sched_ext dispatches tasks from the BPF scheduler from balance_scx() and
thus every pick_task_scx() call must be preceded by balance_scx(). While
this usually holds, due to a bug, there are cases where the fair class's
balance() returns true indicating that it has tasks to run on the CPU and
thus terminating balance() calls but fails to actually find the next task to
run when pick_task() is called. In such cases, pick_task_scx() can be called
without preceding balance_scx().

Detect this condition using SCX_RQ_BAL_PENDING flags. If detected, keep
running the previous task if possible and avoid stalling from entering idle
without balancing.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/Ztj_h5c2LYsdXYbA@slm.duckdns.org
---
Applied to sched_ext/for-6.12-fixes.

 kernel/sched/core.c  | 13 ++++++++-----
 kernel/sched/ext.c   | 44 +++++++++++++++++++++++++++++++-------------
 kernel/sched/sched.h |  5 +++--
 3 files changed, 42 insertions(+), 20 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index aeb595514461..a910a5b4c274 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5914,12 +5914,15 @@ static void prev_balance(struct rq *rq, struct task_struct *prev,
 
 #ifdef CONFIG_SCHED_CLASS_EXT
 	/*
-	 * SCX requires a balance() call before every pick_next_task() including
-	 * when waking up from SCHED_IDLE. If @start_class is below SCX, start
-	 * from SCX instead.
+	 * SCX requires a balance() call before every pick_task() including when
+	 * waking up from SCHED_IDLE. If @start_class is below SCX, start from
+	 * SCX instead. Also, set a flag to detect missing balance() call.
 	 */
-	if (scx_enabled() && sched_class_above(&ext_sched_class, start_class))
-		start_class = &ext_sched_class;
+	if (scx_enabled()) {
+		rq->scx.flags |= SCX_RQ_BAL_PENDING;
+		if (sched_class_above(&ext_sched_class, start_class))
+			start_class = &ext_sched_class;
+	}
 #endif
 
 	/*
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 3bdb08fc2056..19f9cb3a4190 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -2634,7 +2634,7 @@ static int balance_one(struct rq *rq, struct task_struct *prev)
 
 	lockdep_assert_rq_held(rq);
 	rq->scx.flags |= SCX_RQ_IN_BALANCE;
-	rq->scx.flags &= ~SCX_RQ_BAL_KEEP;
+	rq->scx.flags &= ~(SCX_RQ_BAL_PENDING | SCX_RQ_BAL_KEEP);
 
 	if (static_branch_unlikely(&scx_ops_cpu_preempt) &&
 	    unlikely(rq->scx.cpu_released)) {
@@ -2948,12 +2948,11 @@ static struct task_struct *pick_task_scx(struct rq *rq)
 {
 	struct task_struct *prev = rq->curr;
 	struct task_struct *p;
+	bool prev_on_scx = prev->sched_class == &ext_sched_class;
+	bool keep_prev = rq->scx.flags & SCX_RQ_BAL_KEEP;
+	bool kick_idle = false;
 
 	/*
-	 * If balance_scx() is telling us to keep running @prev, replenish slice
-	 * if necessary and keep running @prev. Otherwise, pop the first one
-	 * from the local DSQ.
-	 *
 	 * WORKAROUND:
 	 *
 	 * %SCX_RQ_BAL_KEEP should be set iff $prev is on SCX as it must just
@@ -2962,22 +2961,41 @@ static struct task_struct *pick_task_scx(struct rq *rq)
 	 * which then ends up calling pick_task_scx() without preceding
 	 * balance_scx().
 	 *
-	 * For now, ignore cases where $prev is not on SCX. This isn't great and
-	 * can theoretically lead to stalls. However, for switch_all cases, this
-	 * happens only while a BPF scheduler is being loaded or unloaded, and,
-	 * for partial cases, fair will likely keep triggering this CPU.
+	 * Keep running @prev if possible and avoid stalling from entering idle
+	 * without balancing.
 	 *
-	 * Once fair is fixed, restore WARN_ON_ONCE().
+	 * Once fair is fixed, remove the workaround and trigger WARN_ON_ONCE()
+	 * if pick_task_scx() is called without preceding balance_scx().
 	 */
-	if ((rq->scx.flags & SCX_RQ_BAL_KEEP) &&
-	    prev->sched_class == &ext_sched_class) {
+	if (unlikely(rq->scx.flags & SCX_RQ_BAL_PENDING)) {
+		if (prev_on_scx) {
+			keep_prev = true;
+		} else {
+			keep_prev = false;
+			kick_idle = true;
+		}
+	} else if (unlikely(keep_prev && !prev_on_scx)) {
+		/* only allowed during transitions */
+		WARN_ON_ONCE(scx_ops_enable_state() == SCX_OPS_ENABLED);
+		keep_prev = false;
+	}
+
+	/*
+	 * If balance_scx() is telling us to keep running @prev, replenish slice
+	 * if necessary and keep running @prev. Otherwise, pop the first one
+	 * from the local DSQ.
+	 */
+	if (keep_prev) {
 		p = prev;
 		if (!p->scx.slice)
 			p->scx.slice = SCX_SLICE_DFL;
 	} else {
 		p = first_local_task(rq);
-		if (!p)
+		if (!p) {
+			if (kick_idle)
+				scx_bpf_kick_cpu(cpu_of(rq), SCX_KICK_IDLE);
 			return NULL;
+		}
 
 		if (unlikely(!p->scx.slice)) {
 			if (!scx_rq_bypassing(rq) && !scx_warned_zero_slice) {
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 6085ef50febf..4d79804631e4 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -751,8 +751,9 @@ enum scx_rq_flags {
 	 */
 	SCX_RQ_ONLINE		= 1 << 0,
 	SCX_RQ_CAN_STOP_TICK	= 1 << 1,
-	SCX_RQ_BAL_KEEP		= 1 << 2, /* balance decided to keep current */
-	SCX_RQ_BYPASSING	= 1 << 3,
+	SCX_RQ_BAL_PENDING	= 1 << 2, /* balance hasn't run yet */
+	SCX_RQ_BAL_KEEP		= 1 << 3, /* balance decided to keep current */
+	SCX_RQ_BYPASSING	= 1 << 4,
 
 	SCX_RQ_IN_WAKEUP	= 1 << 16,
 	SCX_RQ_IN_BALANCE	= 1 << 17,
-- 
2.47.0