linux-kernel - Re: [PATCH sched_ext/for-6.11] sched_ext: Reverting @p->sched

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Message-ID: <7d39f26d-3c9f-4ee4-977c-87f9bed0bac1@huawei.com>
Date: Thu, 18 Jul 2024 14:04:47 +0800
From: "Zhangqiao (2012 lab)" <zhangqiao22@...wei.com>
To: Tejun Heo <tj@...nel.org>
CC: <void@...ifault.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH sched_ext/for-6.11] sched_ext: Reverting @p->sched_class
 if @p->disallow is set



在 2024/7/18 1:49, Tejun Heo 写道:
> On Wed, Jul 17, 2024 at 10:01:13AM +0800, Zhangqiao (2012 lab) wrote:
>>> Ah, I see what you mean. I was referring to the classs switching operations
>>> in scx_ops_enable(). You're looking at the fork path. I don't think we can
>>
>> Yes, i was referring to the fork path.
>>
>>> switch sched_class at that point and the .disallow mechanism is there to
>>> allow the scheduler to filter out tasks on scheduler start. I'll update the
>>> code so that .disallow is only allowed during the initial attach.
> 
> So, something like this.
> 

LGTM for this patch.

In addition, the @scx_nr_rejected is only updated while the BPF
scheduler is being loaded and this update behavior is proected by
scx_ops_enable_mutex, so is it appropriate to change the
@scx_nr_rejcted's type from atomic to int ?

> Thanks.
> 
> diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
> index 593d2f4909dd..a4aa516cee7d 100644
> --- a/include/linux/sched/ext.h
> +++ b/include/linux/sched/ext.h
> @@ -181,11 +181,12 @@ struct sched_ext_entity {
>  	 * If set, reject future sched_setscheduler(2) calls updating the policy
>  	 * to %SCHED_EXT with -%EACCES.
>  	 *
> -	 * If set from ops.init_task() and the task's policy is already
> -	 * %SCHED_EXT, which can happen while the BPF scheduler is being loaded
> -	 * or by inhering the parent's policy during fork, the task's policy is
> -	 * rejected and forcefully reverted to %SCHED_NORMAL. The number of
> -	 * such events are reported through /sys/kernel/debug/sched_ext::nr_rejected.
> +	 * Can be set from ops.init_task() while the BPF scheduler is being
> +	 * loaded (!scx_init_task_args->fork). If set and the task's policy is
> +	 * already %SCHED_EXT, the task's policy is rejected and forcefully
> +	 * reverted to %SCHED_NORMAL. The number of such events are reported
> +	 * through /sys/kernel/debug/sched_ext::nr_rejected. Setting this flag
> +	 * during fork is not allowed.
>  	 */
>  	bool			disallow;	/* reject switching into SCX */
>  
> diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
> index da9cac6b6cc2..cf60474efa75 100644
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -3399,18 +3399,17 @@ static int scx_ops_init_task(struct task_struct *p, struct task_group *tg, bool
>  
>  	scx_set_task_state(p, SCX_TASK_INIT);
>  
> -	if (p->scx.disallow) {
> +	if (!fork && p->scx.disallow) {
>  		struct rq *rq;
>  		struct rq_flags rf;
>  
>  		rq = task_rq_lock(p, &rf);
>  
>  		/*
> -		 * We're either in fork or load path and @p->policy will be
> -		 * applied right after. Reverting @p->policy here and rejecting
> -		 * %SCHED_EXT transitions from scx_check_setscheduler()
> -		 * guarantees that if ops.init_task() sets @p->disallow, @p can
> -		 * never be in SCX.
> +		 * We're in the load path and @p->policy will be applied right
> +		 * after. Reverting @p->policy here and rejecting %SCHED_EXT
> +		 * transitions from scx_check_setscheduler() guarantees that if
> +		 * ops.init_task() sets @p->disallow, @p can never be in SCX.
>  		 */
>  		if (p->policy == SCHED_EXT) {
>  			p->policy = SCHED_NORMAL;
> @@ -3418,6 +3417,9 @@ static int scx_ops_init_task(struct task_struct *p, struct task_group *tg, bool
>  		}
>  
>  		task_rq_unlock(rq, p, &rf);
> +	} else if (p->scx.disallow) {
> +		scx_ops_error("ops.init_task() set task->scx.disallow for %s[%d] during fork",
> +			      p->comm, p->pid);
>  	}
>  
>  	p->scx.flags |= SCX_TASK_RESET_RUNNABLE_AT;
>