Am 04.05.21 um 11:47 schrieb Daniel Vetter:
[SNIP]
Yeah, it just takes to long for the preemption to complete to be really
useful for the feature we are discussing here.
As I said when the kernel requests to preempt a queue we can easily expect a
timeout of ~100ms until that comes back. For compute that is even in the
multiple seconds range.
100ms for preempting an idle request sounds like broken hw to me. Of
course preemting something that actually runs takes a while, that's
nothing new. But it's also not the thing we're talking about here. Is this
100ms actual numbers from hw for an actual idle ringbuffer?
Well 100ms is just an example of the scheduler granularity. Let me
explain in a wider context.
The hardware can have X queues mapped at the same time and every Y time
interval the hardware scheduler checks if those queues have changed and
only if they have changed the necessary steps to reload them are started.
Multiple queues can be rendering at the same time, so you can have X as
a high priority queue active and just waiting for a signal to start and
the client rendering one frame after another and a third background
compute task mining bitcoins for you.
As long as everything is static this is perfectly performant. Adding a
queue to the list of active queues is also relatively simple, but taking
one down requires you to wait until we are sure the hardware has seen
the change and reloaded the queues.
Think of it as an RCU grace period. This is simply not something which
is made to be used constantly, but rather just at process termination.
The "preemption" feature is really called suspend and made just for the case
when we want to put a process to sleep or need to forcefully kill it for
misbehavior or stuff like that. It is not meant to be used in normal
operation.
If we only attach it on ->move then yeah maybe a last resort possibility to
do it this way, but I think in that case we could rather stick with kernel
submissions.
Well this is a hybrid userspace ring + kernel augmeted submit mode, so you
can keep dma-fences working. Because the dma-fence stuff wont work with
pure userspace submit, I think that conclusion is rather solid. Once more
even after this long thread here.
When assisted with unload fences, then yes. Problem is that I can't see
how we could implement those performant currently.
Also, if userspace lies to us and keeps pushing crap into the ring
after it's supposed to be idle: Userspace is already allowed to waste
gpu time. If you're too worried about this set a fairly aggressive
preempt timeout on the unload fence, and kill the context if it takes
longer than what preempting an idle ring should take (because that
would indicate broken/evil userspace).
I think you have the wrong expectation here. It is perfectly valid and
expected for userspace to keep writing commands into the ring buffer.
After all when one frame is completed they want to immediately start
rendering the next one.
Sure, for the true userspace direct submit model. But with that you don't
get dma-fence, which means this gpu will not work for 3d accel on any
current linux desktop.
I'm not sure of that. I've looked a bit into how we could add user
fences to dma_resv objects and that isn't that hard after all.
Which sucks, hence some hybrid model of using the userspace ring and
kernel augmented submit is needed. Which was my idea.
Yeah, I think when our firmware folks would really remove the kernel
queue and we still don't have
[SNIP]
Can't find that of hand either, but see the amdgpu_noretry module option.
It basically tells the hardware if retry page faults should be supported or
not because this whole TLB shutdown thing when they are supported is
extremely costly.
Hm so synchronous tlb shootdown is a lot more costly when you allow
retrying of page faults?
Partially correct, yes.
See when you have retry page faults enabled and unmap something you need
to make sure that everybody which could have potentially translated that
page and has a TLB is either invalidated or waited until the access is
completed.
Since every CU could be using a memory location that takes ages to
completed compared to the normal invalidation where you just invalidate
the L1/L2 and are done.
Additional to that the recovery adds some extra overhead to every memory
access, so even without a fault you are quite a bit slower if this is
enabled.
That sounds bad, because for full hmm mode you need to be able to retry
pagefaults. Well at least the PASID/ATS/IOMMU side will do that, and might just
hang your gpu for a long time while it's waiting for the va->pa lookup
response to return. So retrying lookups shouldn't be any different really.
And you also need fairly fast synchronous tlb shootdown for hmm. So if
your hw has a problem with both together that sounds bad.
Completely agree. And since it was my job to validate the implementation
on Vega10 I was also the first one to realize that.
Felix, a couple of others and me are trying to work around those
restrictions ever since.
I was more thinking about handling it all in the kernel.
Yeah can do, just means that you also have to copy the ringbuffer stuff
over from userspace to the kernel.
That is my least worry. The IBs are just addr+length., so no more than
16 bytes for each IB.
It also means that there's more differences in how your userspace works
between full userspace mode (necessary for compute) and legacy dma-fence
mode (necessary for desktop 3d). Which is especially big fun for vulkan,
since that will have to do both.
That is the bigger problem.
Christian.
But then amd is still hanging onto the amdgpu vs amdkfd split, so you're
going for max pain in this area anyway :-P
-Daniel
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