Re: [PATCH 06/20 v3] sched/eevdf: Add min slice check when selecting CPU

[Kayra Cizmeci <kayracizmeci@gmail.com>]

Hi Vincent,


Sashiko reviewed this one so I'll just put here my own concerns instead
of repeating.


On Fri, Nov 06, 2026 at 04:19:33PM +0100, Vincent Guittot wrote:

> Add a new level for selecting CPU when select_task_rq_fair() fails to find
> an idle CPU. This last level compares the min 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@linaro.org>
> ---
>  kernel/sched/core.c |   2 +-
>  kernel/sched/fair.c | 157 ++++++++++++++++++++++++++++----------------
>  2 files changed, 102 insertions(+), 57 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 031efe7d49fb..d71da9069218 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -9012,7 +9012,7 @@ void __init sched_init(void)
>  		rq->sd = NULL;
>  		rq->rd = NULL;
>  		rq->cpu_capacity = SCHED_CAPACITY_SCALE;
> -		rq->min_slice = ULONG_MAX;
> +		rq->min_slice = 0;
>  		rq->balance_callback = &balance_push_callback;
>  		rq->active_balance = 0;
>  		rq->next_balance = jiffies;
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index cfc99b94d9ce..039dce244aaa 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8842,11 +8842,25 @@ static inline int sched_balance_find_dst_cpu(struct sched_domain *sd, struct tas
>  	return new_cpu;
>  }
>  
> -static inline int __select_idle_cpu(int cpu, struct task_struct *p)
> +static inline bool choose_sched_slice_rq(struct rq *rq, struct task_struct *p)
>  {
> -	if (choose_idle_cpu(cpu, p) && sched_cpu_cookie_match(cpu_rq(cpu), p))
> +	if (p->se.slice < get_rq_min_slice(rq))
> +		return true;
> +
> +	return false;
> +}
> +
> +static inline int __select_idle_cpu(int cpu, struct task_struct *p, int *slice_cpu)
> +{
> +	if (!sched_cpu_cookie_match(cpu_rq(cpu), p))
> +		return -1;
> +
> +	if (choose_idle_cpu(cpu, p))
>  		return cpu;
>  
> +	if (*slice_cpu == -1 && choose_sched_slice_rq(cpu_rq(cpu), p))
> +		*slice_cpu = cpu;
> +
>  	return -1;
>  }


Look here:

CPU1: CPU2:
FA    RA
      FC

CPU2 has 1 RT and 1 Fair tasks named RA and FC.
Let's say RA is running on CPU2 and FC's slice is 10 ms.

And let's say FA has 8 ms slice, and a fair task named TC is waking up
with 9 ms slice:

* We look to CPU1, 9 < 8 is false so we return false

* We look to CPU2, 9 < 10 is true so we return it.

But the problem is, CPU2 has a RT task running and we are going to
compete with the RT task. In that case it can be worse than CPU1.

So, I would suggest something like this or so:


(My editor got crazy, so I had to copy this diff :<)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 262626455497..8b0dd858f332 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8876,10 +8876,8 @@ static inline int sched_balance_find_dst_cpu(struct sched_domain *sd, struct tas
 
 static inline bool choose_sched_slice_rq(struct rq *rq, struct task_struct *p)
 {
-	if (p->se.slice < get_rq_min_slice(rq))
-		return true;
-
-	return false;
+	return p->se.slice < get_rq_min_slice(rq) &&
+		READ_ONCE(rq->curr)->sched_class == &fair_sched_class;
 }
 
 static inline int __select_idle_cpu(int cpu, struct task_struct *p, int *slice_cpu)
@@ -9193,7 +9191,6 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
 	bool has_idle_core = sched_smt_active() && test_idle_cores(target);
 	unsigned long task_util, util_min, util_max, best_cap = 0;
 	int fits, best_fits = ASYM_IDLE_THREAD_MISFIT;
-	unsigned long task_slice;
 	int cpu, best_cpu = -1;
 	struct cpumask *cpus;
 	int nr = INT_MAX;
@@ -9204,7 +9201,6 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
 	task_util = task_util_est(p);
 	util_min = uclamp_eff_value(p, UCLAMP_MIN);
 	util_max = uclamp_eff_value(p, UCLAMP_MAX);
-	task_slice = p->se.slice;
 
 	if (sched_feat(SIS_UTIL) && sd->shared) {
 		/*
@@ -9260,7 +9256,7 @@ select_idle_capacity(struct task_struct *p, struct sched_domain *sd, int target)
 			else if (fits > 0)
 				fits = ASYM_IDLE_THREAD_FITS;
 
-		} else if (fits > 0 && task_slice < get_rq_min_slice(cpu_rq(cpu))) {
+		} else if (fits > 0 && choose_sched_slice_rq(cpu_rq(cpu), p)) {
 			fits = ASYM_BUSY_THREAD_FITS;
 			cpu_cap = get_actual_cpu_capacity(cpu);
 		} else {



Cheers,

Kayra :-)