Re: [PATCH] sched_ext: idle: honor built-in idle disablement in node kfuncs

[Andrea Righi]

Hi Joe,

On Tue, Mar 24, 2026 at 03:42:35PM -0400, Joseph Salisbury wrote:
> The node-aware idle kfunc helpers validate per-node idle tracking, but they
> don't check whether built-in idle tracking itself is enabled.
> 
> As a result, when ops.update_idle() disables built-in idle tracking, the
> node helpers can still read per-node idle masks and attempt idle CPU
> selection.  This violates the documented behavior and can expose stale
> idle state to BPF schedulers.
> 
> Fix this by checking check_builtin_idle_enabled() in the node mask getters
> and in scx_bpf_pick_idle_cpu_node(), matching the behavior of the non-node
> helpers.
> 
> scx_bpf_pick_any_cpu_node() is different by: when built-in idle
> tracking is disabled, it should skip idle selection and fall back directly
> to the any-CPU path.  Make it do so and match scx_bpf_pick_any_cpu().
> 
> Fixes: 01059219b0cf ("sched_ext: idle: Introduce node-aware idle cpu kfunc helpers")
> Cc: stable@xxxxxxxxxxxxxxx # v6.15+
> Assisted-by: Codex:GPT-5
> Signed-off-by: Joseph Salisbury <joseph.salisbury@xxxxxxxxxx>

We are already validating this at load time, see validate_ops():
...
       /*
         * SCX_OPS_BUILTIN_IDLE_PER_NODE requires built-in CPU idle
         * selection policy to be enabled.
         */
        if ((ops->flags & SCX_OPS_BUILTIN_IDLE_PER_NODE) &&
            (ops->update_idle && !(ops->flags & SCX_OPS_KEEP_BUILTIN_IDLE))) {
                scx_error(sch, "SCX_OPS_BUILTIN_IDLE_PER_NODE requires CPU idle selection enabled");
                return -EINVAL;
        }
...

In practice you can't have SCX_OPS_BUILTIN_IDLE_PER_NODE set without
built-in idle enabled if a scheduler is running and we are checking for
SCX_OPS_BUILTIN_IDLE_PER_NODE in validate_node(). So I think these extra
checks are not needed.

Thanks,
-Andrea

> ---
>  kernel/sched/ext_idle.c | 17 ++++++++++++++---
>  1 file changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
> index ba298ac3ce6c..948f6b4f8ab5 100644
> --- a/kernel/sched/ext_idle.c
> +++ b/kernel/sched/ext_idle.c
> @@ -1082,6 +1082,9 @@ __bpf_kfunc const struct cpumask *scx_bpf_get_idle_cpumask_node(int node)
>  	if (node < 0)
>  		return cpu_none_mask;
>  
> +	if (!check_builtin_idle_enabled(sch))
> +		return cpu_none_mask;
> +
>  	return idle_cpumask(node)->cpu;
>  }
>  
> @@ -1137,6 +1140,9 @@ __bpf_kfunc const struct cpumask *scx_bpf_get_idle_smtmask_node(int node)
>  	if (node < 0)
>  		return cpu_none_mask;
>  
> +	if (!check_builtin_idle_enabled(sch))
> +		return cpu_none_mask;
> +
>  	if (sched_smt_active())
>  		return idle_cpumask(node)->smt;
>  	else
> @@ -1253,6 +1259,9 @@ __bpf_kfunc s32 scx_bpf_pick_idle_cpu_node(const struct cpumask *cpus_allowed,
>  	if (node < 0)
>  		return node;
>  
> +	if (!check_builtin_idle_enabled(sch))
> +		return -EBUSY;
> +
>  	return scx_pick_idle_cpu(cpus_allowed, node, flags);
>  }
>  
> @@ -1337,9 +1346,11 @@ __bpf_kfunc s32 scx_bpf_pick_any_cpu_node(const struct cpumask *cpus_allowed,
>  	if (node < 0)
>  		return node;
>  
> -	cpu = scx_pick_idle_cpu(cpus_allowed, node, flags);
> -	if (cpu >= 0)
> -		return cpu;
> +	if (static_branch_likely(&scx_builtin_idle_enabled)) {
> +		cpu = scx_pick_idle_cpu(cpus_allowed, node, flags);
> +		if (cpu >= 0)
> +			return cpu;
> +	}
>  
>  	if (flags & SCX_PICK_IDLE_IN_NODE)
>  		cpu = cpumask_any_and_distribute(cpumask_of_node(node), cpus_allowed);
> -- 
> 2.47.3
>