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

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On 2/23/20 4:45 PM, Christian König wrote:
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.


If you have a benchmarking setup available it would be very interesting for future reference to see how changing from WD to WW mutexes affects the roll back frequency. WW is known to cause rollbacks much less frequently but there is more work associated with each rollback.


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.

Hmm. Sounds like we need a per-manager ww_rwsem protecting manager allocation, taken in write-mode then there's thrashing. In read-mode otherwise. That would limit the amount of "unnecessary" locks we'd have to keep and reduce unwanted side-effects, (see below):


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

Yes, I see the problem.

But my first reaction is that this might have undersirable side-effects. Let's say somebody wanted to swap the evicted BOs out? Or cpu-writes to them causing faults, that might also block the mm_sem, which in turn blocks hugepaged?

Still it's a fairly simple solution to a problem that seems otherwise hard to solve efficiently.

Thanks,
Thomas



Regards,
Christian.

-Daniel


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