[PATCH 04/14] sched: Add prepare_switch() class callback

sched_change_begin() only invokes callbacks on the outgoing class before
recording the task queueing state. An incoming class may need to prepare
a task while that state is still live, before the normal dequeue and
class transition take place.

Pass the incoming class to sched_change_begin() and invoke its optional
prepare_switch() callback before recording the queued and running state.

This is a preparatory change to support proxy execution with sched_ext.

Suggested-by: Tejun Heo <[email protected]>
Signed-off-by: Andrea Righi <[email protected]>
---
 kernel/sched/core.c     | 16 +++++++++++-----
 kernel/sched/ext/ext.c  |  7 ++++---
 kernel/sched/ext/sub.c  |  9 ++++++---
 kernel/sched/sched.h    | 10 +++++++---
 kernel/sched/syscalls.c |  4 ++--
 5 files changed, 30 insertions(+), 16 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 9a9d4b5040089..474628f520a29 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2792,7 +2792,7 @@ void set_cpus_allowed_common(struct task_struct *p, struct affinity_context *ctx
 static void
 do_set_cpus_allowed(struct task_struct *p, struct affinity_context *ctx)
 {
-	scoped_guard (sched_change, p, DEQUEUE_SAVE)
+	scoped_guard (sched_change, p, p->sched_class, DEQUEUE_SAVE)
 		p->sched_class->set_cpus_allowed(p, ctx);
 }
 
@@ -7722,7 +7722,7 @@ void rt_mutex_setprio(struct task_struct *p, struct task_struct *pi_task)
 	if (prev_class != next_class)
 		queue_flag |= DEQUEUE_CLASS;
 
-	scoped_guard (sched_change, p, queue_flag) {
+	scoped_guard (sched_change, p, next_class, queue_flag) {
 		/*
 		 * Boosting condition are:
 		 * 1. -rt task is running and holds mutex A
@@ -8399,7 +8399,7 @@ int migrate_task_to(struct task_struct *p, int target_cpu)
 void sched_setnuma(struct task_struct *p, int nid)
 {
 	guard(task_rq_lock)(p);
-	scoped_guard (sched_change, p, DEQUEUE_SAVE)
+	scoped_guard (sched_change, p, p->sched_class, DEQUEUE_SAVE)
 		p->numa_preferred_nid = nid;
 }
 #endif /* CONFIG_NUMA_BALANCING */
@@ -9516,7 +9516,7 @@ void sched_move_task(struct task_struct *tsk, bool for_autogroup)
 	CLASS(task_rq_lock, rq_guard)(tsk);
 	rq = rq_guard.rq;
 
-	scoped_guard (sched_change, tsk, queue_flags) {
+	scoped_guard (sched_change, tsk, tsk->sched_class, queue_flags) {
 		sched_change_group(tsk);
 		if (!for_autogroup)
 			scx_cgroup_move_task(tsk);
@@ -11220,7 +11220,9 @@ static inline void sched_mm_cid_fork(struct task_struct *t) { }
 
 static DEFINE_PER_CPU(struct sched_change_ctx, sched_change_ctx);
 
-struct sched_change_ctx *sched_change_begin(struct task_struct *p, unsigned int flags)
+struct sched_change_ctx *
+sched_change_begin(struct task_struct *p, const struct sched_class *next_class,
+		   unsigned int flags)
 {
 	struct sched_change_ctx *ctx = this_cpu_ptr(&sched_change_ctx);
 	struct rq *rq = task_rq(p);
@@ -11238,6 +11240,10 @@ struct sched_change_ctx *sched_change_begin(struct task_struct *p, unsigned int
 		flags |= DEQUEUE_NOCLOCK;
 	}
 
+	if ((flags & DEQUEUE_CLASS) && next_class != p->sched_class &&
+	    next_class->prepare_switch)
+		next_class->prepare_switch(rq, p);
+
 	if ((flags & DEQUEUE_CLASS) && p->sched_class->switching_from)
 		p->sched_class->switching_from(rq, p);
 
diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 81c2d8eeae410..285bd8422193d 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -5859,7 +5859,8 @@ void scx_bypass(struct scx_sched *sch, bool bypass)
 				continue;
 
 			/* cycling deq/enq is enough, see the function comment */
-			scoped_guard (sched_change, p, DEQUEUE_SAVE | DEQUEUE_MOVE) {
+			scoped_guard (sched_change, p, p->sched_class,
+				      DEQUEUE_SAVE | DEQUEUE_MOVE) {
 				/* nothing */ ;
 			}
 		}
@@ -6141,7 +6142,7 @@ static void scx_root_disable(struct scx_sched *sch)
 		if (old_class != new_class)
 			queue_flags |= DEQUEUE_CLASS;
 
-		scoped_guard (sched_change, p, queue_flags) {
+		scoped_guard (sched_change, p, new_class, queue_flags) {
 			p->sched_class = new_class;
 		}
 
@@ -7477,7 +7478,7 @@ static void scx_root_enable_workfn(struct kthread_work *work)
 		if (old_class != new_class)
 			queue_flags |= DEQUEUE_CLASS;
 
-		scoped_guard (sched_change, p, queue_flags) {
+		scoped_guard (sched_change, p, new_class, queue_flags) {
 			set_task_slice(p, READ_ONCE(sch->slice_dfl));
 			p->sched_class = new_class;
 		}
diff --git a/kernel/sched/ext/sub.c b/kernel/sched/ext/sub.c
index 6da6c91e42870..765b516e06c95 100644
--- a/kernel/sched/ext/sub.c
+++ b/kernel/sched/ext/sub.c
@@ -791,7 +791,8 @@ static void scx_rehome_task(struct scx_sched *to, struct task_struct *p)
 	lockdep_assert_held(&p->pi_lock);
 	lockdep_assert_rq_held(task_rq(p));
 
-	scoped_guard (sched_change, p, DEQUEUE_SAVE | DEQUEUE_MOVE) {
+	scoped_guard (sched_change, p, p->sched_class,
+		      DEQUEUE_SAVE | DEQUEUE_MOVE) {
 		scx_disable_and_exit_task(scx_task_sched(p), p);
 		scx_set_task_state(p, SCX_TASK_INIT_BEGIN);
 		scx_set_task_state(p, SCX_TASK_INIT);
@@ -821,7 +822,8 @@ static void scx_punt_task(struct scx_sched *to, struct task_struct *p)
 	lockdep_assert_rq_held(task_rq(p));
 	WARN_ON_ONCE(!READ_ONCE(to->bypass_depth));
 
-	scoped_guard (sched_change, p, DEQUEUE_SAVE | DEQUEUE_MOVE) {
+	scoped_guard (sched_change, p, p->sched_class,
+		      DEQUEUE_SAVE | DEQUEUE_MOVE) {
 		scx_disable_and_exit_task(scx_task_sched(p), p);
 		scx_set_task_sched(p, to);
 	}
@@ -1471,7 +1473,8 @@ void scx_sub_enable_workfn(struct kthread_work *work)
 		if (!(p->scx.flags & SCX_TASK_SUB_INIT))
 			continue;
 
-		scoped_guard (sched_change, p, DEQUEUE_SAVE | DEQUEUE_MOVE) {
+		scoped_guard (sched_change, p, p->sched_class,
+			      DEQUEUE_SAVE | DEQUEUE_MOVE) {
 			/*
 			 * $p must be either READY or ENABLED. If ENABLED,
 			 * __scx_disabled_and_exit_task() first disables and
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 437c4f68c9e53..ca436a8174d30 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2705,6 +2705,7 @@ struct sched_class {
 	/*
 	 * sched_change
 	 */
+	void (*prepare_switch)(struct rq *this_rq, struct task_struct *task);
 	void (*switching_from)(struct rq *this_rq, struct task_struct *task);
 	void (*switched_from) (struct rq *this_rq, struct task_struct *task);
 	void (*switching_to)  (struct rq *this_rq, struct task_struct *task);
@@ -4214,13 +4215,16 @@ struct sched_change_ctx {
 	bool			running;
 };
 
-struct sched_change_ctx *sched_change_begin(struct task_struct *p, unsigned int flags);
+struct sched_change_ctx *
+sched_change_begin(struct task_struct *p, const struct sched_class *next_class,
+		   unsigned int flags);
 void sched_change_end(struct sched_change_ctx *ctx);
 
 DEFINE_CLASS(sched_change, struct sched_change_ctx *,
 	     sched_change_end(_T),
-	     sched_change_begin(p, flags),
-	     struct task_struct *p, unsigned int flags)
+	     sched_change_begin(p, next_class, flags),
+	     struct task_struct *p, const struct sched_class *next_class,
+	     unsigned int flags)
 
 DEFINE_CLASS_IS_UNCONDITIONAL(sched_change)
 
diff --git a/kernel/sched/syscalls.c b/kernel/sched/syscalls.c
index b215b0ead9a60..bc32ce76ff4fe 100644
--- a/kernel/sched/syscalls.c
+++ b/kernel/sched/syscalls.c
@@ -85,7 +85,7 @@ void set_user_nice(struct task_struct *p, long nice)
 		return;
 	}
 
-	scoped_guard (sched_change, p, DEQUEUE_SAVE) {
+	scoped_guard (sched_change, p, p->sched_class, DEQUEUE_SAVE) {
 		p->static_prio = NICE_TO_PRIO(nice);
 		set_load_weight(p, true);
 		old_prio = p->prio;
@@ -678,7 +678,7 @@ int __sched_setscheduler(struct task_struct *p,
 	if (prev_class != next_class)
 		queue_flags |= DEQUEUE_CLASS;
 
-	scoped_guard (sched_change, p, queue_flags) {
+	scoped_guard (sched_change, p, next_class, queue_flags) {
 
 		if (!(attr->sched_flags & SCHED_FLAG_KEEP_PARAMS)) {
 			__setscheduler_params(p, attr);
-- 
2.55.0