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

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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






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