Commit d8c7bc2e authored by Tejun Heo's avatar Tejun Heo
Browse files

sched: Add sched_class->switching_to() and expose check_class_changing/changed() 



When a task switches to a new sched_class, the prev and new classes are
notified through ->switched_from() and ->switched_to(), respectively, after
the switching is done.

A new BPF extensible sched_class will have callbacks that allow the BPF
scheduler to keep track of relevant task states (like priority and cpumask).
Those callbacks aren't called while a task is on a different sched_class.
When a task comes back, we wanna tell the BPF progs the up-to-date state
before the task gets enqueued, so we need a hook which is called before the
switching is committed.

This patch adds ->switching_to() which is called during sched_class switch
through check_class_changing() before the task is restored. Also, this patch
exposes check_class_changing/changed() in kernel/sched/sched.h. They will be
used by the new BPF extensible sched_class to implement implicit sched_class
switching which is used e.g. when falling back to CFS when the BPF scheduler
fails or unloads.

This is a prep patch and doesn't cause any behavior changes. The new
operation and exposed functions aren't used yet.

v3: Refreshed on top of tip:sched/core.

v2: Improve patch description w/ details on planned use.

Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Reviewed-by: default avatarDavid Vernet <dvernet@meta.com>
Acked-by: default avatarJosh Don <joshdon@google.com>
Acked-by: default avatarHao Luo <haoluo@google.com>
Acked-by: default avatarBarret Rhoden <brho@google.com>
parent e83edbf8

kernel/sched/core.c

+12 −0
Original line number Diff line number Diff line
@@ -2035,6 +2035,17 @@ inline int task_curr(const struct task_struct *p) 
	return cpu_curr(task_cpu(p)) == p;

}



/*

 * ->switching_to() is called with the pi_lock and rq_lock held and must not

 * mess with locking.

 */

void check_class_changing(struct rq *rq, struct task_struct *p,

			  const struct sched_class *prev_class)

{

	if (prev_class != p->sched_class && p->sched_class->switching_to)

		p->sched_class->switching_to(rq, p);

}



/*

 * switched_from, switched_to and prio_changed must _NOT_ drop rq->lock,

 * use the balance_callback list if you want balancing.
@@ -7021,6 +7032,7 @@ void rt_mutex_setprio(struct task_struct *p, struct task_struct *pi_task) 
	}



	__setscheduler_prio(p, prio);

	check_class_changing(rq, p, prev_class);



	if (queued)

		enqueue_task(rq, p, queue_flag);

kernel/sched/sched.h

+3 −0
Original line number Diff line number Diff line
@@ -2322,6 +2322,7 @@ struct sched_class { 
	 * cannot assume the switched_from/switched_to pair is serialized by

	 * rq->lock. They are however serialized by p->pi_lock.

	 */

	void (*switching_to) (struct rq *this_rq, struct task_struct *task);

	void (*switched_from)(struct rq *this_rq, struct task_struct *task);

	void (*switched_to)  (struct rq *this_rq, struct task_struct *task);

	void (*reweight_task)(struct rq *this_rq, struct task_struct *task,
@@ -3608,6 +3609,8 @@ extern void set_load_weight(struct task_struct *p, bool update_load); 
extern void enqueue_task(struct rq *rq, struct task_struct *p, int flags);

extern void dequeue_task(struct rq *rq, struct task_struct *p, int flags);



extern void check_class_changing(struct rq *rq, struct task_struct *p,

				 const struct sched_class *prev_class);

extern void check_class_changed(struct rq *rq, struct task_struct *p,

				const struct sched_class *prev_class,

				int oldprio);

kernel/sched/syscalls.c

+1 −0
Original line number Diff line number Diff line
@@ -797,6 +797,7 @@ int __sched_setscheduler(struct task_struct *p, 
		__setscheduler_prio(p, newprio);

	}

	__setscheduler_uclamp(p, attr);

	check_class_changing(rq, p, prev_class);



	if (queued) {

		/*