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

Thomas Hellström (VMware)
2020-02-23T16:54:24Z

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