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
