Re: [PATCH 6/6] sched_ext: idle: Introduce node-aware idle cpu kfunc helpers

[Tejun Heo <tj@kernel.org>]

Hello,

On Sat, Feb 08, 2025 at 10:19:38AM +0100, Andrea Righi wrote:
...
> > This is contingent on scx_builtin_idle_per_node, right? It's confusing for
> > CPU -> node mapping function to return NUMA_NO_NODE depending on an ops
> > flag. Shouldn't this be a generic mapping function?
> 
> The idea is that BPF schedulers can use this kfunc to determine the right
> idle cpumask to use, for example a typical usage could be:
> 
>   int node = scx_bpf_cpu_node(prev_cpu);
>   s32 cpu = scx_bpf_pick_idle_cpu_in_node(p->cpus_ptr, node, SCX_PICK_IDLE_IN_NODE);
> 
> Or:
> 
>   int node = scx_bpf_cpu_node(prev_cpu);
>   const struct cpumask *idle_cpumask = scx_bpf_get_idle_cpumask_node(node);
> 
> When SCX_OPS_BUILTIN_IDLE_PER_NODE is disabled, we need to point to the
> global idle cpumask, that is identified by NUMA_NO_NODE, so this is why we
> can return NUMA_NO_NODE fro scx_bpf_cpu_node().
> 
> Do you think we should make this more clear / document this better. Or do
> you think we should use a different API?

I think this is too error-prone. It'd be really easy for users to assume
that scx_bpf_cpu_node() always returns the NUMA node for the given CPU which
can lead to really subtle surprises. Why even allow e.g.
scx_bpf_get_idle_cpumask_node() if IDLE_PER_NODE is not enabled?

Thanks.

-- 
tejun