Not going to comment on everything on the first pass...
On Mon, Apr 19, 2021 at 5:48 AM Marek Olšák <mar...@gmail.com> wrote:
Hi,
This is our initial proposal for explicit fences everywhere and new memory
management that doesn't use BO fences. It's a redesign of how Linux graphics
drivers work, and it can coexist with what we have now.
1. Introduction
(skip this if you are already sold on explicit fences)
The current Linux graphics architecture was initially designed for GPUs with
only one graphics queue where everything was executed in the submission order
and per-BO fences were used for memory management and CPU-GPU synchronization,
not GPU-GPU synchronization. Later, multiple queues were added on top, which
required the introduction of implicit GPU-GPU synchronization between queues of
different processes using per-BO fences. Recently, even parallel execution
within one queue was enabled where a command buffer starts draws and compute
shaders, but doesn't wait for them, enabling parallelism between back-to-back
command buffers. Modesetting also uses per-BO fences for scheduling flips. Our
GPU scheduler was created to enable all those use cases, and it's the only
reason why the scheduler exists.
The GPU scheduler, implicit synchronization, BO-fence-based memory management,
and the tracking of per-BO fences increase CPU overhead and latency, and reduce
parallelism. There is a desire to replace all of them with something much
simpler. Below is how we could do it.
2. Explicit synchronization for window systems and modesetting
The producer is an application and the consumer is a compositor or a
modesetting driver.
2.1. The Present request
As part of the Present request, the producer will pass 2 fences (sync objects)
to the consumer alongside the presented DMABUF BO:
- The submit fence: Initially unsignalled, it will be signalled when the
producer has finished drawing into the presented buffer.
- The return fence: Initially unsignalled, it will be signalled when the
consumer has finished using the presented buffer.
I'm not sure syncobj is what we want. In the Intel world we're trying
to go even further to something we're calling "userspace fences" which
are a timeline implemented as a single 64-bit value in some
CPU-mappable BO. The client writes a higher value into the BO to
signal the timeline.
The kernel then provides some helpers for
waiting on them reliably and without spinning. I don't expect
everyone to support these right away but, If we're going to re-plumb
userspace for explicit synchronization, I'd like to make sure we take
this into account so we only have to do it once.
Deadlock mitigation to recover from segfaults:
- The kernel knows which process is obliged to signal which fence. This
information is part of the Present request and supplied by userspace.
This isn't clear to me. Yes, if we're using anything dma-fence based
like syncobj, this is true. But it doesn't seem totally true as a
general statement.
- If the producer crashes, the kernel signals the submit fence, so that the
consumer can make forward progress.
- If the consumer crashes, the kernel signals the return fence, so that the
producer can reclaim the buffer.
- A GPU hang signals all fences. Other deadlocks will be handled like GPU hangs.
What do you mean by "all"? All fences that were supposed to be
signaled by the hung context?
Other window system requests can follow the same idea.
Merged fences where one fence object contains multiple fences will be
supported. A merged fence is signalled only when its fences are signalled. The
consumer will have the option to redefine the unsignalled return fence to a
merged fence.
2.2. Modesetting
Since a modesetting driver can also be the consumer, the present ioctl will
contain a submit fence and a return fence too. One small problem with this is
that userspace can hang the modesetting driver, but in theory, any later
present ioctl can override the previous one, so the unsignalled presentation is
never used.
3. New memory management
The per-BO fences will be removed and the kernel will not know which buffers
are busy. This will reduce CPU overhead and latency. The kernel will not need
per-BO fences with explicit synchronization, so we just need to remove their
last user: buffer evictions. It also resolves the current OOM deadlock.
Is this even really possible? I'm no kernel MM expert (trying to
learn some) but my understanding is that the use of per-BO dma-fence
runs deep. I would like to stop using it for implicit synchronization
to be sure, but I'm not sure I believe the claim that we can get rid
of it entirely. Happy to see someone try, though.
3.1. Evictions
If the kernel wants to move a buffer, it will have to wait for everything to go
idle, halt all userspace command submissions, move the buffer, and resume
everything. This is not expected to happen when memory is not exhausted. Other
more efficient ways of synchronization are also possible (e.g. sync only one
process), but are not discussed here.
3.2. Per-process VRAM usage quota
Each process can optionally and periodically query its VRAM usage quota and
change domains of its buffers to obey that quota. For example, a process
allocated 2 GB of buffers in VRAM, but the kernel decreased the quota to 1 GB.
The process can change the domains of the least important buffers to GTT to get
the best outcome for itself. If the process doesn't do it, the kernel will
choose which buffers to evict at random. (thanks to Christian Koenig for this
idea)
This is going to be difficult. On Intel, we have some resources that
have to be pinned to VRAM and can't be dynamically swapped out by the
kernel. In GL, we probably can deal with it somewhat dynamically. In
Vulkan, we'll be entirely dependent on the application to use the
appropriate Vulkan memory budget APIs.
--Jason
3.3. Buffer destruction without per-BO fences
When the buffer destroy ioctl is called, an optional fence list can be passed
to the kernel to indicate when it's safe to deallocate the buffer. If the fence
list is empty, the buffer will be deallocated immediately. Shared buffers will
be handled by merging fence lists from all processes that destroy them.
Mitigation of malicious behavior:
- If userspace destroys a busy buffer, it will get a GPU page fault.
- If userspace sends fences that never signal, the kernel will have a timeout
period and then will proceed to deallocate the buffer anyway.
3.4. Other notes on MM
Overcommitment of GPU-accessible memory will cause an allocation failure or
invoke the OOM killer. Evictions to GPU-inaccessible memory might not be
supported.
Kernel drivers could move to this new memory management today. Only buffer
residency and evictions would stop using per-BO fences.
4. Deprecating implicit synchronization
It can be phased out by introducing a new generation of hardware where the
driver doesn't add support for it (like a driver fork would do), assuming
userspace has all the changes for explicit synchronization. This could
potentially create an isolated part of the kernel DRM where all drivers only
support explicit synchronization.
Marek
_______________________________________________
dri-devel mailing list
dri-de...@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/dri-devel
_______________________________________________
mesa-dev mailing list
mesa-dev@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/mesa-dev
