Re: [PATCH 8/8] sched/eevdf: Add min slice check when selecting CPU

[Peter Zijlstra]

On Mon, Sep 21, 2026 at 05:22:38PM +0200, Vincent Guittot wrote:
> Add a new level for selecting CPU when select_task_rq_fair() fails to find
> an idle CPU. This last level will compare the slice to select a CPU where
> the task could run 1st.
> This helps a waking task to select a CPU where a longer slice runs
> instead of one where a task with the same or shorter slice already runs.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 63 ++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 62 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 452da94289fe..5c9add3e853c 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9023,6 +9023,59 @@ static inline bool asym_fits_cpu(unsigned long util,
>  	return true;
>  }
>  
> +static int select_slice_cpu(struct task_struct *p, struct sched_domain *sd, int target)
> +{
> +	unsigned long task_util, util_min, util_max;
> +	int cpu, nr = INT_MAX;
> +	u64 slice = p->se.slice;
> +	struct cpumask *cpus;
> +
> +	if (sched_feat(SIS_UTIL) && sd->shared) {
> +		/*
> +		 * Same nr_idle_scan hint as select_idle_cpu(), nr only limits
> +		 * the scan when not preferring an idle core.
> +		 */
> +		nr = READ_ONCE(sd->shared->nr_idle_scan) + 1;
> +		/* overloaded domain is unlikely to have idle cpu/core */
> +		if (nr == 1)
> +			return -1;
> +	}
> +
> +	cpus = this_cpu_cpumask_var_ptr(select_rq_mask);
> +	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> +
> +	if (sched_asym_cpucap_active()) {
> +		task_util = task_util_est(p);
> +		util_min = uclamp_eff_value(p, UCLAMP_MIN);
> +		util_max = uclamp_eff_value(p, UCLAMP_MAX);
> +	}

This all seems duplicated from select_idle_siblings(), and while I
appreciated the breaking into functions, I do worry about the duplicate
work done too.

> +	/* Those CPUs have been tested not being idle and fiting */
> +	for_each_cpu_wrap(cpu, cpus, target) {
> +		/*
> +		 * Stop when the nr_idle_scan is exhausted (mirrors
> +		 * select_idle_cpu() logic).
> +		 */
> +		if (--nr <= 0)
> +			return -1;
> +
> +		if (slice >= get_rq_min_slice(cpu_rq(cpu)))
> +			continue;
> +
> +		if (sched_asym_cpucap_active()) {
> +			int fits = util_fits_cpu(task_util, util_min, util_max, cpu);
> +
> +			/* Perfect fit: capacity satisfies util + uclamp */
> +			if (fits > 0)
> +				return cpu;
> +		} else {
> +			return cpu;
> +		}
> +	}

And it does seem like a waste to re-scan the CPUs we've already visited.
Can't we keep track of the minimal slice CPU that was not idle during
our initial scan?


diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2ad46fb2eafe..d3ab945f50ba 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9179,6 +9179,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 	struct sched_domain *sd;
 	unsigned long task_util, util_min, util_max;
 	int i, recent_used_cpu, prev_aff = -1;
+	int best = target;
 
 	/*
 	 * On asymmetric system, update task utilization because we will check
@@ -9263,8 +9264,10 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 		 * capacity path.
 		 */
 		if (sd) {
-			i = select_idle_capacity(p, sd, target);
-			return ((unsigned)i < nr_cpumask_bits) ? i : target;
+			i = select_idle_capacity(p, sd, target, &best)
+			if ((unsigned)i < nr_cpumask_bits)
+				return i;
+			return best;
 		}
 	}
 
@@ -9282,7 +9285,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 		}
 	}
 
-	i = select_idle_cpu(p, sd, has_idle_core, target);
+	i = select_idle_cpu(p, sd, has_idle_core, target, &best);
 	if ((unsigned)i < nr_cpumask_bits)
 		return i;
 
@@ -9297,7 +9300,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 	if ((unsigned int)recent_used_cpu < nr_cpumask_bits)
 		return recent_used_cpu;
 
-	return target;
+	return best;
 }
 
 /**