linux-kernel - Re: [PATCH sched_ext/for-6.19] sched_ext: Use SCX_TASK_READY test instead of tryget_task

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Message-ID: <aQEyeiN_HEqRtQw6@gpd4>
Date: Tue, 28 Oct 2025 22:15:38 +0100
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
	linux-kernel@...r.kernel.org, sched-ext@...ts.linux.dev
Subject: Re: [PATCH sched_ext/for-6.19] sched_ext: Use SCX_TASK_READY test
 instead of tryget_task_struct() during class switch

Hi Tejun,

On Tue, Oct 28, 2025 at 11:04:18AM -1000, Tejun Heo wrote:
> ddf7233fcab6 ("sched/ext: Fix invalid task state transitions on class
> switch") added tryget_task_struct() test during scx_enable()'s class
> switching loop. The reason for the addition was to avoid enabling tasks which
> skipped prep in the previous loop due to being dead.
> 
> While tryget_task_struct() does work for this purpose as tasks that fail
> tryget always will fail it, it's a bit roundabout. A more direct way is
> testing whether the task is in READY state. Switch to testing SCX_TASK_READY
> directly.
> 
> Cc: Andrea Righi <arighi@...dia.com>
> Signed-off-by: Tejun Heo <tj@...nel.org>

Looks like a better check.

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

Thanks,
-Andrea

> ---
>  kernel/sched/ext.c |    3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -4662,7 +4662,7 @@ static int scx_enable(struct sched_ext_o
>  			__setscheduler_class(p->policy, p->prio);
>  		struct sched_enq_and_set_ctx ctx;
> 
> -		if (!tryget_task_struct(p))
> +		if (scx_get_task_state(p) != SCX_TASK_READY)
>  			continue;
> 
>  		if (old_class != new_class && p->se.sched_delayed)
> @@ -4677,7 +4677,6 @@ static int scx_enable(struct sched_ext_o
>  		sched_enq_and_set_task(&ctx);
> 
>  		check_class_changed(task_rq(p), p, old_class, p->prio);
> -		put_task_struct(p);
>  	}
>  	scx_task_iter_stop(&sti);
>  	percpu_up_write(&scx_fork_rwsem);