On Wednesday, May 8th, 2024 at 17:49, Daniel Vetter <dan...@ffwll.ch> wrote:
> On Wed, May 08, 2024 at 09:38:33AM +0100, Daniel Stone wrote: > > > On Wed, 8 May 2024 at 09:33, Daniel Vetter dan...@ffwll.ch wrote: > > > > > On Wed, May 08, 2024 at 06:46:53AM +0100, Daniel Stone wrote: > > > > > > > That would have the unfortunate side effect of making sandboxed apps > > > > less efficient on some platforms, since they wouldn't be able to do > > > > direct scanout anymore ... > > > > > > I was assuming that everyone goes through pipewire, and ideally that is > > > the only one that can even get at these special chardev. > > > > > > If pipewire is only for sandboxed apps then yeah this aint great :-/ > > > > No, PipeWire is fine, I mean graphical apps. > > > > Right now, if your platform requires CMA for display, then the app > > needs access to the GPU render node and the display node too, in order > > to allocate buffers which the compositor can scan out directly. If it > > only has access to the render nodes and not the display node, it won't > > be able to allocate correctly, so its content will need a composition > > pass, i.e. performance penalty for sandboxing. But if it can allocate > > correctly, then hey, it can exhaust CMA just like heaps can. > > > > Personally I think we'd be better off just allowing access and > > figuring out cgroups later. It's not like the OOM story is great > > generally, and hey, you can get there with just render nodes ... > > Imo the right fix is to ask the compositor to allocate the buffers in this > case, and then maybe have some kind of revoke/purge behaviour on these > buffers. Compositor has an actual idea of who's a candidate for direct > scanout after all, not the app. Or well at least force migrate the memory > from cma to shmem. > > If you only whack cgroups on this issue you're still stuck in the world > where either all apps together can ddos the display or no one can > realistically direct scanout. > > So yeah on the display side the problem isn't solved either, but we knew > that already. What makes scanout memory so special? The way I see it, any kind of memory will always be a limited resource: regular programs can exhaust system memory, as well as GPU VRAM, as well as scanout memory. I think we need to have ways to limit/control/arbiter the allocations regardless, and I don't think scanout memory should be a special case here.
