Re: [Intel-gfx] [PATCH 5/5] drm/amdgpu: implement amdgpu_gem_prime_move_notify v2

Christian König
2020-02-24T18:13:35Z

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