linux-kernel - Re: [PATCH] sched/fair: Update stale comments referencing last/skip buddy

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Message-ID: <22e5bd44-1bba-4cc4-be21-04bbd9702da8@linux.ibm.com>
Date: Thu, 7 Aug 2025 11:51:54 +0530
From: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
To: Zhongqiu Han <quic_zhonhan@...cinc.com>
Cc: mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        vschneid@...hat.com, Adam Li <adamli@...amperecomputing.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair: Update stale comments referencing last/skip
 buddy

Hi Zhongqiu,

On 05/08/25 17:53, Zhongqiu Han wrote:
> Since the integration of EEVDF, the last/skip buddy scheduling features
> have been removed. This patch updates outdated comments that still
> reference these legacy behaviors to avoid inconsistencies.
> 
> Fixes: 5e963f2bd465 ("sched/fair: Commit to EEVDF")
> Signed-off-by: Zhongqiu Han <quic_zhonhan@...cinc.com>
> ---
>  kernel/sched/fair.c | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index b173a059315c..b3618aa075ec 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5500,11 +5500,11 @@ 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
> + * Pick the next sched_entity to run from cfs_rq.
> + *
> + * Prefer ->next buddy if sched_feat(PICK_BUDDY) is enabled and it's eligible,
> + * to improve cache locality.
> + * Otherwise, pick the entity via EEVDF for fairness and latency control.
>   */
>  static struct sched_entity *
>  pick_next_entity(struct rq *rq, struct cfs_rq *cfs_rq)
> @@ -8673,9 +8673,9 @@ 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. This
> +	 * prevents further preemption handling, including checks and potential
> +	 * reschedule triggering.
>  	 */
>  	if (test_tsk_need_resched(rq->curr))
>  		return;

Sent similar patch here https://lore.kernel.org/lkml/20250314090128.25141-1-vineethr@linux.ibm.com/
[PATCH] sched/fair: Remove stale references to LAST_BUDDY and SKIP_BUDDY

Thanks,
Madadi Vineeth Reddy