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

[Maarten Lankhorst <maarten.lankhorst@linux.intel.com>]

Hey,

On 2023-07-26 21:44, Tejun Heo wrote:
> Hello,
> 
> On Wed, Jul 26, 2023 at 12:14:24PM +0200, Maarten Lankhorst wrote:
>>> 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.
>>
>> 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..
> 
> I don't have any idea on this front. The basic idea of making high level
> distribution decisions in core code and letting individual drivers enforce
> that in a way which fits them the best makes sense to me but I don't know
> enough to have an opinion here.
> 
>> 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
> 
> For persistent resources, that's the general rule. Whoever instantiates a
> resource gets to own it until the resource gets freed. There is an exception
> with the pid controller and there are discussions around whether we want
> some sort of migration behavior with memcg but yes by and large instantiator
> being the owner is the general model cgroup follows.
> 
>> attempt to create the scheduler controls only on the first time the fd is
>> used, you could probably get rid of all the tracking.
>> 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.
> 
> So, my dumb understanding:
> 
> * Ownership of an fd can be established on the first ioctl call and doesn't
>    need to be migrated afterwards. There are no persistent resources to
>    migration on the first call.
> 
> * Memory then can be tracked in a similar way to memcg. Memory gets charged
>    to the initial instantiator and doesn't need to be moved around
>    afterwards. There may be some discrepancies around stolen memory but the
>    magnitude of inaccuracy introduced that way is limited and bound and can
>    be safely ignored.
> 
> Is that correct?

Hey,

Yeah mostly, I think we can stop tracking stolen memory. I stopped doing 
that for Xe, there is literally nothing to control for userspace in there.

Cheers,
Maarten