[Linaro-mm-sig] [RFC] Cross-driver ww transaction lock lists

[Thomas Hellström (Intel)]

On 4/15/21 4:40 PM, Daniel Vetter wrote:
> On Thu, Apr 15, 2021 at 4:02 PM Thomas Hellström (Intel)
> <thomas_os at shipmail.org> wrote:
>>
>> On 4/15/21 3:37 PM, Daniel Vetter wrote:
>>> On Tue, Apr 13, 2021 at 09:57:06AM +0200, Christian König wrote:
>>>> Am 13.04.21 um 09:50 schrieb Thomas Hellström:
>>>>> Hi!
>>>>>
>>>>> During the dma_resv conversion of the i915 driver, we've been using ww
>>>>> transaction lock lists to keep track of ww_mutexes that are locked
>>>>> during the transaction so that they can be batch unlocked at suitable
>>>>> locations. Including also the LMEM/VRAM eviction we've ended up with
>>>>> two static lists per transaction context; one typically unlocked at the
>>>>> end of transaction and one initialized before and unlocked after each
>>>>> buffer object validate. This enables us to do sleeping locking at
>>>>> eviction and keep objects locked on the eviction list until we
>>>>> eventually succeed allocating memory (modulo minor flaws of course).
>>>>>
>>>>> It would be beneficial with the i915 TTM conversion to be able to
>>>>> introduce a similar functionality that would work in ttm but also
>>>>> cross-driver in, for example move_notify. It would also be beneficial
>>>>> to be able to put any dma_resv ww mutex on the lists, and not require
>>>>> it to be embedded in a particular object type.
>>>>>
>>>>> I started scetching on some utilities for this. For TTM, for example,
>>>>> the idea would be to pass a list head for the ww transaction lock list
>>>>> in the ttm_operation_ctx. A function taking a ww_mutex could then
>>>>> either attach a grabbed lock to the list for batch unlocking, or be
>>>>> responsible for unlocking when the lock's scope is exited. It's also
>>>>> possible to create sublists if so desired. I believe the below would be
>>>>> sufficient to cover the i915 functionality.
>>>>>
>>>>> Any comments and suggestions appreciated!
>>>> ah yes Daniel and I haven been discussing something like this for years.
>>>>
>>>> I also came up with rough implementation, but we always ran into lifetime
>>>> issues.
>>>>
>>>> In other words the ww_mutexes which are on the list would need to be kept
>>>> alive until unlocked.
>>>>
>>>> Because of this we kind of backed up and said we would need this on the GEM
>>>> level instead of working with dma_resv objects.
>>> Yeah there's a few funny concerns here that make this awkward:
>>> - For simplicity doing these helpers at the gem level should make things a
>>>     bit easier, because then we have a standard way to drop the reference.
>>>     It does mean that the only thing you can lock like this are gem objects,
>>>     but I think that's fine. At least for a first cut.
>>>
>>> - This is a bit awkward for vmwgfx, but a) Zack has mentioned he's looking
>>>     into adopting gem bo internally to be able to drop a pile of code and
>>>     stop making vmwgfx the only special-case we have b) drivers which don't
>>>     need this won't need this, so should be fine.
>>>
>>>     The other awkward thing I guess is that ttm would need to use the
>>>     embedded kref from the gem bo, but that should be transparent I think.
>>>
>>> - Next up is dma-buf: For i915 we'd like to do the same eviction trick
>>>     also through p2p dma-buf callbacks, so that this works the same as
>>>     eviction/reservation within a gpu. But for these internal bo you might
>>>     not have a dma-buf, so we can't just lift the trick to the dma-buf
>>>     level. But I think if we pass e.g. a struct list_head and a callback to
>>>     unreference/unlock all the buffers in there to the exporter, plus
>>>     similar for the slowpath lock, then that should be doable without
>>>     glorious layering inversions between dma-buf and gem.
>>>
>>>     I think for dma-buf it should even be ok if this requires that we
>>>     allocate an entire structure with kmalloc or something, allocating
>>>     memory while holding dma_resv is ok.
>> Yes, the thing here with the suggested helpers is that you would just
>> embed a trans_lockitem struct in the gem object (and defines the gem
>> object ops). Otherwise and for passing to dma-buf this is pretty much
>> exactly what you are suggesting, but the huge benefit of encapsulating
>> the needed members like this is that when we need to change something we
>> change it in just one place.
>>
>> For anything that doesn't have a gem object (dma-buf, vmwgfx or
>> whatever) you have the choice of either allocating a struct
>> trans_lockitem or embed it wherever you prefer. In particular, this is
>> beneficial where you have a single dma-resv class ww-mutex sitting
>> somewhere in the way and you don't want to needlessly have a gem object
>> that embeds it.
> The thing is, everyone who actually uses dma_resv_lock has a
> gem_buffer_object underneath. So it feels a bit like flexibility for
> no real need, and I think it would make it slightly more awkard for
> gem drivers to neatly integrate into their cs path. The lockitem
> struct works, but it is a bit cumbersome.

Well that's partly because of it's impossible to use a standalone 
ww_mutex in a locking transaction that can only add gem objects to the 
list :/. Already in the i915 driver we have and may want to add various 
places where we have dead gem objects sitting because of this.

Also, more importantly, If we pass a list down the dma-buf 
move_notify(), a trans_lockitem is pretty much exactly what we expect 
back (except of course for the private pointer). It would be odd if we'd 
expect all list items to be gem objects when it's a dma-buf interface?

>
> Also if we add some wrappers to e.g. add a gem_bo to the ctx, then if
> we decide to slip the lockitem in there, we still only need to touch
> the helper code, and not all drivers.

Well, yes assuming we always have an embedding gem object for a dma_resv 
that might be true, but either way I don't really expect the gem helpers 
to look very different. We will need the ops anyway and a specialized 
context so if the only thing we're debating is whether or not to embed a 
struct in the gem object, unless you really insist on using the gem 
object initially, I suggest we try this and if it becomes awkward, just 
s/trans_lockitem/drm_gem_object/

/Thomas


> -Daniel