Re: [PATCH 1/2] sched/core: Allow newidle for core-sched

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jun 25, 2026 at 05:26:03AM +0530, K Prateek Nayak wrote:

> ... or we can return a RETRY_TASK when newidle balance succeeds too to
> force a core-wide pick when core scheduling is enabled like:
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0d212bf04885..e393aed58bfa 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9946,8 +9946,16 @@ struct task_struct *pick_task_fair(struct rq *rq, struct rq_flags *rf)
>  	if (rq_modified_above(rq, &fair_sched_class))
>  		return RETRY_TASK;
>  
> -	if (cfs_rq->nr_queued)
> +	if (cfs_rq->nr_queued) {
> +		/*
> +		 * Force a core-wide pick if newidle
> +		 * balance managed to pull a task since
> +		 * the lock was dropped.
> +		 */
> +		if (sched_core_enabled(rq))
> +			return RETRY_TASK;
>  		goto again;
> +	}
>  
>  	return NULL;
>  }

This has forward progress issues. You can ping-pong he one task back and
forth and never actually pick anything.