[PATCH 06/12] sched_ext: Fix ops.running/stopping() pairing for proxy-exec donors

[Andrea Righi]

With proxy-exec, pick_next_task() can return a task with blocked_on set
(a proxy donor); put_prev_set_next_task() then calls set_next_task_scx()
on this "ghost" task, which fires ops.running(). However, the task never
actually runs.

If we simply short-circuit set_next_task_scx() for blocked tasks, we
break DSQ bookkeeping. If we only skip ops.running(), we create an
ops.enqueue() -> ops.stopping() pair without running, because
ops.stopping() is still called in put_prev_task_scx().

Fix this by introducing a new flag SCX_TASK_IS_RUNNING to track whether
ops.running() was actually called. Skip ops.running() for blocked tasks,
and only call ops.stopping() if SCX_TASK_IS_RUNNING is set. This ensures
that running and stopping callbacks are perfectly paired even when a
blocked task is picked as a proxy donor.

Signed-off-by: Andrea Righi <arighi@xxxxxxxxxx>
---
 include/linux/sched/ext.h |  2 ++
 kernel/sched/core.c       |  2 +-
 kernel/sched/ext/ext.c    | 14 +++++++++++---
 kernel/sched/ext/ext.h    |  6 ++++++
 4 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index 75cb8b119fb79..e599bb86f8acd 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -102,6 +102,8 @@ enum scx_ent_flags {
 	SCX_TASK_SUB_INIT	= 1 << 4, /* task being initialized for a sub sched */
 	SCX_TASK_IMMED		= 1 << 5, /* task is on local DSQ with %SCX_ENQ_IMMED */
 
+	SCX_TASK_IS_RUNNING	= 1 << 6, /* ops.running() has been called */
+
 	/*
 	 * Bits 8 to 10 are used to carry task state:
 	 *
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2e4c98b4ea6b0..6aedb26c08ee7 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7189,7 +7189,7 @@ static void __sched notrace __schedule(int sched_mode)
 			 * sched_ext tracks curr/donor itself; re-entering set_next_task_scx
 			 * here dispatches through a stale/NULL BPF ops vtable.
 			 */
-			if (donor->sched_class != &ext_sched_class) {
+			if (!is_ext_class(donor)) {
 				donor->sched_class->put_prev_task(rq, donor, donor);
 				donor->sched_class->set_next_task(rq, donor, true);
 			}
diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 189ba9c42043a..b0ec579e3a3ef 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -1985,9 +1985,11 @@ static bool dequeue_task_scx(struct rq *rq, struct task_struct *p, int core_deq_
 	 * information meaningful to the BPF scheduler and can be suppressed by
 	 * skipping the callbacks if the task is !QUEUED.
 	 */
-	if (SCX_HAS_OP(sch, stopping) && task_current(rq, p)) {
+	if (SCX_HAS_OP(sch, stopping) && task_current(rq, p) &&
+	    (p->scx.flags & SCX_TASK_IS_RUNNING)) {
 		update_curr_scx(rq);
 		SCX_CALL_OP_TASK(sch, stopping, rq, p, false);
+		p->scx.flags &= ~SCX_TASK_IS_RUNNING;
 	}
 
 	if (SCX_HAS_OP(sch, quiescent) && !task_on_rq_migrating(p))
@@ -2725,8 +2727,11 @@ static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
 	p->se.exec_start = rq_clock_task(rq);
 
 	/* see dequeue_task_scx() on why we skip when !QUEUED */
-	if (SCX_HAS_OP(sch, running) && (p->scx.flags & SCX_TASK_QUEUED))
+	if (SCX_HAS_OP(sch, running) && (p->scx.flags & SCX_TASK_QUEUED) &&
+	    !task_is_blocked(p)) {
 		SCX_CALL_OP_TASK(sch, running, rq, p);
+		p->scx.flags |= SCX_TASK_IS_RUNNING;
+	}
 
 	clr_task_runnable(p, true);
 
@@ -2815,8 +2820,11 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p,
 	update_curr_scx(rq);
 
 	/* see dequeue_task_scx() on why we skip when !QUEUED */
-	if (SCX_HAS_OP(sch, stopping) && (p->scx.flags & SCX_TASK_QUEUED))
+	if (SCX_HAS_OP(sch, stopping) && (p->scx.flags & SCX_TASK_QUEUED) &&
+	    (p->scx.flags & SCX_TASK_IS_RUNNING)) {
 		SCX_CALL_OP_TASK(sch, stopping, rq, p, true);
+		p->scx.flags &= ~SCX_TASK_IS_RUNNING;
+	}
 
 	if (p->scx.flags & SCX_TASK_QUEUED) {
 		set_task_runnable(rq, p);
diff --git a/kernel/sched/ext/ext.h b/kernel/sched/ext/ext.h
index 0b7fc46aee08c..c7fa4d06ac7d3 100644
--- a/kernel/sched/ext/ext.h
+++ b/kernel/sched/ext/ext.h
@@ -35,6 +35,11 @@ static inline bool task_on_scx(const struct task_struct *p)
 	return scx_enabled() && p->sched_class == &ext_sched_class;
 }
 
+static inline bool is_ext_class(const struct task_struct *p)
+{
+	return p->sched_class == &ext_sched_class;
+}
+
 #ifdef CONFIG_SCHED_CORE
 bool scx_prio_less(const struct task_struct *a, const struct task_struct *b,
 		   bool in_fi);
@@ -53,6 +58,7 @@ static inline void scx_rq_activate(struct rq *rq) {}
 static inline void scx_rq_deactivate(struct rq *rq) {}
 static inline int scx_check_setscheduler(struct task_struct *p, int policy) { return 0; }
 static inline bool task_on_scx(const struct task_struct *p) { return false; }
+static inline bool is_ext_class(const struct task_struct *p) { return false; }
 static inline bool scx_allow_ttwu_queue(const struct task_struct *p) { return true; }
 static inline void init_sched_ext_class(void) {}
 
-- 
2.55.0