linux-kernel - Re: [PATCH] sched/fair: Only increment deadline once on yield

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Message-ID: <e28db863-3547-4a20-83da-26d8f7be2b03@linux.ibm.com>
Date: Sun, 13 Apr 2025 09:30:07 +0530
From: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
To: Fernand Sieber <sieberf@...zon.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org, nh-open-source@...zon.com,
        Madadi Vineeth Reddy <vineethr@...ux.ibm.com>
Subject: Re: [PATCH] sched/fair: Only increment deadline once on yield

Hi Fernand,

On 01/04/25 18:06, Fernand Sieber wrote:
> If a task yields, the scheduler may decide to pick it again. The task in
> turn may decide to yield immediately or shortly after, leading to a tight
> loop of yields.
> 
> If there's another runnable task as this point, the deadline will be
> increased by the slice at each loop. This can cause the deadline to runaway
> pretty quickly, and subsequent elevated run delays later on as the task
> doesn't get picked again. The reason the scheduler can pick the same task
> again and again despite its deadline increasing is because it may be the
> only eligible task at that point.
> 
> Fix this by updating the deadline only to one slice ahead.
> 
> Note, we might want to consider iterating on the implementation of yield as
> follow up:
> * the yielding task could be forfeiting its remaining slice by
>   incrementing its vruntime correspondingly
> * in case of yield_to the yielding task could be donating its remaining
>   slice to the target task
> 
> Signed-off-by: Fernand Sieber <sieberf@...zon.com>
> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e43993a4e580..c1eff68d8ffc 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9024,7 +9024,7 @@ static void yield_task_fair(struct rq *rq)
>  	 */
>  	rq_clock_skip_update(rq);
> 
> -	se->deadline += calc_delta_fair(se->slice, se);
> +	se->deadline = se->vruntime + calc_delta_fair(se->slice, se);
>  }

Makes sense. Deadline would be rapidly incremented in the scenario of
having other tasks not eligible and the current task calls yield
repeatedly for a very short run time.

Reviewed-by: Madadi Vineeth Reddy <vineethr@...ux.ibm.com>

Thanks,
Madadi Vineeth Reddy

> 
>  static bool yield_to_task_fair(struct rq *rq, struct task_struct *p)
> --
> 2.47.1
> 
> 
> 
> 
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