On Tue, Aug 1, 2023 at 8:30 PM David Vernet <void@xxxxxxxxxxxxx> wrote: > I agree that this is the correct way to generalize this. The only thing > that we'll have to figure out is how to generalize treating const struct > cpumask * objects as kptrs. In sched_ext [0] we export > scx_bpf_get_idle_cpumask() and scx_bpf_get_idle_smtmask() kfuncs to > return trusted global cpumask kptrs that can then be "released" in > scx_bpf_put_idle_cpumask(). scx_bpf_put_idle_cpumask() is empty and > exists only to appease the verifier that the trusted cpumask kptrs > aren't being leaked and are having their references "dropped". why is it KF_ACQUIRE ? I think it can just return a trusted pointer without acquire. > [0]: https://lore.kernel.org/all/20230711011412.100319-13-tj@xxxxxxxxxx/ > > I'd imagine that we have 2 ways forward if we want to enable progs to > fetch other global cpumasks with static lifetimes (e.g. > __cpu_possible_mask or nohz.idle_cpus_mask): > > 1. The most straightforward thing to do would be to add a new kfunc in > kernel/bpf/cpumask.c that's a drop-in replacment for > scx_bpf_put_idle_cpumask(): > > void bpf_global_cpumask_drop(const struct cpumask *cpumask) > {} > > 2. Another would be to implement something resembling what Yonghong > suggested in [1], where progs can link against global allocated kptrs > like: > > const struct cpumask *__cpu_possible_mask __ksym; > > [1]: https://lore.kernel.org/all/3f56b3b3-9b71-f0d3-ace1-406a8eeb64c0@xxxxxxxxx/#t > > In my opinion (1) is more straightforward, (2) is a better UX. 1 = adding few kfuncs. 2 = teaching pahole to emit certain global vars. nm vmlinux|g -w D|g -v __SCK_|g -v __tracepoint_|wc -l 1998 imo BTF increase trade off is acceptable.
