[PATCH 2/2] dma-buf: cleanup shared fence removal

[Koenig, Christian]

Am 27.06.19 um 19:10 schrieb Daniel Vetter:
> On Thu, Jun 27, 2019 at 03:48:06PM +0000, Koenig, Christian wrote:
>> Am 27.06.19 um 17:34 schrieb Daniel Vetter:
>>> On Thu, Jun 27, 2019 at 3:19 PM Christian König
>>> <ckoenig.leichtzumerken at gmail.com> wrote:
>>>> Am 27.06.19 um 12:43 schrieb Daniel Vetter:
>>>>> On Thu, Jun 27, 2019 at 12:18 PM Christian König
>>>>> <ckoenig.leichtzumerken at gmail.com> wrote:
>>>>>> While freeing up memory it is easier to remove a fence from a reservation
>>>>>> object instead of signaling it and installing a new one in all other objects.
>>>>>>
>>>>>> Clean this up by adding the removal function to the core reservation object
>>>>>> code instead of messing with the internal in amdgpu.
>>>>>>
>>>>>> No functional change.
>>>>>>
>>>>>> Signed-off-by: Christian König <christian.koenig at amd.com>
>>>>>> ---
>>>>>>     drivers/dma-buf/reservation.c                 | 62 +++++++++++++++++++
>>>>>>     .../gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c  | 45 +-------------
>>>>>>     include/linux/reservation.h                   |  3 +-
>>>>>>     3 files changed, 65 insertions(+), 45 deletions(-)
>>>>>>
>>>>>> diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c
>>>>>> index ef710effedfa..e43a316a005d 100644
>>>>>> --- a/drivers/dma-buf/reservation.c
>>>>>> +++ b/drivers/dma-buf/reservation.c
>>>>>> @@ -231,6 +231,68 @@ void reservation_object_add_shared_fence(struct reservation_object *obj,
>>>>>>     }
>>>>>>     EXPORT_SYMBOL(reservation_object_add_shared_fence);
>>>>>>
>>>>>> +/**
>>>>>> + * reservation_object_remove_shared_fence - remove shared fences
>>>>>> + * @obj: the reservation object
>>>>>> + * @context: the context of the fences to remove
>>>>>> + *
>>>>>> + * Remove shared fences based on their fence context.
>>>>>> + */
>>>>> This needs a serious explanation of "why?". Removing fences without
>>>>> guaranteeing they're all signaled is a good way to create havoc. Your
>>>>> commit message has a few words about what you're doing here, but it
>>>>> still doesn't explain why this is safe and when exactly it should be
>>>>> used.
>>>> Yeah, I'm not very keen about this either.
>>>>
>>>> The key point is the memory is not accessible by the hardware any more
>>>> because it is freed and removed from the page tables.
>>>>
>>>> So further access is prevented and in this special case it is actually
>>>> valid to do this even if the operation represented by the fence is still
>>>> ongoing.
>>> Hm, why do you have to remove these fences then? Can't you just let
>>> them signal and get collected as usual? As soon as you share buffers
>>> these fences can get anywhere, so you need to correctly unwind them no
>>> matter what.
>>>
>>> You're kinda still just describing what you're doing, not why.
>> It is simply more efficient to remove the fence from one reservation
>> object than to add a new fence to all other reservation objects in the
>> same process.
> Ok, you still talk in riddles and don't explain what the goal of this
> entire dance is, so I went and read the code. Assuming I didn't misread
> too much:
>
> 1. you create a fake fence on a per-process timeline.
> 2. you attach this liberally to all the bo you're creating on that
> process
> 3. the fence never signals on its own, but it has a very magic
> ->enable_signaling callback which is the only thing that makes this fence
> switch to signalled in a finite time. Before that it's stuck forever. So
> quite a bit a schrödinger fence: It's not a real fence (because it fails
> to signal in bounded time) except when you start to look at it.
> 4. Looking at the fence triggers eviction, at that point we replace this
> magic eviction fence with the next set, reacquire buffers and then unblock
> the kfd process once everything is in shape again.
>
> This is soooooooooooooooooo magic that I really don't think we should
> encourage people without clue to maybe use this and totally break all
> fences guarantees.

Yeah, that is correct. But this is completely unrelated to why we want 
to remove the fence.

> If you do want to make sure an optimized version within
> reservation_object.c, then it should be code which replaces fences iff:
> - they're the same context
> - later in the ordering within that context
> - of the same type (i.e. safe vs shared)
>
> That would actually be safe thing to do.

No, that won't work because there is no replacement for the fence in 
question.

See we want to remove the fence because the memory is freed up.

> Also, the above is what I expected when asking "why do you need this", not
> "we replace fences, its more efficient" I kinda got that from the code :-)

Well I explained the short version why we do this. What you dug up here 
is correct as well, but completely unrelated to removing the fence.

Again, the reason to remove the fence from one reservation object is 
simply that it is faster to remove it from one object than to attach a 
new fence to all other objects.

It's just an optimization,
Christian.

>
> Plus reading this now with (at least the believe of) understanding what
> you're doing, replacing the fences and reattaching the next one of these
> magic fences doesn't feel like it's actually making anything faster. Just
> more obscure ...
>
> Looking at reservation_object_add_shared_fence it seems to dtrt already.
> -Daniel
>