linux-kernel - Re: [PATCH sched_ext/for-6.15] sched_ext: initialize built-in idle state before ops.init()

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Message-ID: <1fcbaeee8b49b1c5f81cd01ba59a00a2@igalia.com>
Date: Mon, 24 Mar 2025 21:48:23 +0900
From: Changwoo Min <changwoo@...lia.com>
To: Andrea Righi <arighi@...dia.com>
Cc: Tejun Heo <tj@...nel.org>, David Vernet <void@...ifault.com>, Joel
 Fernandes <joelagnelf@...dia.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH sched_ext/for-6.15] sched_ext: initialize built-in idle
 state before ops.init()

Hi Andrea,

This is a nice catch! Looks good to me.

Regards,
Changwoo Min

On 2025-03-24 17:57, Andrea Righi wrote:
> A BPF scheduler may want to use the built-in idle cpumasks in ops.init()
> before the scheduler is fully initialized, either directly or through a
> BPF timer for example.
> 
> However, this would result in an error, since the idle state has not
> been properly initialized yet.
> 
> This can be easily verified by modifying scx_simple to call
> scx_bpf_get_idle_cpumask() in ops.init():
> 
> $ sudo scx_simple
> 
> DEBUG DUMP
> ===========================================================================
> 
> scx_simple[121] triggered exit kind 1024:
>   runtime error (built-in idle tracking is disabled)
> ...
> 
> Fix this by properly initializing the idle state before ops.init() is
> called. With this change applied:
> 
> $ sudo scx_simple
> local=2 global=0
> local=19 global=11
> local=23 global=11
> ...
> 
> Fixes: d73249f88743d ("sched_ext: idle: Make idle static keys private")
> Signed-off-by: Andrea Righi <arighi@...dia.com>
> ---
>  kernel/sched/ext.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 06561d6717c9a..1ba02755ae8ad 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -5361,6 +5361,8 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
>  	 */
>  	cpus_read_lock();
>  
> +	scx_idle_enable(ops);
> +
>  	if (scx_ops.init) {
>  		ret = SCX_CALL_OP_RET(SCX_KF_UNLOCKED, init);
>  		if (ret) {
> @@ -5427,8 +5429,6 @@ static int scx_ops_enable(struct sched_ext_ops *ops, struct bpf_link *link)
>  	if (scx_ops.cpu_acquire || scx_ops.cpu_release)
>  		static_branch_enable(&scx_ops_cpu_preempt);
>  
> -	scx_idle_enable(ops);
> -
>  	/*
>  	 * Lock out forks, cgroup on/offlining and moves before opening the
>  	 * floodgate so that they don't wander into the operations prematurely.