Riley (CCed) and I will be at Plumbers in a couple weeks.
There is a session on sync planned in the Android track, and of course we'll be available to chat.
On Thu, Oct 2, 2014 at 1:44 PM, Daniel Vetter daniel@ffwll.ch wrote:
On Thu, Oct 02, 2014 at 05:59:51PM +0300, Lauri Peltonen wrote:
+Rom who seems to be presenting about mainlining android sync at linux
plumbers
Also add Greg KH as fyi that we're working on de-stage one of the android subsystems.
On Wed, Oct 01, 2014 at 05:58:52PM +0200, Maarten Lankhorst wrote:
You could neuter implicit fences by always attaching the fences as shared when explicit syncing is used. This would work correctly with eviction, and wouldn't cause any unneeded syncing. :)
Yes, that will probably work! So, just to reiterate that I understood
you and
Daniel correctly:
- de-stage sync_fence and it's user space API (the tedious part)
- add dma-buf ioctls for extracting and attaching explicit fences
- Nouveau specific: allow flagging gem buffers for explicit sync
- do not check pre-fences from explicitly synced buffers at submit
- continue to attach a shared fence after submit so that pinning and unmapping continue to work
- Have explicit in/out fences for the pushbuf ioctl is missing I guess in this step?I also think we need some kind of demonstration vehicle using nouveau to satisfy Dave Airlie's open-source userspace requirements for new interfaces. Might be good to chat with him to make sure we have that covered (and how much needs to be there really).
Then working sets and getting rid of locking all buffers individually can be dealt with later as an optimization.
Yeah, sounds like a good plan.
On Wed, Oct 01, 2014 at 07:27:21PM +0200, Daniel Vetter wrote:
On Wed, Oct 01, 2014 at 06:14:16PM +0300, Lauri Peltonen wrote:
Implicit fences attached to individual buffers are one way for
residency
management. Do you think a working set based model could work in
the DRM
framework? For example, something like this:
- Allow user space to create "working set objects" and associate
buffers with
them. If the user space doesn't want to manage working sets
explicitly, it
could also use an implicit default working set that contains all
buffers that
are mapped to the channel vm (on Android we could always use the
default
working set since we don't need to manage residency). The working
sets are
initially marked as dirty.
- User space tells which working sets are referenced by each work
submission.
Kernel locks these working sets, pins all buffers in dirty working
sets, and
resets the dirty bits. After kicking off work, kernel stores the
fence to
the _working sets_, and then releases the locks (if an implicit
default
working set is used, then this would be roughly equivalent to
storing a fence
to channel vm that tells "this is the last hw operation that might
have
touched buffers in this address space").
- If swapping doesn't happen, then we just need to check the working
set dirty
bits at each submit.
- When a buffer is swapped out, all working sets that refer to it
need to be
marked as dirty.
- When a buffer is swapped out or unmapped, we need to wait for the
fences from
all working sets that refer to the buffer.
Initially one might think of working sets as a mere optimization -
we now need
to process a few working sets at every submit instead of many
individual
buffers. However, it makes a huge difference because of fences:
fences that
are attached to buffers are used for implicitly synchronizing work
across
different channels and engines. They are in the performance
critical path, and
we want to carefully manage them (that's the idea of explicit
synchronization).
The working set fences, on the other hand, would only be used to
guarantee that
we don't swap out or unmap something that the GPU might be
accessing. We never
need to wait for those fences (except when swapping or unmapping),
so we can be
conservative without hurting performance.
Yeah, within the driver (i.e. for private objects which are never
exported
to dma_buf) we can recently do stuff like this. And your above idea is roughly one of the things we're tossing around for i915.
But the cool stuff with drm is that cmd submission is driver-specific,
so
you can just go wild with nouveau. Of course you have to coninvce the nouveau guys (and also have open-source users for the new interface).
For shared buffers I think we should stick with the implicit fences
for a
while simply because I'm not sure whether it's really worth the fuzz.
And
reworking all the drivers and dma-buf for some working sets is a lot of fuzz ;-) Like Maarten said you can mostly short-circuit the implicit fencing by only attaching shared fences.
Yes, I'll try to do that.
In case you're curious: The idea is to have a 1:1 association between ppgtt address spaces and what you call the working set above, to
implement
the buffer svm model in ocl2. Mostly because we expect that
applications
won't get the more fine-grained buffer list right anyway. And this
kind of
gang-scheduling of working set sizes should be more efficient for the usual case where everything fits.
If I understood correctly, this would be exactly the same as what I
called the
"default working set" above. On Android we don't care much about finer
grained
working sets either, because of UMA and no swapping.
Yeah, that's pretty much the idea. Well with ocl the SVM buffer working set sizes are attached to an ocl context, but I don't think there'll be more than one of those around really (except maybe when different libraries all use ocl, but don't work together on the same data).
Imo de-staging the android syncpt stuff needs to happen first,
before drivers
can use it. Since non-staging stuff really shouldn't depend upon
code from
staging.
Fully agree. I thought the best way towards that would be to show
some driver
code that _would_ use it. :)
Oh, there's the usual chicken&egg where we need a full-blown prototype before we can start merging. Interface work on upstream is super-hard,
but
given the ridiculous backwards compat guarantees Linus expects us to
keep
up totally justified. Mistakes are really expensive. So I'm happy to
see
you charge ahead here.
Given that I'm not used to working with upstream, don't expect too much
from my
"charging ahead". :) I'm still secretly hoping that the Android guys at
would jump in to help, now that we seem to agree that we could de-stage sync_fence.
Forget that, imo the android guys are 100% absorbed with their own stuff, at least on the gfx side. Occasionally they pipe up with "btw this is what we're doing now".
Also, the problem is that to actually push android stuff out of staging you need a use-case in upstream, which means an open-source gpu driver. There's not a lot of companies who have both that and ship android, and definitely not the nexus/android lead platforms.
Display side would be easier since there's a bunch of kms drivers now upstream. But given that google decided to go ahead with their own adf instead of drm-kms that's also a non-starter.
Heck, I have a hard time draggin our own android folks here at intel into upstream work ...
I'm all for adding explicit syncing. Our plans are roughly. - Add
both an in
and and out fence to execbuf to sync with other rendering and give
userspace
a fence back. Needs to different flags probably.
- Maybe add an ioctl to dma-bufs to get at the current implicit
fences
attached to them (both an exclusive and non-exclusive version).
This
should help with making explicit and implicit sync work together
nicely.
- Add fence support to kms. Probably only worth it together with
the new
atomic stuff. Again we need an in fence to wait for (one for each buffer) and an out fence. The later can easily be implemented by extending struct drm_event, which means not a single driver code
line
needs to be changed for this.
- For de-staging android syncpts we need to de-clutter the internal interfaces and also review all the ioctls exposed. Like you say
it
should be just the userspace interface for struct drm_fence.
Also, it
needs testcases and preferrably manpages.
This all sounds very similar to what we'd like to do! Maybe we can
move
forward with these parts, and continue to attach fences at submit
until we have
a satisfactory solution for the pinning problem?
Yeah, that's our plan for i915 too. First add explicit fences, then
figure
out whether we need to be better at neutering the implicit fences, in
case
and only where it really gets in the way.
I'd like to understand what are the concrete steps to de-stage struct sync_fence, since that's the first thing that needs to be done. For
example,
what do you mean by "de-cluttering the internal interfaces"? Just
that we'd
move the sync_fence parts from drivers/staging/android/sync.c to,
say,
drivers/dma-buf/sync-fence.c ? Would we still leave a copy of the
existing
full driver to staging/android?
Yeah I guess that would be an approach. Personally I think we should
also
have basic ioctl testcase for all the ioctls exposed by syncpt fds. And reviewing the kerneldoc for the driver-internal interfaces (which
includes
removing everything that's no made obsolete by struct fence). Bonus
points
for documenting the ioctls. We could throw the test binary into libdrm maybe, there's a bunch other like it already there.
I'm not sure whether/how much google has already for this.
The simplest way to add tests is if we allow user space to create and
trigger
fences. We don't want to enable that from kernel by default, because
that
opens possibilities for deadlocks (e.g. a process could deadlock a
compositor
by passing a fence that it never triggers). Android sync driver solves
this by
having a separate CONFIG_SW_SYNC_USER that can be used for testing.
Here's a simple test by Google:
https://android.googlesource.com/platform/system/core/+/master/libsync/sync_...
Hm, usually we expose such test interfaces through debugfs - that way production system won't ever ship with it (since there's too many exploits in there, especially with secure boot). But since you need it for validation tests (at least for the i915 suite) it should always be there when you need it.
Exposing this as a configurable driver in dev is imo a no-go. But we should be able to easily convert this into a few debugfs files, so not too much fuzz hopefully.
And this is their userspace wrapper lib:
https://android.googlesource.com/platform/system/core/+/master/libsync/sync....
https://android.googlesource.com/platform/system/core/+/master/libsync/inclu...
Would that go to libdrm now...?
Imo it would make sense. At least we kinda want to use fences outside of Android, and a separate library project feels like overkill.
Aside: Will you be at XDC or linux plumbers? Either would be a perfect place to discuss plans and ideas - I'll attend both.
I wasn't going to, but let's see. The former is pretty soon and the
latter is
sold out. At least Andy Ritger from Nvidia is coming to XDC for sure,
and he's
been involved in our internal discussions around these topics. So I
suggest you
have a chat with him at least! :)
I'll definitely have a chat (and some beers) with Andy, been a while I've last seen him ;-)
Cheers, Daniel
Daniel Vetter Software Engineer, Intel Corporation +41 (0) 79 365 57 48 - http://blog.ffwll.ch