linux-kernel - Re: [PATCH v2 3/3] sched/fair: Update comments regarding last and skip buddy

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Message-ID: <3e31e4b7-d4de-4e23-a208-50effb4894a5@linux.ibm.com>
Date: Thu, 13 Mar 2025 14:00:34 +0530
From: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
To: Adam Li <adamli@...amperecomputing.com>
Cc: peterz@...radead.org, mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        vschneid@...hat.com, linux-kernel@...r.kernel.org,
        patches@...erecomputing.com, cl@...ux.com, christian.loehle@....com,
        Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
Subject: Re: [PATCH v2 3/3] sched/fair: Update comments regarding last and
 skip buddy

Hi Adam,

On 27/11/24 11:26, Adam Li wrote:
> Commit 5e963f2bd465 ("sched/fair: Commit to EEVDF") removed the "last"
> and "skip" buddy. Update comments in pick_next_entity().
> 
> Signed-off-by: Adam Li <adamli@...amperecomputing.com>
> Reviewed-by: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
> ---
>  kernel/sched/fair.c | 8 +-------
>  1 file changed, 1 insertion(+), 7 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d5a3b5589e4e..259c56dcdff6 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5602,17 +5602,11 @@ set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
>  
>  static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags);
>  
> -/*
> - * Pick the next process, keeping these things in mind, in this order:
> - * 1) keep things fair between processes/task groups
> - * 2) pick the "next" process, since someone really wants that to run
> - * 3) pick the "last" process, for cache locality
> - * 4) do not run the "skip" process, if something else is available
> - */
>  static struct sched_entity *
>  pick_next_entity(struct rq *rq, struct cfs_rq *cfs_rq)
>  {
>  	/*
> +	 * Pick the "next" buddy, since someone really wants that to run.
>  	 * Enabling NEXT_BUDDY will affect latency but not fairness.
>  	 */
>  	if (sched_feat(NEXT_BUDDY) &&

There is one more reference to LAST_BUDDY in check_preempt_wakeup_fair.

Regarding pick_next_entity, the first two points are still valid, so only
points 3 and 4 could be removed?

Something like below

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 9dafb374d76d..379dbcbb24e9 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5578,8 +5578,6 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags);
  * Pick the next process, keeping these things in mind, in this order:
  * 1) keep things fair between processes/task groups
  * 2) pick the "next" process, since someone really wants that to run
- * 3) pick the "last" process, for cache locality
- * 4) do not run the "skip" process, if something else is available
  */
 static struct sched_entity *
 pick_next_entity(struct rq *rq, struct cfs_rq *cfs_rq)
@@ -8780,9 +8778,7 @@ static void check_preempt_wakeup_fair(struct rq *rq, struct task_struct *p, int
         *
         * Note: this also catches the edge-case of curr being in a throttled
         * group (e.g. via set_curr_task), since update_curr() (in the
-        * enqueue of curr) will have resulted in resched being set.  This
-        * prevents us from potentially nominating it as a false LAST_BUDDY
-        * below.
+        * enqueue of curr) will have resulted in resched being set.
         */
        if (test_tsk_need_resched(rq->curr))
                return;

Thanks,
Madadi Vineeth Reddy