Re: [PATCH 01/12] sched: Employ sched_change guards

[Andrea Righi]

Hi Peter,

On Mon, Oct 06, 2025 at 12:44:03PM +0200, Peter Zijlstra wrote:
> As proposed a long while ago -- and half done by scx -- wrap the
> scheduler's 'change' pattern in a guard helper.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Acked-by: Tejun Heo <tj@xxxxxxxxxx>
> ---
...
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -3885,23 +3885,22 @@ extern void check_class_changed(struct r
>  extern struct balance_callback *splice_balance_callbacks(struct rq *rq);
>  extern void balance_callbacks(struct rq *rq, struct balance_callback *head);
>  
> -#ifdef CONFIG_SCHED_CLASS_EXT
> -/*
> - * Used by SCX in the enable/disable paths to move tasks between sched_classes
> - * and establish invariants.
> - */
> -struct sched_enq_and_set_ctx {

Not necessarily for this patch, we can add it later, but I kinda liked the
comment that briefly explained how the context is used. Maybe having
something along these lines could be helpful?

/*
 * Used to ensure the correct sequence of task state transitions, such as
 * switching between sched_classes, changing CPU affinity, priority, or
 * updating the queued/running state.
 */

> +struct sched_change_ctx {
>  	struct task_struct	*p;
> -	int			queue_flags;
> +	int			flags;
>  	bool			queued;
>  	bool			running;
>  };
>  
> -void sched_deq_and_put_task(struct task_struct *p, int queue_flags,
> -			    struct sched_enq_and_set_ctx *ctx);
> -void sched_enq_and_set_task(struct sched_enq_and_set_ctx *ctx);
> +struct sched_change_ctx *sched_change_begin(struct task_struct *p, unsigned int flags);
> +void sched_change_end(struct sched_change_ctx *ctx);
>  
> -#endif /* CONFIG_SCHED_CLASS_EXT */
> +DEFINE_CLASS(sched_change, struct sched_change_ctx *,
> +	     sched_change_end(_T),
> +	     sched_change_begin(p, flags),
> +	     struct task_struct *p, unsigned int flags)
> +
> +DEFINE_CLASS_IS_UNCONDITIONAL(sched_change)
>  
>  #include "ext.h"
>  
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -64,7 +64,6 @@ static int effective_prio(struct task_st
>  
>  void set_user_nice(struct task_struct *p, long nice)
>  {
> -	bool queued, running;
>  	struct rq *rq;
>  	int old_prio;
>  
> @@ -90,22 +89,12 @@ void set_user_nice(struct task_struct *p
>  		return;
>  	}
>  
> -	queued = task_on_rq_queued(p);
> -	running = task_current_donor(rq, p);
> -	if (queued)
> -		dequeue_task(rq, p, DEQUEUE_SAVE | DEQUEUE_NOCLOCK);
> -	if (running)
> -		put_prev_task(rq, p);
> -
> -	p->static_prio = NICE_TO_PRIO(nice);
> -	set_load_weight(p, true);
> -	old_prio = p->prio;
> -	p->prio = effective_prio(p);
> -
> -	if (queued)
> -		enqueue_task(rq, p, ENQUEUE_RESTORE | ENQUEUE_NOCLOCK);
> -	if (running)
> -		set_next_task(rq, p);
> +	scoped_guard (sched_change, p, DEQUEUE_SAVE | DEQUEUE_NOCLOCK) {
> +		p->static_prio = NICE_TO_PRIO(nice);
> +		set_load_weight(p, true);
> +		old_prio = p->prio;
> +		p->prio = effective_prio(p);
> +	}
>  
>  	/*
>  	 * If the task increased its priority or is running and
> @@ -515,7 +504,7 @@ int __sched_setscheduler(struct task_str
>  			 bool user, bool pi)
>  {
>  	int oldpolicy = -1, policy = attr->sched_policy;
> -	int retval, oldprio, newprio, queued, running;
> +	int retval, oldprio, newprio;
>  	const struct sched_class *prev_class, *next_class;
>  	struct balance_callback *head;
>  	struct rq_flags rf;
> @@ -698,33 +687,25 @@ int __sched_setscheduler(struct task_str
>  	if (prev_class != next_class && p->se.sched_delayed)
>  		dequeue_task(rq, p, DEQUEUE_SLEEP | DEQUEUE_DELAYED | DEQUEUE_NOCLOCK);
>  
> -	queued = task_on_rq_queued(p);
> -	running = task_current_donor(rq, p);
> -	if (queued)
> -		dequeue_task(rq, p, queue_flags);
> -	if (running)
> -		put_prev_task(rq, p);
> -
> -	if (!(attr->sched_flags & SCHED_FLAG_KEEP_PARAMS)) {
> -		__setscheduler_params(p, attr);
> -		p->sched_class = next_class;
> -		p->prio = newprio;
> -	}
> -	__setscheduler_uclamp(p, attr);
> -	check_class_changing(rq, p, prev_class);
> +	scoped_guard (sched_change, p, queue_flags) {
>  
> -	if (queued) {
> -		/*
> -		 * We enqueue to tail when the priority of a task is
> -		 * increased (user space view).
> -		 */
> -		if (oldprio < p->prio)
> -			queue_flags |= ENQUEUE_HEAD;
> +		if (!(attr->sched_flags & SCHED_FLAG_KEEP_PARAMS)) {
> +			__setscheduler_params(p, attr);
> +			p->sched_class = next_class;
> +			p->prio = newprio;
> +		}
> +		__setscheduler_uclamp(p, attr);
> +		check_class_changing(rq, p, prev_class);
>  
> -		enqueue_task(rq, p, queue_flags);
> +		if (scope->queued) {
> +			/*
> +			 * We enqueue to tail when the priority of a task is
> +			 * increased (user space view).
> +			 */
> +			if (oldprio < p->prio)
> +				scope->flags |= ENQUEUE_HEAD;
> +		}
>  	}
> -	if (running)
> -		set_next_task(rq, p);
>  
>  	check_class_changed(rq, p, prev_class, oldprio);
>  
> 
> 

Thanks,
-Andrea