Re: [PATCH 05/16] sched_ext: Make scx_enable() take scx_enable_cmd

[Cheng-Yang Chou]

Hi Tejun,

On Mon, Apr 20, 2026 at 09:19:34PM -1000, Tejun Heo wrote:
> Pass struct scx_enable_cmd to scx_enable() rather than unpacking @ops
> at every call site and re-packing into a fresh cmd inside. bpf_scx_reg()
> now builds the cmd on its stack and hands it in; scx_enable() just
> wires up the kthread work and waits.

Verified stack lifetime.

Reviewed-by: Cheng-Yang Chou <yphbchou0911@xxxxxxxxx>

> 
> Relocate struct scx_enable_cmd above scx_alloc_and_add_sched() so
> upcoming patches that also want the cmd can see it.
> 
> No behavior change.
> 
> Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
> ---
>  kernel/sched/ext.c | 46 +++++++++++++++++++++++-----------------------
>  1 file changed, 23 insertions(+), 23 deletions(-)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 74e4271e44e9..62aab432dbf4 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -6424,6 +6424,19 @@ static struct scx_sched_pnode *alloc_pnode(struct scx_sched *sch, int node)
>  	return pnode;
>  }
>  
> +/*
> + * scx_enable() is offloaded to a dedicated system-wide RT kthread to avoid
> + * starvation. During the READY -> ENABLED task switching loop, the calling
> + * thread's sched_class gets switched from fair to ext. As fair has higher
> + * priority than ext, the calling thread can be indefinitely starved under
> + * fair-class saturation, leading to a system hang.
> + */
> +struct scx_enable_cmd {
> +	struct kthread_work	work;
> +	struct sched_ext_ops	*ops;
> +	int			ret;
> +};
> +
>  /*
>   * Allocate and initialize a new scx_sched. @cgrp's reference is always
>   * consumed whether the function succeeds or fails.
> @@ -6655,19 +6668,6 @@ static int validate_ops(struct scx_sched *sch, const struct sched_ext_ops *ops)
>  	return 0;
>  }
>  
> -/*
> - * scx_enable() is offloaded to a dedicated system-wide RT kthread to avoid
> - * starvation. During the READY -> ENABLED task switching loop, the calling
> - * thread's sched_class gets switched from fair to ext. As fair has higher
> - * priority than ext, the calling thread can be indefinitely starved under
> - * fair-class saturation, leading to a system hang.
> - */
> -struct scx_enable_cmd {
> -	struct kthread_work	work;
> -	struct sched_ext_ops	*ops;
> -	int			ret;
> -};
> -
>  static void scx_root_enable_workfn(struct kthread_work *work)
>  {
>  	struct scx_enable_cmd *cmd = container_of(work, struct scx_enable_cmd, work);
> @@ -7243,11 +7243,10 @@ static s32 __init scx_cgroup_lifetime_notifier_init(void)
>  core_initcall(scx_cgroup_lifetime_notifier_init);
>  #endif	/* CONFIG_EXT_SUB_SCHED */
>  
> -static s32 scx_enable(struct sched_ext_ops *ops, struct bpf_link *link)
> +static s32 scx_enable(struct scx_enable_cmd *cmd, struct bpf_link *link)
>  {
>  	static struct kthread_worker *helper;
>  	static DEFINE_MUTEX(helper_mutex);
> -	struct scx_enable_cmd cmd;
>  
>  	if (!cpumask_equal(housekeeping_cpumask(HK_TYPE_DOMAIN),
>  			   cpu_possible_mask)) {
> @@ -7271,16 +7270,15 @@ static s32 scx_enable(struct sched_ext_ops *ops, struct bpf_link *link)
>  	}
>  
>  #ifdef CONFIG_EXT_SUB_SCHED
> -	if (ops->sub_cgroup_id > 1)
> -		kthread_init_work(&cmd.work, scx_sub_enable_workfn);
> +	if (cmd->ops->sub_cgroup_id > 1)
> +		kthread_init_work(&cmd->work, scx_sub_enable_workfn);
>  	else
>  #endif	/* CONFIG_EXT_SUB_SCHED */
> -		kthread_init_work(&cmd.work, scx_root_enable_workfn);
> -	cmd.ops = ops;
> +		kthread_init_work(&cmd->work, scx_root_enable_workfn);
>  
> -	kthread_queue_work(READ_ONCE(helper), &cmd.work);
> -	kthread_flush_work(&cmd.work);
> -	return cmd.ret;
> +	kthread_queue_work(READ_ONCE(helper), &cmd->work);
> +	kthread_flush_work(&cmd->work);
> +	return cmd->ret;
>  }
>  
>  
> @@ -7452,7 +7450,9 @@ static int bpf_scx_check_member(const struct btf_type *t,
>  
>  static int bpf_scx_reg(void *kdata, struct bpf_link *link)
>  {
> -	return scx_enable(kdata, link);
> +	struct scx_enable_cmd cmd = { .ops = kdata };
> +
> +	return scx_enable(&cmd, link);
>  }
>  
>  static void bpf_scx_unreg(void *kdata, struct bpf_link *link)
> -- 
> 2.53.0
> 
> 

-- 
Cheers,
Cheng-Yang