Re: [PATCH drm-misc-next v8 11/12] drm/nouveau: implement new VM_BIND uAPI

[Danilo Krummrich]

On 7/25/23 18:43, Danilo Krummrich wrote:
> On 7/25/23 18:16, Faith Ekstrand wrote:
> > Thanks for the detailed write-up! That would definitely explain it. If 
> > I remember, I'll try to do a single-threaded run or two. If your 
> > theory is correct, there should be no real perf difference when 
> > running single-threaded. Those runs will take a long time, though, so 
> > I'll have to run them over night. I'll let you know in a few days once 
> > I have the results.
>
> I can also push a separate branch where I just print out a warning 
> whenever we run into such a condition including the time we were waiting 
> for things to complete. I can probably push something later today.

https://gitlab.freedesktop.org/nouvelles/kernel/-/tree/new-uapi-drm-next-track-stalls

It prints out the duration of every wait as well as the total wait time 
since boot.

- Danilo

> > If this theory holds, then I'm not concerned about the performance of 
> > the API itself. It would still be good to see if we can find a way to 
> > reduce the cross-process drag in the implementation but that's a perf 
> > optimization we can do later.
>
>  From the kernel side I think the only thing we could really do is to 
> temporarily run a secondary drm_gpu_scheduler instance, one for VM_BINDs 
> and one for EXECs until we got the new page table handling in place.
>
> However, the UMD could avoid such conditions more effectively, since it 
> controls the address space. Namely, avoid re-using the same region of 
> the address space right away in certain cases. For instance, instead of 
> replacing a sparse region A[0x0, 0x4000000] with a larger sparse region 
> B[0x0, 0x8000000], replace it with B'[0x4000000, 0xC000000] if possible.
>
> However, just mentioning this for completeness. The UMD surely shouldn't 
> probably even temporarily work around such a kernel limitation.
>
> Anyway, before doing any of those, let's see if the theory holds and 
> we're actually running into such cases.
>
> > Does it actually matter? Yes, it kinda does. No, it probably doesn't 
> > matter for games because you're typically only running one game at a 
> > time. From a development PoV, however, if it makes CI take longer then 
> > that slows down development and that's not good for the users, either.
>
> Fully agree.
>
> - Danilo
>
> > ~Faith
> >
> >     - Danilo
> >
> >      >
> >      > ~Faith
> >      >