[PATCH 06/32] sched: Add sched_class->switching_to() and expose check_class_changing/changed()

Tejun Heo
2023-03-17T21:38:30Z

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.

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

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>
---
kernel/sched/core.c | 20 +++++++++++++++++---
kernel/sched/sched.h | 7 +++++++
2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5a6f4884384e..a378e8e09061 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2223,6 +2223,17 @@ 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.
@@ -2230,9 +2241,9 @@ inline int task_curr(const struct task_struct *p)
* this means any call to check_class_changed() must be followed by a call to
* balance_callback().
*/
-static inline void check_class_changed(struct rq *rq, struct task_struct *p,
- const struct sched_class *prev_class,
- int oldprio)
+void check_class_changed(struct rq *rq, struct task_struct *p,
+ const struct sched_class *prev_class,
+ int oldprio)
{
if (prev_class != p->sched_class) {
if (prev_class->switched_from)
@@ -7139,6 +7150,7 @@ void rt_mutex_setprio(struct task_struct *p, struct task_struct *pi_task)
}

__setscheduler_prio(p, prio);
+ check_class_changing(rq, p, prev_class);
}

check_class_changed(rq, p, prev_class, oldprio);
@@ -7748,6 +7760,8 @@ static int __sched_setscheduler(struct task_struct *p,
}
__setscheduler_uclamp(p, attr);

+ check_class_changing(rq, p, prev_class);
+
/*
* We enqueue to tail when the priority of a task is
* increased (user space view).
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 7934b597053d..1545779c5db8 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2211,6 +2211,7 @@ 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,
@@ -2447,6 +2448,12 @@ static inline void sub_nr_running(struct rq *rq, unsigned count)
extern void activate_task(struct rq *rq, struct task_struct *p, int flags);
extern void deactivate_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);
+
extern void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags);

#ifdef CONFIG_PREEMPT_RT
--
2.39.2