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: 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>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 48f9d00..b088fbe 100644 (file)
--- 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 a2399cc..0ed4271 100644 (file)
--- 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 ae1b427..cf189bc 100644 (file)
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -797,6 +797,7 @@ change:
                __setscheduler_prio(p, newprio);
        }
        __setscheduler_uclamp(p, attr);
+       check_class_changing(rq, p, prev_class);
 
        if (queued) {
                /*