[PATCH 04/30] sched: Add sched_class->switching_to() and expose check_class_changing/changed()

Tejun Heo
2024-06-18T21:21:12Z

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: 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 | 12 ++++++++++++
kernel/sched/sched.h | 3 +++
kernel/sched/syscalls.c | 1 +
3 files changed, 16 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 48f9d00d0666..b088fbeaf26d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -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);
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index a2399ccf259a..0ed4271cedf5 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -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);
diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index ae1b42775ef9..cf189bc3dd18 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -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) {
/*
--
2.45.2