[PATCH 16/17] cgroup/drm: Expose memory stats

[Maarten Lankhorst]

Hey,

On 2023-07-26 13:41, Tvrtko Ursulin wrote:
> 
> On 26/07/2023 11:14, Maarten Lankhorst wrote:
>> Hey,
>>
>> On 2023-07-22 00:21, Tejun Heo wrote:
>>> On Wed, Jul 12, 2023 at 12:46:04PM +0100, Tvrtko Ursulin wrote:
>>>>    $ cat drm.memory.stat
>>>>    card0 region=system total=12898304 shared=0 active=0 
>>>> resident=12111872 purgeable=167936
>>>>    card0 region=stolen-system total=0 shared=0 active=0 resident=0 
>>>> purgeable=0
>>>>
>>>> Data is generated on demand for simplicty of implementation ie. no 
>>>> running
>>>> totals are kept or accounted during migrations and such. Various
>>>> optimisations such as cheaper collection of data are possible but
>>>> deliberately left out for now.
>>>>
>>>> Overall, the feature is deemed to be useful to container orchestration
>>>> software (and manual management).
>>>>
>>>> Limits, either soft or hard, are not envisaged to be implemented on 
>>>> top of
>>>> this approach due on demand nature of collecting the stats.
>>>
>>> So, yeah, if you want to add memory controls, we better think through 
>>> how
>>> the fd ownership migration should work.
>> I've taken a look at the series, since I have been working on cgroup 
>> memory eviction.
>>
>> The scheduling stuff will work for i915, since it has a purely 
>> software execlist scheduler, but I don't think it will work for GuC 
>> (firmware) scheduling or other drivers that use the generic drm 
>> scheduler.
> 
> It actually works - I used to have a blurb in the cover letter about it 
> but apparently I dropped it. Just a bit less well with many clients, 
> since there are fewer priority levels.
> 
> All that the design requires from the invididual drivers is some way to 
> react to the "you are over budget by this much" signal. The rest is 
> driver and backend specific.

What I mean is that this signal may not be applicable since the drm 
scheduler just schedules jobs that run. Adding a weight might be done in 
hardware, since it's responsible for  scheduling which context gets to 
run. The over budget signal is useless in that case, and you just need 
to set a scheduling priority for the hardware instead.

>> For something like this,  you would probably want it to work inside 
>> the drm scheduler first. Presumably, this can be done by setting a 
>> weight on each runqueue, and perhaps adding a callback to update one 
>> for a running queue. Calculating the weights hierarchically might be 
>> fun..
> 
> It is not needed to work in drm scheduler first. In fact drm scheduler 
> based drivers can plug into what I have since it already has the notion 
> of scheduling priorities.
> 
> They would only need to implement a hook which allow the cgroup 
> controller to query client GPU utilisation and another to received the 
> over budget signal.
> 
> Amdgpu and msm AFAIK could be easy candidates because they both support 
> per client utilisation and priorities.
> 
> Looks like I need to put all this info back into the cover letter.
> 
> Also, hierarchic weights and time budgets are all already there. What 
> could be done later is make this all smarter and respect the time budget 
> with more precision. That would however, in many cases including Intel, 
> require co-operation with the firmware. In any case it is only work in 
> the implementation, while the cgroup control interface remains the same.
> 
>> I have taken a look at how the rest of cgroup controllers change 
>> ownership when moved to a different cgroup, and the answer was: not at 
>> all. If we attempt to create the scheduler controls only on the first 
>> time the fd is used, you could probably get rid of all the tracking.
> 
> Can you send a CPU file descriptor from process A to process B and have 
> CPU usage belonging to process B show up in process' A cgroup, or 
> vice-versa? Nope, I am not making any sense, am I? My point being it is 
> not like-to-like, model is different.
> 
> No ownership transfer would mean in wide deployments all GPU utilisation 
> would be assigned to Xorg and so there is no point to any of this. No 
> way to throttle a cgroup with un-important GPU clients for instance.
If you just grab the current process' cgroup when a drm_sched_entity is 
created, you don't have everything charged to X.org. No need for 
complicated ownership tracking in drm_file. The same equivalent should 
be done in i915 as well when a context is created as it's not using the 
drm scheduler.

>> This can be done very easily with the drm scheduler.
>>
>> WRT memory, I think the consensus is to track system memory like 
>> normal memory. Stolen memory doesn't need to be tracked. It's kernel 
>> only memory, used for internal bookkeeping  only.
>>
>> The only time userspace can directly manipulate stolen memory, is by 
>> mapping the pinned initial framebuffer to its own address space. The 
>> only allocation it can do is when a framebuffer is displayed, and 
>> framebuffer compression creates some stolen memory. Userspace is not
>> aware of this though, and has no way to manipulate those contents.
> 
> Stolen memory is irrelevant and not something cgroup controller knows 
> about. Point is drivers say which memory regions they have and their 
> utilisation.
> 
> Imagine instead of stolen it said vram0, or on Intel multi-tile it shows 
> local0 and local1. People working with containers are interested to see 
> this breakdown. I guess the parallel and use case here is closer to 
> memory.numa_stat.
Correct, but for the same reason, I think it might be more useful to 
split up the weight too.

A single scheduling weight for the global GPU might be less useful than 
per engine, or per tile perhaps..

Cheers,
~Maarten