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

From: Changwoo Min
Date: Mon Mar 24 2025 - 08:49:15 EST


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@xxxxxxxxxx>
> ---
> 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.