Am Mittwoch, dem 26.11.2025 um 16:44 +0100 schrieb Philipp Stanner: > On Wed, 2025-11-26 at 16:03 +0100, Christian König wrote: > > > > [...] > > > My hope would be that in the mid-term future we'd get firmware > > > rings > > > that can be preempted through a firmware call for all major > > > hardware. > > > Then a huge share of our problems would disappear. > > > > At least on AMD HW pre-emption is actually horrible unreliable as > > well. > > Do you mean new GPUs with firmware scheduling, or what is "HW pre- > emption"? > > With firmware interfaces, my hope would be that you could simply tell > > stop_running_ring(nr_of_ring) > // time slice for someone else > start_running_ring(nr_of_ring) > > Thereby getting real scheduling and all that. And eliminating many > other problems we know well from drm/sched.
It doesn't really matter if you have firmware scheduling or not for preemption to be a hard problem on GPUs. CPUs have limited software visible state that needs to be saved/restored on a context switch and even there people start complaining now that they need to context switch the AVX512 register set. GPUs have megabytes of software visible state. Which needs to be saved/restored on the context switch if you want fine grained preemption with low preemption latency. There might be points in the command execution where you can ignore most of that state, but reaching those points can have basically unbounded latency. So either you can reliably save/restore lots of state or you are limited to very coarse grained preemption with all the usual issues of timeouts and DoS vectors. I'm not totally up to speed with the current state across all relevant GPUs, but until recently NVidia was the only vendor to have real reliable fine-grained preemption. Regards, Lucas
