Re: [RFC] Linux Graphics Next: Explicit fences everywhere and no BO fences - initial proposal

["Marek Olšák" <maraeo@gmail.com>]

[-- Attachment #1.1: Type: text/plain, Size: 9600 bytes --]

We already don't have accurate BO fences in some cases. Instead, BOs can
have fences which are equal to the last seen command buffer for each queue.
It's practically the same as if the kernel had no visibility into command
submissions and just added a fence into all queues when it needed to wait
for idle. That's already one alternative to BO fences that would work
today. The only BOs that need accurate BO fences are shared buffers, and
those use cases can be converted to explicit fences.

Removing memory management from all command buffer submission logic would
be one of the benefits that is quite appealing.

You don't need to depend on apps for budgeting and placement determination.
You can sort buffers according to driver usage, e.g. scratch/spill buffers,
shader IO rings, MSAA images, other images, and buffers. Alternatively, you
can have just internal buffers vs app buffers. Then you assign VRAM from
left to right until you reach the quota. This is optional, so this part can
be ignored.

>> - 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?

Yes, that's one of the possibilities. Any GPU hang followed by a GPU reset
can clear VRAM, so all processes should recreate their contexts and
reinitialize resources. A deadlock caused by userspace could be handled
similarly.

I don't know how timeline fences would work across processes and how
resilient they would be to segfaults.

Marek

On Mon, Apr 19, 2021 at 11:48 AM Jason Ekstrand <jason@jlekstrand.net>
wrote:

> Not going to comment on everything on the first pass...
>
> On Mon, Apr 19, 2021 at 5:48 AM Marek Olšák <maraeo@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-devel@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
>

[-- Attachment #1.2: Type: text/html, Size: 10784 bytes --]

[-- Attachment #2: Type: text/plain, Size: 160 bytes --]

_______________________________________________
dri-devel mailing list
dri-devel@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/dri-devel