[Intel-gfx] [Linaro-mm-sig] [PATCH 1/2] dma-buf.rst: Document why indefinite fences are a bad idea

[Thomas Hellström (Intel)]

On 2020-07-21 15:59, Christian König wrote:
> Am 21.07.20 um 12:47 schrieb Thomas Hellström (Intel):
...
>> Yes, we can't do magic. As soon as an indefinite batch makes it to 
>> such hardware we've lost. But since we can break out while the batch 
>> is stuck in the scheduler waiting, what I believe we *can* do with 
>> this approach is to avoid deadlocks due to locally unknown 
>> dependencies, which has some bearing on this documentation patch, and 
>> also to allow memory allocation in dma-fence (not memory-fence) 
>> critical sections, like gpu fault- and error handlers without 
>> resorting to using memory pools.
>
> Avoiding deadlocks is only the tip of the iceberg here.
>
> When you allow the kernel to depend on user space to proceed with some 
> operation there are a lot more things which need consideration.
>
> E.g. what happens when an userspace process which has submitted stuff 
> to the kernel is killed? Are the prepared commands send to the 
> hardware or aborted as well? What do we do with other processes 
> waiting for that stuff?
>
> How to we do resource accounting? When processes need to block when 
> submitting to the hardware stuff which is not ready we have a process 
> we can punish for blocking resources. But how is kernel memory used 
> for a submission accounted? How do we avoid deny of service attacks 
> here were somebody eats up all memory by doing submissions which can't 
> finish?
>
Hmm. Are these problems really unique to user-space controlled 
dependencies? Couldn't you hit the same or similar problems with 
mis-behaving shaders blocking timeline progress?

/Thomas