Re: [PATCH bpf-next v4 00/10] bpf: Support private stack for bpf progs

[Tejun Heo <tj@kernel.org>]

Hello,

On Thu, Oct 10, 2024 at 10:55:52AM -0700, Yonghong Song wrote:
> The main motivation for private stack comes from nested scheduler in
> sched-ext from Tejun. The basic idea is that
>  - each cgroup will its own associated bpf program,
>  - bpf program with parent cgroup will call bpf programs
>    in immediate child cgroups.
> 
> Let us say we have the following cgroup hierarchy:
>   root_cg (prog0):
>     cg1 (prog1):
>       cg11 (prog11):
>         cg111 (prog111)
>         cg112 (prog112)
>       cg12 (prog12):
>         cg121 (prog121)
>         cg122 (prog122)
>     cg2 (prog2):
>       cg21 (prog21)
>       cg22 (prog22)
>       cg23 (prog23)

Thank you so much for working on this. I have some basic and a bit
tangential questions around how stacks are allocated. So, for sched_ext,
each scheduler would be represented by struct_ops and I think the interface
to load them would be attaching a struct_ops to a cgroup.

- I suppose each operation in a struct_ops would count as a separate program
  and would thus allocate 512 * nr_cpus stacks, right?

- If the same scheduler implementation is attached to more than one cgroups,
  would each instance be treated as a separate set of programs or would they
  share the stack?

- Most struct_ops operations won't need to be nested and thus wouldn't need
  to use a private stack. Would it be possible to indicate which one should
  use a private stack?

Thanks.

-- 
tejun