On Tue, Jun 14, 2022 at 10:04:00AM +0300, Lionel Landwerlin wrote:
On 13/06/2022 21:02, Niranjana Vishwanathapura wrote:
On Mon, Jun 13, 2022 at 06:33:07AM -0700, Zeng, Oak wrote:
Regards,
Oak
-----Original Message-----
From: Intel-gfx <intel-gfx-bounces@xxxxxxxxxxxxxxxxxxxxx> On
Behalf Of Niranjana
Vishwanathapura
Sent: June 10, 2022 1:43 PM
To: Landwerlin, Lionel G <lionel.g.landwerlin@xxxxxxxxx>
Cc: Intel GFX <intel-gfx@xxxxxxxxxxxxxxxxxxxxx>; Maling list -
DRI developers <dri-
devel@xxxxxxxxxxxxxxxxxxxxx>; Hellstrom, Thomas
<thomas.hellstrom@xxxxxxxxx>;
Wilson, Chris P <chris.p.wilson@xxxxxxxxx>; Vetter, Daniel
<daniel.vetter@xxxxxxxxx>; Christian König <christian.koenig@xxxxxxx>
Subject: Re: [Intel-gfx] [RFC v3 1/3] drm/doc/rfc: VM_BIND
feature design
document
On Fri, Jun 10, 2022 at 11:18:14AM +0300, Lionel Landwerlin wrote:
On 10/06/2022 10:54, Niranjana Vishwanathapura wrote:
On Fri, Jun 10, 2022 at 09:53:24AM +0300, Lionel Landwerlin wrote:
On 09/06/2022 22:31, Niranjana Vishwanathapura wrote:
On Thu, Jun 09, 2022 at 05:49:09PM +0300, Lionel Landwerlin wrote:
On 09/06/2022 00:55, Jason Ekstrand wrote:
On Wed, Jun 8, 2022 at 4:44 PM Niranjana Vishwanathapura
<niranjana.vishwanathapura@xxxxxxxxx> wrote:
On Wed, Jun 08, 2022 at 08:33:25AM +0100, Tvrtko
Ursulin wrote:
>
>
>On 07/06/2022 22:32, Niranjana Vishwanathapura wrote:
>>On Tue, Jun 07, 2022 at 11:18:11AM -0700, Niranjana
Vishwanathapura
wrote:
>>>On Tue, Jun 07, 2022 at 12:12:03PM -0500, Jason
Ekstrand wrote:
>>>> On Fri, Jun 3, 2022 at 6:52 PM Niranjana
Vishwanathapura
>>>> <niranjana.vishwanathapura@xxxxxxxxx> wrote:
>>>>
>>>> On Fri, Jun 03, 2022 at 10:20:25AM +0300, Lionel
Landwerlin
wrote:
>>>> > On 02/06/2022 23:35, Jason Ekstrand wrote:
>>>> >
>>>> > On Thu, Jun 2, 2022 at 3:11 PM Niranjana
Vishwanathapura
>>>> > <niranjana.vishwanathapura@xxxxxxxxx> wrote:
>>>> >
>>>> > On Wed, Jun 01, 2022 at 01:28:36PM
-0700, Matthew
>>>>Brost wrote:
>>>> > >On Wed, Jun 01, 2022 at 05:25:49PM
+0300, Lionel
Landwerlin
>>>> wrote:
>>>> > >> On 17/05/2022 21:32, Niranjana Vishwanathapura
wrote:
>>>> > >> > +VM_BIND/UNBIND ioctl will immediately start
>>>> binding/unbinding
>>>> > the mapping in an
>>>> > >> > +async worker. The binding and
unbinding will
>>>>work like a
>>>> special
>>>> > GPU engine.
>>>> > >> > +The binding and unbinding operations are
serialized and
>>>> will
>>>> > wait on specified
>>>> > >> > +input fences before the operation
and will signal
the
>>>> output
>>>> > fences upon the
>>>> > >> > +completion of the operation. Due to
serialization,
>>>> completion of
>>>> > an operation
>>>> > >> > +will also indicate that all
previous operations
>>>>are also
>>>> > complete.
>>>> > >>
>>>> > >> I guess we should avoid saying "will
immediately
start
>>>> > binding/unbinding" if
>>>> > >> there are fences involved.
>>>> > >>
>>>> > >> And the fact that it's happening in an async
>>>>worker seem to
>>>> imply
>>>> > it's not
>>>> > >> immediate.
>>>> > >>
>>>> >
>>>> > Ok, will fix.
>>>> > This was added because in earlier design
binding was
deferred
>>>> until
>>>> > next execbuff.
>>>> > But now it is non-deferred (immediate in
that sense).
>>>>But yah,
>>>> this is
>>>> > confusing
>>>> > and will fix it.
>>>> >
>>>> > >>
>>>> > >> I have a question on the behavior of the bind
>>>>operation when
>>>> no
>>>> > input fence
>>>> > >> is provided. Let say I do :
>>>> > >>
>>>> > >> VM_BIND (out_fence=fence1)
>>>> > >>
>>>> > >> VM_BIND (out_fence=fence2)
>>>> > >>
>>>> > >> VM_BIND (out_fence=fence3)
>>>> > >>
>>>> > >>
>>>> > >> In what order are the fences going to
be signaled?
>>>> > >>
>>>> > >> In the order of VM_BIND ioctls? Or out
of order?
>>>> > >>
>>>> > >> Because you wrote "serialized I assume
it's : in
order
>>>> > >>
>>>> >
>>>> > Yes, in the order of VM_BIND/UNBIND
ioctls. Note that
>>>>bind and
>>>> unbind
>>>> > will use
>>>> > the same queue and hence are ordered.
>>>> >
>>>> > >>
>>>> > >> One thing I didn't realize is that
because we only
get one
>>>> > "VM_BIND" engine,
>>>> > >> there is a disconnect from the Vulkan
specification.
>>>> > >>
>>>> > >> In Vulkan VM_BIND operations are
serialized but
>>>>per engine.
>>>> > >>
>>>> > >> So you could have something like this :
>>>> > >>
>>>> > >> VM_BIND (engine=rcs0, in_fence=fence1,
out_fence=fence2)
>>>> > >>
>>>> > >> VM_BIND (engine=ccs0, in_fence=fence3,
out_fence=fence4)
>>>> > >>
>>>> > >>
>>>> > >> fence1 is not signaled
>>>> > >>
>>>> > >> fence3 is signaled
>>>> > >>
>>>> > >> So the second VM_BIND will proceed before the
>>>>first VM_BIND.
>>>> > >>
>>>> > >>
>>>> > >> I guess we can deal with that scenario in
>>>>userspace by doing
>>>> the
>>>> > wait
>>>> > >> ourselves in one thread per engines.
>>>> > >>
>>>> > >> But then it makes the VM_BIND input
fences useless.
>>>> > >>
>>>> > >>
>>>> > >> Daniel : what do you think? Should be
rework this or
just
>>>> deal with
>>>> > wait
>>>> > >> fences in userspace?
>>>> > >>
>>>> > >
>>>> > >My opinion is rework this but make the
ordering via
>>>>an engine
>>>> param
>>>> > optional.
>>>> > >
>>>> > >e.g. A VM can be configured so all binds
are ordered
>>>>within the
>>>> VM
>>>> > >
>>>> > >e.g. A VM can be configured so all binds
accept an
engine
>>>> argument
>>>> > (in
>>>> > >the case of the i915 likely this is a
gem context
>>>>handle) and
>>>> binds
>>>> > >ordered with respect to that engine.
>>>> > >
>>>> > >This gives UMDs options as the later
likely consumes
>>>>more KMD
>>>> > resources
>>>> > >so if a different UMD can live with
binds being
>>>>ordered within
>>>> the VM
>>>> > >they can use a mode consuming less resources.
>>>> > >
>>>> >
>>>> > I think we need to be careful here if we
are looking
for some
>>>> out of
>>>> > (submission) order completion of vm_bind/unbind.
>>>> > In-order completion means, in a batch of
binds and
>>>>unbinds to be
>>>> > completed in-order, user only needs to specify
>>>>in-fence for the
>>>> > first bind/unbind call and the our-fence
for the last
>>>> bind/unbind
>>>> > call. Also, the VA released by an unbind
call can be
>>>>re-used by
>>>> > any subsequent bind call in that
in-order batch.
>>>> >
>>>> > These things will break if
binding/unbinding were to
>>>>be allowed
>>>> to
>>>> > go out of order (of submission) and user
need to be
extra
>>>> careful
>>>> > not to run into pre-mature triggereing of
out-fence and
bind
>>>> failing
>>>> > as VA is still in use etc.
>>>> >
>>>> > Also, VM_BIND binds the provided
mapping on the
specified
>>>> address
>>>> > space
>>>> > (VM). So, the uapi is not engine/context
specific.
>>>> >
>>>> > We can however add a 'queue' to the uapi
which can be
>>>>one from
>>>> the
>>>> > pre-defined queues,
>>>> > I915_VM_BIND_QUEUE_0
>>>> > I915_VM_BIND_QUEUE_1
>>>> > ...
>>>> > I915_VM_BIND_QUEUE_(N-1)
>>>> >
>>>> > KMD will spawn an async work queue for
each queue which
will
>>>> only
>>>> > bind the mappings on that queue in the
order of
submission.
>>>> > User can assign the queue to per engine
or anything
>>>>like that.
>>>> >
>>>> > But again here, user need to be
careful and not
>>>>deadlock these
>>>> > queues with circular dependency of fences.
>>>> >
>>>> > I prefer adding this later an as
extension based on
>>>>whether it
>>>> > is really helping with the implementation.
>>>> >
>>>> > I can tell you right now that having
everything on a
single
>>>> in-order
>>>> > queue will not get us the perf we want.
What vulkan
>>>>really wants
>>>> is one
>>>> > of two things:
>>>> > 1. No implicit ordering of VM_BIND
ops. They just
happen in
>>>> whatever
>>>> > their dependencies are resolved and we
ensure ordering
>>>>ourselves
>>>> by
>>>> > having a syncobj in the VkQueue.
>>>> > 2. The ability to create multiple VM_BIND
queues. We
need at
>>>> least 2
>>>> > but I don't see why there needs to be a
limit besides
>>>>the limits
>>>> the
>>>> > i915 API already has on the number of
engines. Vulkan
could
>>>> expose
>>>> > multiple sparse binding queues to the
client if it's not
>>>> arbitrarily
>>>> > limited.
>>>>
>>>> Thanks Jason, Lionel.
>>>>
>>>> Jason, what are you referring to when you say
"limits the i915
API
>>>> already
>>>> has on the number of engines"? I am not sure if
there is such
an uapi
>>>> today.
>>>>
>>>> There's a limit of something like 64 total engines
today based on
the
>>>> number of bits we can cram into the exec flags in
execbuffer2. I
think
>>>> someone had an extended version that allowed more
but I ripped it
out
>>>> because no one was using it. Of course,
execbuffer3 might not
>>>>have that
>>>> problem at all.
>>>>
>>>
>>>Thanks Jason.
>>>Ok, I am not sure which exec flag is that, but yah,
execbuffer3
probably
>>>will not have this limiation. So, we need to define a
VM_BIND_MAX_QUEUE
>>>and somehow export it to user (I am thinking of
embedding it in
>>>I915_PARAM_HAS_VM_BIND. bits[0]->HAS_VM_BIND,
bits[1-3]->'n'
meaning 2^n
>>>queues.
>>
>>Ah, I think you are waking about I915_EXEC_RING_MASK
(0x3f) which
execbuf3
Yup! That's exactly the limit I was talking about.
>>will also have. So, we can simply define in vm_bind/unbind
structures,
>>
>>#define I915_VM_BIND_MAX_QUEUE 64
>> __u32 queue;
>>
>>I think that will keep things simple.
>
>Hmmm? What does execbuf2 limit has to do with how
many engines
>hardware can have? I suggest not to do that.
>
>Change with added this:
>
> if (set.num_engines > I915_EXEC_RING_MASK + 1)
> return -EINVAL;
>
>To context creation needs to be undone and so let users
create engine
>maps with all hardware engines, and let execbuf3 access
them all.
>
Earlier plan was to carry I915_EXEC_RING_MAP (0x3f) to
execbuff3 also.
Hence, I was using the same limit for VM_BIND queues
(64, or 65 if we
make it N+1).
But, as discussed in other thread of this RFC series, we
are planning
to drop this I915_EXEC_RING_MAP in execbuff3. So,
there won't be
any uapi that limits the number of engines (and hence
the vm_bind
queues
need to be supported).
If we leave the number of vm_bind queues to be
arbitrarily large
(__u32 queue_idx) then, we need to have a hashmap for
queue (a wq,
work_item and a linked list) lookup from the user
specified queue
index.
Other option is to just put some hard limit (say 64 or
65) and use
an array of queues in VM (each created upon first use).
I prefer this.
I don't get why a VM_BIND queue is any different from any
other queue or
userspace-visible kernel object. But I'll leave those
details up to
danvet or whoever else might be reviewing the implementation.
--Jason
I kind of agree here. Wouldn't be simpler to have the bind
queue created
like the others when we build the engine map?
For userspace it's then just matter of selecting the right
queue ID when
submitting.
If there is ever a possibility to have this work on the GPU,
it would be
all ready.
I did sync offline with Matt Brost on this.
We can add a VM_BIND engine class and let user create VM_BIND
engines (queues).
The problem is, in i915 engine creating interface is bound to
gem_context.
So, in vm_bind ioctl, we would need both context_id and
queue_idx for proper
lookup of the user created engine. This is bit ackward as
vm_bind is an
interface to VM (address space) and has nothing to do with
gem_context.
A gem_context has a single vm object right?
Set through I915_CONTEXT_PARAM_VM at creation or given a default
one if not.
So it's just like picking up the vm like it's done at execbuffer
time right now : eb->context->vm
Are you suggesting replacing 'vm_id' with 'context_id' in the
VM_BIND/UNBIND
ioctl and probably call it CONTEXT_BIND/UNBIND, because VM can be
obtained
from the context?
Yes, because if we go for engines, they're associated with a context
and so also associated with the VM bound to the context.
Hmm...context doesn't sould like the right interface. It should be
VM and engine (independent of context). Engine can be virtual or soft
engine (kernel thread), each with its own queue. We can add an
interface
to create such engines (independent of context). But we are anway
implicitly creating it when user uses a new queue_idx. If in future
we have hardware engines for VM_BIND operation, we can have that
explicit inteface to create engine instances and the queue_index
in vm_bind/unbind will point to those engines.
Anyone has any thoughts? Daniel?
Exposing gem_context or intel_context to user space is a strange
concept to me. A context represent some hw resources that is used
to complete certain task. User space should care allocate some
resources (memory, queues) and submit tasks to queues. But user
space doesn't care how certain task is mapped to a HW context -
driver/guc should take care of this.
So a cleaner interface to me is: user space create a vm, create
gem object, vm_bind it to a vm; allocate queues (internally
represent compute or blitter HW. Queue can be virtual to user) for
this vm; submit tasks to queues. User can create multiple queues
under one vm. One queue is only for one vm.
I915 driver/guc manage the hw compute or blitter resources which
is transparent to user space. When i915 or guc decide to schedule
a queue (run tasks on that queue), a HW engine will be pick up and
set up properly for the vm of that queue (ie., switch to page
tables of that vm) - this is a context switch.
From vm_bind perspective, it simply bind a gem_object to a vm.
Engine/queue is not a parameter to vm_bind, as any engine can be
pick up by i915/guc to execute a task using the vm bound va.
I didn't completely follow the discussion here. Just share some
thoughts.
Yah, I agree.
Lionel,
How about we define the queue as
union {
__u32 queue_idx;
__u64 rsvd;
}
If required, we can extend by expanding the 'rsvd' field to <ctx_id,
queue_idx> later
with a flag.
Niranjana
I did not really understand Oak's comment nor what you're suggesting
here to be honest.
First the GEM context is already exposed to userspace. It's explicitly
created by userpace with DRM_IOCTL_I915_GEM_CONTEXT_CREATE.
We give the GEM context id in every execbuffer we do with
drm_i915_gem_execbuffer2::rsvd1.
It's still in the new execbuffer3 proposal being discussed.
Second, the GEM context is also where we set the VM with
I915_CONTEXT_PARAM_VM.
Third, the GEM context also has the list of engines with
I915_CONTEXT_PARAM_ENGINES.
Yes, the execbuf and engine map creation are tied to gem_context.
(which probably is not the best interface.)
So it makes sense to me to dispatch the vm_bind operation to a GEM
context, to a given vm_bind queue, because it's got all the
information required :
- the list of new vm_bind queues
- the vm that is going to be modified
But the operation is performed here on the address space (VM) which
can have multiple gem_contexts referring to it. So, VM is the right
interface here. We need not 'gem_context'ify it.
All we need is multiple queue support for the address space (VM).
Going to gem_context for that just because we have engine creation
support there seems unnecessay and not correct to me.
Otherwise where do the vm_bind queues live?
In the i915/drm fd object?
That would mean that all the GEM contexts are sharing the same vm_bind
queues.
Not all, only the gem contexts that are using the same address space (VM).
But to me the right way to describe would be that "VM will be using those
queues".
Niranjana
intel_context or GuC are internal details we're not concerned about.
I don't really see the connection with the GEM context.
Maybe Oak has a different use case than Vulkan.
-Lionel
Regards,
Oak
Niranjana
I think the interface is clean as a interface to VM. It is
only that we
don't have a clean way to create a raw VM_BIND engine (not
associated with
any context) with i915 uapi.
May be we can add such an interface, but I don't think that is
worth it
(we might as well just use a queue_idx in VM_BIND/UNBIND ioctl as I
mentioned
above).
Anyone has any thoughts?
Another problem is, if two VMs are binding with the same defined
engine,
binding on VM1 can get unnecessary blocked by binding on VM2
(which may be
waiting on its in_fence).
Maybe I'm missing something, but how can you have 2 vm objects
with a single gem_context right now?
No, we don't have 2 VMs for a gem_context.
Say if ctx1 with vm1 and ctx2 with vm2.
First vm_bind call was for vm1 with q_idx 1 in ctx1 engine map.
Second vm_bind call was for vm2 with q_idx 2 in ctx2 engine map. If
those two queue indicies points to same underlying vm_bind engine,
then the second vm_bind call gets blocked until the first
vm_bind call's
'in' fence is triggered and bind completes.
With per VM queues, this is not a problem as two VMs will not endup
sharing same queue.
BTW, I just posted a updated PATCH series.
https://www.spinics.net/lists/dri-devel/msg350483.html
Niranjana
So, my preference here is to just add a 'u32 queue' index in
vm_bind/unbind
ioctl, and the queues are per VM.
Niranjana
Thanks,
-Lionel
Niranjana
>Regards,
>
>Tvrtko
>
>>
>>Niranjana
>>
>>>
>>>> I am trying to see how many queues we need and
don't want it to
be
>>>> arbitrarily
>>>> large and unduely blow up memory usage and
complexity in i915
driver.
>>>>
>>>> I expect a Vulkan driver to use at most 2 in the
vast majority
>>>>of cases. I
>>>> could imagine a client wanting to create more
than 1 sparse
>>>>queue in which
>>>> case, it'll be N+1 but that's unlikely. As far as
complexity
>>>>goes, once
>>>> you allow two, I don't think the complexity is
going up by
>>>>allowing N. As
>>>> for memory usage, creating more queues means more
memory. That's
a
>>>> trade-off that userspace can make. Again, the
expected number
>>>>here is 1
>>>> or 2 in the vast majority of cases so I don't think
you need to
worry.
>>>
>>>Ok, will start with n=3 meaning 8 queues.
>>>That would require us create 8 workqueues.
>>>We can change 'n' later if required.
>>>
>>>Niranjana
>>>
>>>>
>>>> > Why? Because Vulkan has two basic kind of bind
>>>>operations and we
>>>> don't
>>>> > want any dependencies between them:
>>>> > 1. Immediate. These happen right after BO
creation or
>>>>maybe as
>>>> part of
>>>> > vkBindImageMemory() or VkBindBufferMemory(). These
>>>>don't happen
>>>> on a
>>>> > queue and we don't want them serialized
with anything. To
>>>> synchronize
>>>> > with submit, we'll have a syncobj in the
VkDevice which
is
>>>> signaled by
>>>> > all immediate bind operations and make
submits wait on
it.
>>>> > 2. Queued (sparse): These happen on a
VkQueue which may
be the
>>>> same as
>>>> > a render/compute queue or may be its own
queue. It's up
to us
>>>> what we
>>>> > want to advertise. From the Vulkan API
PoV, this is like
any
>>>> other
>>>> > queue. Operations on it wait on and signal
semaphores. If we
>>>> have a
>>>> > VM_BIND engine, we'd provide syncobjs to
wait and
>>>>signal just like
>>>> we do
>>>> > in execbuf().
>>>> > The important thing is that we don't want
one type of
>>>>operation to
>>>> block
>>>> > on the other. If immediate binds are
blocking on sparse
binds,
>>>> it's
>>>> > going to cause over-synchronization issues.
>>>> > In terms of the internal implementation, I
know that
>>>>there's going
>>>> to be
>>>> > a lock on the VM and that we can't actually
do these
things in
>>>> > parallel. That's fine. Once the dma_fences have
signaled and
>>>> we're
>>>>
>>>> Thats correct. It is like a single VM_BIND
engine with
>>>>multiple queues
>>>> feeding to it.
>>>>
>>>> Right. As long as the queues themselves are
independent and
>>>>can block on
>>>> dma_fences without holding up other queues, I think
we're fine.
>>>>
>>>> > unblocked to do the bind operation, I don't care if
>>>>there's a bit
>>>> of
>>>> > synchronization due to locking. That's
expected. What
>>>>we can't
>>>> afford
>>>> > to have is an immediate bind operation
suddenly blocking
on a
>>>> sparse
>>>> > operation which is blocked on a compute job
that's going
to run
>>>> for
>>>> > another 5ms.
>>>>
>>>> As the VM_BIND queue is per VM, VM_BIND on one VM
doesn't block
the
>>>> VM_BIND
>>>> on other VMs. I am not sure about usecases
here, but just
wanted to
>>>> clarify.
>>>>
>>>> Yes, that's what I would expect.
>>>> --Jason
>>>>
>>>> Niranjana
>>>>
>>>> > For reference, Windows solves this by allowing
arbitrarily many
>>>> paging
>>>> > queues (what they call a VM_BIND
engine/queue). That
>>>>design works
>>>> > pretty well and solves the problems in
question. >>>>Again, we could
>>>> just
>>>> > make everything out-of-order and require
using syncobjs
>>>>to order
>>>> things
>>>> > as userspace wants. That'd be fine too.
>>>> > One more note while I'm here: danvet said
something on
>>>>IRC about
>>>> VM_BIND
>>>> > queues waiting for syncobjs to
materialize. We don't
really
>>>> want/need
>>>> > this. We already have all the machinery in
userspace to
handle
>>>> > wait-before-signal and waiting for syncobj
fences to
>>>>materialize
>>>> and
>>>> > that machinery is on by default. It
would actually
>>>>take MORE work
>>>> in
>>>> > Mesa to turn it off and take advantage of
the kernel
>>>>being able to
>>>> wait
>>>> > for syncobjs to materialize. Also, getting
that right is
>>>> ridiculously
>>>> > hard and I really don't want to get it
wrong in kernel
>>>>space. �� When we
>>>> > do memory fences, wait-before-signal will
be a thing. We
don't
>>>> need to
>>>> > try and make it a thing for syncobj.
>>>> > --Jason
>>>> >
>>>> > Thanks Jason,
>>>> >
>>>> > I missed the bit in the Vulkan spec that
we're allowed to
have a
>>>> sparse
>>>> > queue that does not implement either graphics
or compute
>>>>operations
>>>> :
>>>> >
>>>> > "While some implementations may include
>>>> VK_QUEUE_SPARSE_BINDING_BIT
>>>> > support in queue families that also include
>>>> >
>>>> > graphics and compute support, other
implementations may
only
>>>> expose a
>>>> > VK_QUEUE_SPARSE_BINDING_BIT-only queue
>>>> >
>>>> > family."
>>>> >
>>>> > So it can all be all a vm_bind engine that
just does
bind/unbind
>>>> > operations.
>>>> >
>>>> > But yes we need another engine for the
immediate/non-sparse
>>>> operations.
>>>> >
>>>> > -Lionel
>>>> >
>>>> > >
>>>> > Daniel, any thoughts?
>>>> >
>>>> > Niranjana
>>>> >
>>>> > >Matt
>>>> > >
>>>> > >>
>>>> > >> Sorry I noticed this late.
>>>> > >>
>>>> > >>
>>>> > >> -Lionel
>>>> > >>
>>>> > >>
