Re: [Intel-gfx] [RFC v2 0/5] Waitboost drm syncobj waits

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 20/02/2023 15:52, Rob Clark wrote:
> On Mon, Feb 20, 2023 at 3:33 AM Tvrtko Ursulin
> <tvrtko.ursulin@linux.intel.com> wrote:
>>
>>
>> On 17/02/2023 20:45, Rodrigo Vivi wrote:

[snip]

>> Yeah I agree. And as not all media use cases are the same, as are not
>> all compute contexts someone somewhere will need to run a series of
>> workloads for power and performance numbers. Ideally that someone would
>> be the entity for which it makes sense to look at all use cases, from
>> server room to client, 3d, media and compute for both. If we could get
>> the capability to run this in some automated fashion, akin to CI, we
>> would even have a chance to keep making good decisions in the future.
>>
>> Or we do some one off testing for this instance, but we still need a
>> range of workloads and parts to do it properly..
>>
>>>> I also think the "arms race" scenario isn't really as much of a
>>>> problem as you think.  There aren't _that_ many things using the GPU
>>>> at the same time (compared to # of things using CPU).   And a lot of
>>>> mobile games throttle framerate to avoid draining your battery too
>>>> quickly (after all, if your battery is dead you can't keep buying loot
>>>> boxes or whatever).
>>>
>>> Very good point.
>>
>> On this one I still disagree from the point of view that it does not
>> make it good uapi if we allow everyone to select themselves for priority
>> handling (one flavour or the other).
> 
> There is plenty of precedent for userspace giving hints to the kernel
> about scheduling and freq mgmt.  Like schedutil uclamp stuff.
> Although I think that is all based on cgroups.

I knew about SCHED_DEADLINE and that it requires CAP_SYS_NICE, but I did 
not know about uclamp. Quick experiment with uclampset suggests it 
indeed does not require elevated privilege. If that is indeed so, it is 
good enough for me as a precedent.

It appears to work using sched_setscheduler so maybe could define 
something similar in i915/xe, per context or per client, not sure.

Maybe it would start as a primitive implementation but the uapi would 
not preclude making it smart(er) afterwards. Or passing along to GuC to 
do it's thing with it.

> In the fence/syncobj case, I think we need per-wait hints.. because
> for a single process the driver will be doing both housekeeping waits
> and potentially urgent waits.  There may also be some room for some
> cgroup or similar knobs to control things like what max priority an
> app can ask for, and whether or how aggressively the kernel responds
> to the "deadline" hints.  So as far as "arms race", I don't think I'd

Per wait hints are okay I guess even with "I am important" in their name 
if sched_setscheduler allows raising uclamp.min just like that. In which 
case cgroup limits to mimick cpu uclamp also make sense.

> change anything about my "fence deadline" proposal.. but that it might
> just be one piece of the overall puzzle.

That SCHED_DEADLINE requires CAP_SYS_NICE does not worry you?

Regards,

Tvrtko