multi-GPU mappings, and the final wait
> for completion, into multiple ioctl calls. That would result in
> additional system call overhead. I'd argue that the end result is the
> same for user mode, so I don't see why I'd use multiple ioctls over a
> single one.

Again stop worrying about ioctl overhead, this isn't Windows. If you
can show the overhead as being a problem then address it, but I
think it's premature worrying about it at this stage.

Dave.