Re: [PATCH sched_ext/for-7.1] sched_ext: idle: Prioritize idle SMT sibling

[Cheng-Yang Chou]

Hi Andrea,

On Wed, Mar 18, 2026 at 01:38:42AM +0100, Andrea Righi wrote:
> In the default built-in idle CPU selection policy, when @prev_cpu is
> busy and no fully idle core is available, try to place the task on its
> SMT sibling if that sibling is idle, before searching any other idle CPU
> in the same LLC.
> 
> Migration to the sibling is cheap and keeps the task on the same core,
> preserving L1 cache and reducing wakeup latency.
> 
> On large SMT systems this appears to consistently boost throughput by
> roughly 2-3% on CPU-bound workloads (running a number of tasks equal to
> the number of SMT cores).
> 
> Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
> ---
>  kernel/sched/ext_idle.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
> index c7e4052626979..e0c57355b33b8 100644
> --- a/kernel/sched/ext_idle.c
> +++ b/kernel/sched/ext_idle.c
> @@ -616,6 +616,18 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags,
>  		goto out_unlock;
>  	}
>  
> +	/*
> +	 * Use @prev_cpu's sibling if it's idle.
> +	 */
> +	if (sched_smt_active()) {
> +		for_each_cpu_and(cpu, cpu_smt_mask(prev_cpu), allowed) {
> +			if (cpu == prev_cpu)
> +				continue;
> +			if (scx_idle_test_and_clear_cpu(cpu))
> +				goto out_unlock;
> +		}
> +	}
> +
>  	/*
>  	 * Search for any idle CPU in the same LLC domain.
>  	 */
> -- 
> 2.53.0
> 

Overall looks good, just a nit:

The block comment at the top of scx_select_cpu_dfl() still lists 5
steps. With this patch a new step should be added and the numbering
updated accordingly.

-- 
Thanks,
Cheng-Yang