linux-kernel - Re: [PATCH drm-misc-next v8 11/12] drm/nouveau: implement new VM

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Message-ID: <21ea63fe-33d7-a075-7291-35dd1b2a9b64@redhat.com>
Date:   Tue, 25 Jul 2023 23:00:23 +0200
From:   Danilo Krummrich <dakr@...hat.com>
To:     Faith Ekstrand <faith@...strand.net>
Cc:     airlied@...il.com, daniel@...ll.ch, tzimmermann@...e.de,
        mripard@...nel.org, corbet@....net, christian.koenig@....com,
        bskeggs@...hat.com, Liam.Howlett@...cle.com,
        matthew.brost@...el.com, boris.brezillon@...labora.com,
        alexdeucher@...il.com, ogabbay@...nel.org, bagasdotme@...il.com,
        willy@...radead.org, jason@...kstrand.net,
        donald.robson@...tec.com, dri-devel@...ts.freedesktop.org,
        nouveau@...ts.freedesktop.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH drm-misc-next v8 11/12] drm/nouveau: implement new VM_BIND
 uAPI

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
>>      >
>>