Re: [PATCH] sched_ext: pass the initial cpu.idle state in scx_cgroup_init_args

[Andrea Righi]

Hi Tao,

On Mon, Aug 24, 2026 at 09:39:54PM +0800, Tao Cui wrote:
> From: Tao Cui <cuitao@xxxxxxxxxx>
> 
> scx_cgroup_init_args carries the initial weight and bandwidth control
> parameters of a cgroup to ops.cgroup_init(), but not its cpu.idle
> state. A cgroup that was already configured idle before the scheduler
> was loaded (or before it was onlined under it) is presented as
> non-idle, and the BPF scheduler only learns about it if cpu.idle is
> written again later.
> 
> Add the idle state to scx_cgroup_init_args and fill it in both places
> that build the args -- scx_tg_online() for cgroups created under the
> scheduler and scx_cgroup_init() for cgroups that already exist when
> the scheduler is loaded.
> 
> Verified in a VM with a probe scheduler printing the init args: a
> cgroup configured cpu.idle=1 before loading shows idle=1 in
> ops.cgroup_init(), the default shows 0, and later cpu.idle writes
> still come through ops.cgroup_set_idle().
> 
> Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>
> ---
>  kernel/sched/ext/ext.c      | 4 +++-
>  kernel/sched/ext/internal.h | 3 +++
>  2 files changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
> index b646711a45fe..a7218314dcfb 100644
> --- a/kernel/sched/ext/ext.c
> +++ b/kernel/sched/ext/ext.c
> @@ -4764,7 +4764,8 @@ int scx_tg_online(struct task_group *tg)
>  				{ .weight = tg->scx.weight,
>  				  .bw_period_us = tg->scx.bw_period_us,
>  				  .bw_quota_us = tg->scx.bw_quota_us,
> -				  .bw_burst_us = tg->scx.bw_burst_us };
> +				  .bw_burst_us = tg->scx.bw_burst_us,
> +				  .idle = tg->scx.idle };
>  
>  			ret = SCX_CALL_OP_RET(sch, cgroup_init,
>  					      NULL, tg->css.cgroup, &args);
> @@ -5185,6 +5186,7 @@ static int scx_cgroup_init(struct scx_sched *sch)
>  				.bw_period_us = tg->scx.bw_period_us,
>  				.bw_quota_us = tg->scx.bw_quota_us,
>  				.bw_burst_us = tg->scx.bw_burst_us,
> +				.idle = tg->scx.idle,
>  			};
>  
>  			ret = SCX_CALL_OP_RET(sch, cgroup_init, NULL, css->cgroup, &args);
> diff --git a/kernel/sched/ext/internal.h b/kernel/sched/ext/internal.h
> index 53e136a47924..aa149a9c29f7 100644
> --- a/kernel/sched/ext/internal.h
> +++ b/kernel/sched/ext/internal.h
> @@ -259,6 +259,9 @@ struct scx_cgroup_init_args {
>  	u64			bw_period_us;
>  	u64			bw_quota_us;
>  	u64			bw_burst_us;
> +
> +	/* whether the cgroup is configured idle via cpu.idle */
> +	bool			idle;
>  };

It looks like there are two more places in kernel/sched/ext/sub.c,
scx_cgroup_claim_subtree() and scx_cgroup_return_subtree(), they should also
initialize idle. Otherwise the field defaults to false and an idle cgroup would
be presented as non-idle when it's handed to a sub-scheduler.

Thanks,
-Andrea