Re: [PATCH] sched_ext: don't deliver duplicate ops.cgroup_set_idle() for same value

[Andrea Righi]

Hi Tao,

On Tue, Sep 01, 2026 at 11:11:01AM +0800, Tao Cui wrote:
> From: Tao Cui <cuitao@xxxxxxxxxx>
> 
> ops.cgroup_set_idle() is documented to be invoked when a cgroup
> transitions between idle and non-idle states, and scx_group_set_weight()
> already skips value-preserving writes. scx_group_set_idle() delivers
> every write unconditionally, so rewriting an already-correct cpu.idle
> value feeds the BPF scheduler a transition callback each time, which
> toggle- or accounting-based schedulers miscount. Mirror the weight
> guard and only deliver on an actual change.
> 
> Verified with a probe scheduler printing each callback: rewriting
> cpu.idle=1 twice on an already-idle cgroup delivered two callbacks
> before and none after.
> 
> Fixes: 347ed2d566da ("sched/ext: Implement cgroup_set_idle() callback")
> Link: https://lore.kernel.org/r/b53c61a1-4d7d-4232-941f-d48b0563d4ed

This link seems broken, I think the right one is:

Link: https://lore.kernel.org/r/b53c61a1-4d7d-4232-941f-d48b0563d4ed@xxxxxxxxx

> Signed-off-by: Tao Cui <cuitao@xxxxxxxxxx>

Other than that looks good to me.

Reviewed-by: Andrea Righi <arighi@xxxxxxxxxx>

Thanks,
-Andrea

> ---
>  kernel/sched/ext/ext.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
> index 8041c87a3562..8b3625107b72 100644
> --- a/kernel/sched/ext/ext.c
> +++ b/kernel/sched/ext/ext.c
> @@ -4933,7 +4933,8 @@ void scx_group_set_idle(struct task_group *tg, bool idle)
>  	percpu_down_read(&scx_cgroup_ops_rwsem);
>  	sch = scx_tg_knob_sched(tg);
>  
> -	if (scx_cgroup_enabled && sch && SCX_HAS_OP(sch, cgroup_set_idle))
> +	if (scx_cgroup_enabled && sch && SCX_HAS_OP(sch, cgroup_set_idle) &&
> +	    tg->scx.sched_idle != idle)
>  		SCX_CALL_OP(sch, cgroup_set_idle, NULL, tg_cgrp(tg), idle);
>  
>  	/* Update the task group's idle state */
> -- 
> 2.43.0
>