Re: [PATCH] sched_ext: idle: Skip cross-node search with !CONFIG_NUMA

[Yury Norov <yury.norov@gmail.com>]

Hi Andrea!

On Tue, Jun 03, 2025 at 10:22:01AM +0200, Andrea Righi wrote:
> In the idle CPU selection logic, attempting cross-node searches adds
> unnecessary complexity when CONFIG_NUMA is disabled.
> 
> Since there's no meaningful concept of nodes in this case, simplify the
> logic by restricting the idle CPU search to the current node only.
> 
> Fixes: 48849271e6611 ("sched_ext: idle: Per-node idle cpumasks")
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---
>  kernel/sched/ext_idle.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
> index 66da03cc0b338..8660d9ae40169 100644
> --- a/kernel/sched/ext_idle.c
> +++ b/kernel/sched/ext_idle.c
> @@ -138,6 +138,7 @@ static s32 pick_idle_cpu_in_node(const struct cpumask *cpus_allowed, int node, u
>  		goto retry;
>  }
>  
> +#ifdef CONFIG_NUMA

It would be more natural if you move this inside the function body,
and not duplicate the function declaration.

>  /*
>   * Tracks nodes that have not yet been visited when searching for an idle
>   * CPU across all available nodes.
> @@ -186,6 +187,13 @@ static s32 pick_idle_cpu_from_online_nodes(const struct cpumask *cpus_allowed, i
>  
>  	return cpu;
>  }
> +#else
> +static inline s32
> +pick_idle_cpu_from_online_nodes(const struct cpumask *cpus_allowed, int node, u64 flags)
> +{
> +	return -EBUSY;
> +}

This is misleading errno. The system is nut busy, it is disabled. If
it was a syscall, I would say you should return ENOSYS. ENODATA is
another candidate. Or you have a special policy for the subsystem/

The above pick_idle_cpu_in_node() doesn't have CONFIG_NUMA protection
as well. Is it safe against CONFIG_NUMA?

> +#endif
>  
>  /*
>   * Find an idle CPU in the system, starting from @node.
> -- 
> 2.49.0