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

From: Tvrtko Ursulin
Date: Thu Jul 27 2023 - 12:43:47 EST


On 27/07/2023 14:42, Maarten Lankhorst wrote:
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.

Yes, keyword is "can". Trouble is migration may or may not happen.

One may choose "Plasma X.org" session type in your login manager and all DRM fds would be under Xorg if not migrated. Or one may choose "Plasma Wayland" and migration wouldn't matter. But former is I think has a huge deployed base so that not supporting implicit migration would be a significant asterisk next to the controller.

* 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.

Right, but for reporting stolen is a red-herring. In this RFC I simply report on all memory regions known by the driver. As I said in the other reply, imagine the keys are 'system' and 'vram0'. Point was just to illustrate multiplicity of regions.

Regards,

Tvrtko