Asynchronous eviction [WAS Re: [PATCH] drm/ttm: add minimum residency constraint for bo eviction]

[Jerome Glisse]

On Fri, Nov 30, 2012 at 06:43:36PM +0100, Thomas Hellstrom wrote:
> On 11/30/2012 06:18 PM, Jerome Glisse wrote:
> >On Fri, Nov 30, 2012 at 12:08 PM, Thomas Hellstrom <thomas at shipmail.org> wrote:
> >>On 11/30/2012 05:30 PM, Jerome Glisse wrote:
> >>>On Fri, Nov 30, 2012 at 4:39 AM, Thomas Hellstrom <thomas at shipmail.org>
> >>>wrote:
> >>>>On 11/29/2012 10:58 PM, Marek Olšák wrote:
> >>>>>
> >>>>>What I tried to point out was that the synchronization shouldn't be
> >>>>>needed, because the CPU shouldn't do anything with the contents of
> >>>>>evicted buffers. The GPU moves the buffers, not the CPU. What does the
> >>>>>CPU do besides updating some kernel structures?
> >>>>>
> >>>>>Also, buffer deletion is something where you don't need to wait for
> >>>>>the buffer to become idle if you know the memory area won't be
> >>>>>mapped by the CPU, ever. The memory can be reclaimed right away. It
> >>>>>would be the GPU to move new data in and once that happens, the old
> >>>>>buffer will be trivially idle, because single-ring GPUs execute
> >>>>>commands in order.
> >>>>>
> >>>>>Marek
> >>>>
> >>>>Actually asynchronous eviction / deletion is something I have been
> >>>>prototyping for a while but never gotten around to implement in TTM:
> >>>>
> >>>>There are a few minor caveats:
> >>>>
> >>>>With buffer deletion, what you say is true for fixed memory, but not for
> >>>>TT
> >>>>memory where pages are reclaimed by the system after buffer destruction.
> >>>>That means that we don't have to wait for idle to free GPU space, but we
> >>>>need to wait before pages are handed back to the system.
> >>>>
> >>>>Swapout needs to access the contents of evicted buffers, but
> >>>>synchronizing
> >>>>doesn't need to happen until just before swapout.
> >>>>
> >>>>Multi-ring - CPU support: If another ring / engine or the CPU is about to
> >>>>move in buffer contents to VRAM or a GPU aperture that was previously
> >>>>evicted by another ring, it needs to sync with that eviction, but doesn't
> >>>>know what buffer or even which buffers occupied the space previously.
> >>>>Trivially one can attach a sync object to the memory type manager that
> >>>>represents the last eviction from that memory type, and *any* engine (CPU
> >>>>or
> >>>>GPU) that moves buffer contents in needs to order that movement with
> >>>>respect
> >>>>to that fence. As you say, with a single ring and no CPU fallbacks, that
> >>>>ordering is a no-op, but any common (non-driver based) implementation
> >>>>needs
> >>>>to support this.
> >>>>
> >>>>A single fence attached to the memory type manager is the simplest
> >>>>solution,
> >>>>but a solution with a fence for each free region in the free list is also
> >>>>possible. Then TTM needs a driver callback to be able order fences w r t
> >>>>echother.
> >>>>
> >>>>/Thomas
> >>>>
> >>>Radeon already handle multi-ring and ttm interaction with what we call
> >>>semaphore. Semaphore are created to synchronize with fence accross
> >>>different ring. I think the easiest solution is to just remove the bo
> >>>wait in ttm and let driver handle this.
> >>
> >>The wait can be removed, but only conditioned on a driver flag that says it
> >>supports unsynchronous buffer moves.
> >>
> >>The multi-ring case I'm talking about is:
> >>
> >>Ring 1 evicts buffer A, emits fence 0
> >>Ring 2 evicts buffer B, emits fence 1
> >>..Other eviction takes place by various rings, perhaps including ring 1 and
> >>ring 2.
> >>Ring 3 moves buffer C into the space which happens bo be the union of the
> >>space prevously occupied buffer A and buffer B.
> >>
> >>Question is: which fence do you want to order this move with?
> >>The answer is whichever of fence 0 and 1 signals last.
> >>
> >>I think it's a reasonable thing for TTM to keep track of this, but in order
> >>to do so it needs a driver callback that
> >>can order two fences, and can order a job in the current ring w r t a fence.
> >>In radeon's case that driver callback
> >>would probably insert a barrier / semaphore. In the case of simpler hardware
> >>it would wait on one of the fences.
> >>
> >>/Thomas
> >>
> >I don't think we can order fence easily with a clean api, i would
> >rather see ttm provide a list of fence to driver and tell to the
> >driver before moving this object all the fence on this list need to be
> >completed. I think it's as easy as associating fence with drm_mm (well
> >nouveau as its own mm stuff) but idea would basicly be that fence are
> >both associated with bo and with mm object so you know when a segment
> >of memory is idle/available for use.
> >
> >Cheers,
> >Jerome
> 
> 
> Hmm. Agreed that would save a lot of barriers.
> 
> Even if TTM tracks fences by free mm regions or a single fence for
> the whole memory type, it's a simple fact that fences from the same
> ring are trivially ordered, which means such a list should contain at
> most as many fences as there are rings.

Yes, one function callback is needed to know which fence is necessary,
also ttm needs to know the number of rings (note that i think newer
hw will have somethings like 1024 rings or even more, even today hw
might have as many as i think nvidia channel is pretty much what i
define to be a ring).

But i think most case will be few fence accross few rings. Like 1
ring is the dma ring and then you have a ring for one of the GL
context that using the memory and another ring for the new context
that want to use the memory.

> 
> So, whatever approach is chosen, TTM needs to be able to determine
> that trivial ordering, and I think the upcoming cross-device fencing
> work will face the exact same problem.
> 
> My proposed ordering API would look something like
> 
> struct fence *order_fences(struct fence *fence_a, struct fence
> *fence_b, bool trivial_order, bool interruptible, bool no_wait_gpu)
> 
> Returns which of the fences @fence_a and @fence_b that when
> signaled, guarantees that also the other
> fence has signaled. If @quick_order is true, and the driver cannot
> trivially order the fences, it may return ERR_PTR(-EAGAIN),
> if @interruptible is true, any wait should be performed
> interruptibly and if no_wait_gpu is true, the function is not
> allowed to wait on gpu but should issue ERR_PTR(-EBUSY) if it needs
> to do so to order fences.
> 
> (Hardware without semaphores can't order fences without waiting on them).
> 
> The list approach you suggest would use @trivial_order = true,
> Single fence approach would use @trivial_order = false.
> 
> And a first simple implementation in TTM would perhaps use your list
> approach with a single list for the whole memory type.
> 
> /Thomas

I would rather add a callback like :

ttm_reduce_fences(unsigned *nfences, fence **fencearray)

So each mm block has an array of fence, each time a new fence is added to
the array you callback to reduce the array of fence, it remove all the
fence that can be removed from the array. It might even remove all of them
if they are signaled.

In the ttm bo move callback you provide the list list of mm block (each
having its array of fence) and the move callback is responsible to
use what ever mecanism it wants to properly schedule and synchronize the
move.

One thing i am not sure is should we merge free mm block and merge/reduce
their fence array or should we provide a list of mm block to the move
callback. I think here there is tradeoff you probably want to merge small
mm block up to a certain point but you don't want to merge so much that
any allocation will have to wait on a zillions fences.

In the memcpy fallback you just wait on each fence in the array.

This move all the synchronization into the driver and i think it's the
easiest way to do thing.

Cheers,
Jerome