linux-kernel - Re: [PATCH v2] sched_ext: switch class when preempted by higher priority scheduler

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Message-ID: <Z34_3FSZwj0E1aUC@gpd3>
Date: Wed, 8 Jan 2025 10:05:32 +0100
From: Andrea Righi <arighi@...dia.com>
To: Honglei Wang <jameshongleiwang@....com>
Cc: tj@...nel.org, void@...ifault.com, changwoo@...lia.com,
	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, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched_ext: switch class when preempted by higher
 priority scheduler

Hi Honglei,

On Wed, Jan 08, 2025 at 10:33:28AM +0800, Honglei Wang wrote:
> ops.cpu_release() function, if defined, must be invoked when preempted by
> a higher priority scheduler class task. This scenario was skipped in
> commit f422316d7466 ("sched_ext: Remove switch_class_scx()"). Let's fix
> it.
> 
> Fixes: f422316d7466 ("sched_ext: Remove switch_class_scx()")
> Signed-off-by: Honglei Wang <jameshongleiwang@....com>

Thanks a lot for catching this, I've also reproduced a stall (due to the
missing ops.cpu_release() invocation) and verified that your fixes resolve
the problem, so:

Acked-by: Andrea Righi <arighi@...dia.com>

> ---
>  kernel/sched/ext.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index 19d2699cf638..ef0e3f2c1e6f 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -3034,7 +3034,7 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p,
>  		 */
>  		if (p->scx.slice && !scx_rq_bypassing(rq)) {
>  			dispatch_enqueue(&rq->scx.local_dsq, p, SCX_ENQ_HEAD);
> -			return;
> +			goto switch_class;
>  		}
>  
>  		/*
> @@ -3051,6 +3051,7 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p,
>  		}
>  	}
>  
> +switch_class:
>  	if (next && next->sched_class != &ext_sched_class)
>  		switch_class(rq, next);
>  }
> -- 
> 2.45.2
> 

-Andrea