sched_ext: Handle cases where pick_task_scx() is called without preceding balance_scx()

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.

Tejun Heo committed 2 years ago

Commit a6250aa1 parent a759bf03 files · +42 −20Browse files

Unified · Split

3 files

kernel/sched/core.c +8 −5
kernel/sched/ext.c +31 −13
kernel/sched/sched.h +3 −2

kernel/sched/core.c+8−5

 #ifdef CONFIG_SCHED_CLASS_EXT
 	/*
-	 * SCX requires a balance() call before every pick_next_task() including
+	 * SCX requires a balance() call before every pick_task() including when
-	 * when waking up from SCHED_IDLE. If @start_class is below SCX, start
+	 * waking up from SCHED_IDLE. If @start_class is below SCX, start from
-	 * from SCX instead.
+	 * SCX instead. Also, set a flag to detect missing balance() call.
 	 */
-	if (scx_enabled() && sched_class_above(&ext_sched_class, start_class))
+	if (scx_enabled()) {
-		start_class = &ext_sched_class;
+		rq->scx.flags |= SCX_RQ_BAL_PENDING;
+		if (sched_class_above(&ext_sched_class, start_class))
+			start_class = &ext_sched_class;
+	}
 #endif
 	/*

kernel/sched/ext.c+31−13

 	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)) {
 {
 	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
 	 * 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
+	 * Keep running @prev if possible and avoid stalling from entering idle
-	 * can theoretically lead to stalls. However, for switch_all cases, this
+	 * without balancing.
-	 * happens only while a BPF scheduler is being loaded or unloaded, and,
-	 * for partial cases, fair will likely keep triggering this CPU.
 	 *
-	 * 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) &&
+	if (unlikely(rq->scx.flags & SCX_RQ_BAL_PENDING)) {
-	    prev->sched_class == &ext_sched_class) {
+		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) {

kernel/sched/sched.h+3−2

 	 */
 	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_BAL_PENDING	= 1 << 2, /* balance hasn't run yet */
-	SCX_RQ_BYPASSING	= 1 << 3,
+	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,