[PATCH 5/5] drm/amdgpu: implement amdgpu_gem_prime_move_notify v2

[Christian König]

Am 21.02.20 um 18:12 schrieb Daniel Vetter:
> [SNIP]
> Yeah the Great Plan (tm) is to fully rely on ww_mutex slowly degenerating
> into essentially a global lock. But only when there's actual contention
> and thrashing.

Yes exactly. A really big problem in TTM is currently that we drop the 
lock after evicting BOs because they tend to move in again directly 
after that.

 From practice I can also confirm that there is exactly zero benefit 
from dropping locks early and reacquire them for example for the VM page 
tables. That's just makes it more likely that somebody needs to roll 
back and this is what we need to avoid in the first place.

Contention on BO locks during command submission is perfectly fine as 
long as this is as lightweight as possible while we don't have trashing. 
When we have trashing multi submission performance is best archived to 
just favor a single process to finish its business and block everybody else.

Because of this I would actually vote for forbidding to release 
individual ww_mutex() locks in a context.

Regards,
Christian.

> -Daniel