[tip: sched/core] sched/ext: Fold balance_scx() into pick_task_scx()

["tip-bot2 for Peter Zijlstra" <tip-bot2@linutronix.de>]

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     4c95380701f58b8112f0b891de8d160e4199e19d
Gitweb:        https://git.kernel.org/tip/4c95380701f58b8112f0b891de8d160e4199e19d
Author:        Peter Zijlstra <peterz@infradead.org>
AuthorDate:    Wed, 01 Oct 2025 20:41:36 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Thu, 16 Oct 2025 11:13:55 +02:00

sched/ext: Fold balance_scx() into pick_task_scx()

With pick_task() having an rf argument, it is possible to do the
lock-break there, get rid of the weird balance/pick_task hack.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Tejun Heo <tj@kernel.org>
---
 kernel/sched/core.c  | 13 +-------
 kernel/sched/ext.c   | 78 ++++++-------------------------------------
 kernel/sched/sched.h |  1 +-
 3 files changed, 12 insertions(+), 80 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a75d456..096e8d0 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5845,19 +5845,6 @@ static void prev_balance(struct rq *rq, struct task_struct *prev,
 	const struct sched_class *start_class = prev->sched_class;
 	const struct sched_class *class;
 
-#ifdef CONFIG_SCHED_CLASS_EXT
-	/*
-	 * 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()) {
-		rq->scx.flags |= SCX_RQ_BAL_PENDING;
-		if (sched_class_above(&ext_sched_class, start_class))
-			start_class = &ext_sched_class;
-	}
-#endif
-
 	/*
 	 * We must do the balancing pass before put_prev_task(), such
 	 * that when we release the rq->lock the task is in the same
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index dc743ca..49f4a9e 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -2013,7 +2013,7 @@ 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_PENDING | SCX_RQ_BAL_KEEP);
+	rq->scx.flags &= ~SCX_RQ_BAL_KEEP;
 
 	if ((sch->ops.flags & SCX_OPS_HAS_CPU_PREEMPT) &&
 	    unlikely(rq->scx.cpu_released)) {
@@ -2119,40 +2119,6 @@ has_tasks:
 	return true;
 }
 
-static int balance_scx(struct rq *rq, struct task_struct *prev,
-		       struct rq_flags *rf)
-{
-	int ret;
-
-	rq_unpin_lock(rq, rf);
-
-	ret = balance_one(rq, prev);
-
-#ifdef CONFIG_SCHED_SMT
-	/*
-	 * When core-sched is enabled, this ops.balance() call will be followed
-	 * by pick_task_scx() on this CPU and the SMT siblings. Balance the
-	 * siblings too.
-	 */
-	if (sched_core_enabled(rq)) {
-		const struct cpumask *smt_mask = cpu_smt_mask(cpu_of(rq));
-		int scpu;
-
-		for_each_cpu_andnot(scpu, smt_mask, cpumask_of(cpu_of(rq))) {
-			struct rq *srq = cpu_rq(scpu);
-			struct task_struct *sprev = srq->curr;
-
-			WARN_ON_ONCE(__rq_lockp(rq) != __rq_lockp(srq));
-			update_rq_clock(srq);
-			balance_one(srq, sprev);
-		}
-	}
-#endif
-	rq_repin_lock(rq, rf);
-
-	return ret;
-}
-
 static void process_ddsp_deferred_locals(struct rq *rq)
 {
 	struct task_struct *p;
@@ -2335,38 +2301,19 @@ static struct task_struct *first_local_task(struct rq *rq)
 static struct task_struct *pick_task_scx(struct rq *rq, struct rq_flags *rf)
 {
 	struct task_struct *prev = rq->curr;
+	bool keep_prev, kick_idle = false;
 	struct task_struct *p;
-	bool keep_prev = rq->scx.flags & SCX_RQ_BAL_KEEP;
-	bool kick_idle = false;
 
-	/*
-	 * WORKAROUND:
-	 *
-	 * %SCX_RQ_BAL_KEEP should be set iff $prev is on SCX as it must just
-	 * have gone through balance_scx(). Unfortunately, there currently is a
-	 * bug where fair could say yes on balance() but no on pick_task(),
-	 * which then ends up calling pick_task_scx() without preceding
-	 * balance_scx().
-	 *
-	 * Keep running @prev if possible and avoid stalling from entering idle
-	 * without balancing.
-	 *
-	 * Once fair is fixed, remove the workaround and trigger WARN_ON_ONCE()
-	 * if pick_task_scx() is called without preceding balance_scx().
-	 */
-	if (unlikely(rq->scx.flags & SCX_RQ_BAL_PENDING)) {
-		if (prev->scx.flags & SCX_TASK_QUEUED) {
-			keep_prev = true;
-		} else {
-			keep_prev = false;
-			kick_idle = true;
-		}
-	} else if (unlikely(keep_prev &&
-			    prev->sched_class != &ext_sched_class)) {
-		/*
-		 * Can happen while enabling as SCX_RQ_BAL_PENDING assertion is
-		 * conditional on scx_enabled() and may have been skipped.
-		 */
+	rq_modified_clear(rq);
+	rq_unpin_lock(rq, rf);
+	balance_one(rq, prev);
+	rq_repin_lock(rq, rf);
+	if (rq_modified_above(rq, &ext_sched_class))
+		return RETRY_TASK;
+
+	keep_prev = rq->scx.flags & SCX_RQ_BAL_KEEP;
+	if (unlikely(keep_prev &&
+		     prev->sched_class != &ext_sched_class)) {
 		WARN_ON_ONCE(scx_enable_state() == SCX_ENABLED);
 		keep_prev = false;
 	}
@@ -3243,7 +3190,6 @@ DEFINE_SCHED_CLASS(ext) = {
 
 	.wakeup_preempt		= wakeup_preempt_scx,
 
-	.balance		= balance_scx,
 	.pick_task		= pick_task_scx,
 
 	.put_prev_task		= put_prev_task_scx,
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 8946294..e7718f1 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -779,7 +779,6 @@ enum scx_rq_flags {
 	 */
 	SCX_RQ_ONLINE		= 1 << 0,
 	SCX_RQ_CAN_STOP_TICK	= 1 << 1,
-	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_CLK_VALID	= 1 << 5, /* RQ clock is fresh and valid */