Re: [PATCH 8/8] drm/i915: Expose RPCS (SSEU) configuration to userspace

[Lionel Landwerlin <lionel.g.landwerlin@intel.com>]

On 03/05/18 17:04, Joonas Lahtinen wrote:
> Quoting Lionel Landwerlin (2018-04-26 13:22:30)
>> On 26/04/18 11:00, Joonas Lahtinen wrote:
>>> Quoting Lionel Landwerlin (2018-04-25 14:45:21)
>>>> From: Chris Wilson <chris@chris-wilson.co.uk>
>>>>
>>>> We want to allow userspace to reconfigure the subslice configuration for
>>>> its own use case. To do so, we expose a context parameter to allow
>>>> adjustment of the RPCS register stored within the context image (and
>>>> currently not accessible via LRI). If the context is adjusted before
>>>> first use, the adjustment is for "free"; otherwise if the context is
>>>> active we flush the context off the GPU (stalling all users) and forcing
>>>> the GPU to save the context to memory where we can modify it and so
>>>> ensure that the register is reloaded on next execution.
>>>>
>>>> The overhead of managing additional EU subslices can be significant,
>>>> especially in multi-context workloads. Non-GPGPU contexts should
>>>> preferably disable the subslices it is not using, and others should
>>>> fine-tune the number to match their workload.
>>> This hit a dead end last time due to the system wide policy needed to
>>> avoid two parties fighting over the slice count (and going back and
>>> forth between two slice counts would counter the benefits received from
>>> this).
>>>
>>> Do we now have a solution for the contention? I don't see code to
>>> negotiate a global value, just raw setter.
>>>
>>> Regards, Joonas
>> I've tried to come up with some numbers about the cost of the back &
>> forth (see igt series).
>>
>> Other than that, I don't think we can expect the kernel to workaround
>> the inefficient use of the hardware by userspace.
> Well, I'm pretty sure we should not try to make the situation too
> miserable for the basic usecases.
>
> If we allow two contexts to fight over the slice count, countering any
> perceived benefit, I don't think that'll be a good default.
>
> My recollection of the last round of discussion was that reasonable
> thing to do would be to only disable slices when everyone is willing to
> let go of. Then it would become a system maintainer level decision to
> only run on two slices for when they see fit (configuring the userspace).

How would you detect that everybody is willing to let go?
We don't appear to have a mechanism to detect that.
Wouldn't that require to teach all userspace drivers?

>
> More advanced tactics would include scheduling work so that we try to
> avoid the slice count changes and deduct the switching time from the
> execution budget of the app requesting less slices (if we had fair
> time slicing).

That sounds more workable, although fairly complicated.
Maybe we could tweak the priority based on the slice count and say :
     for the next (1second / number of slice config), priority bumped 
for the configuration with x slices
     and rotate every (1second / number of slice config)

Would that resolve the back & forth issue completely though?

Moving a particular context back & forth between different 
configurations is costly too.
You need to drain the context, then pin the context before you can edit 
and resubmit.

Thanks for your feedback,

-
Lionel

>
> Regards, Joonas
>

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