[PATCH 03/10] sched_ext: Split curr|donor references properly

[Andrea Righi <arighi@nvidia.com>]

With proxy execution, the task selected by the scheduler and the task
physically executing can differ. A blocked mutex waiter donates its
scheduling context to the lock owner:

  D -----------------> M -------------> O ----------------> T
  [donor] blocked on [mutex] owned by [owner] preempted by [task]
     \_________________________________^
          donates scheduling context

where:

  D = blocked donor
  M = mutex
  O = mutex owner
  T = competing runnable task

During a proxy execution switch, D supplies the scheduling class,
priority, and runtime budget, while O supplies the execution context: O
is the task whose code physically executes. T is a competing runnable
task which may preempt the D/O proxy execution.

Consider FAIR and EXT tasks with sched_ext running in partial mode. FAIR
can be replaced with a higher scheduling class such as RT or deadline
without changing the class interaction described here. The possible
combinations are:

  1. D is EXT, O is EXT, T is EXT

     D can interrupt T according to BPF scheduling policy. O executes
     with D's EXT priority and runtime budget, while T waits in EXT.

  2. D is EXT, O is EXT, T is FAIR

     D is visible to the BPF scheduler, but cannot preempt T because
     EXT is below FAIR. Once T stops, BPF can dispatch D and O executes
     with D's EXT priority and runtime budget. If T becomes runnable
     again, it preempts the D/O proxy execution.

  3. D is EXT, O is FAIR, T is EXT

     This cannot represent T preempting O because EXT is below FAIR.

  4. D is EXT, O is FAIR, T is FAIR

     D cannot boost O above T because EXT is below FAIR. O and T
     continue competing under FAIR. Once O releases M, D wakes and
     resumes normal EXT scheduling.

  5. D is FAIR, O is EXT, T is EXT

     D preempts T as the higher-class scheduling context. O executes
     with D's FAIR priority and runtime budget, while T waits in EXT.
     D is not visible to the BPF scheduler.

  6. D is FAIR, O is EXT, T is FAIR

     D competes with T according to its FAIR deadline. When D is
     selected, O executes with D's FAIR priority and runtime budget.
     D is not visible to the BPF scheduler.

  7. D is FAIR, O is FAIR, T is EXT

     This cannot represent T preempting O because EXT is below FAIR.

  8. D is FAIR, O is FAIR, T is FAIR

     O, T, and D all have FAIR scheduling contexts. D remains runnable
     as a blocked proxy donor. When CFS selects D, O executes using D's
     FAIR scheduling context. When CFS selects O, O executes using its
     own FAIR context, and when CFS selects T, T executes normally. D
     is not visible to the BPF scheduler.

Thus, sched_ext policy and accounting must generally use rq->donor, the
scheduler-selected task which supplies the scheduling context, rather
than rq->curr, the task whose code physically executes. Without proxy
execution they are the same task.

On nohz_full CPUs, a blocked proxy donor must retain the scheduler tick
even when it has an infinite slice. Otherwise, a full dynticks CPU could
stop the tick while rq->curr and rq->donor differ, violating assumptions
made by the remote NOHZ tick path.

This is a conservative compromise that keeps the change local to
sched_ext, at the cost of a periodic tick while a blocked proxy donor is
selected. Allowing blocked proxy donors to run tickless would require
making the core scheduler's remote tick handling aware that rq->curr and
rq->donor can differ.

Moreover, extend scx_dump_state() to report both contexts. Each CPU
record now includes a donor= line. If an EXT donor differs from
rq->curr, also emit its detailed task record. The existing '*' marker
continues to identify rq->curr, while the donor= line identifies the
otherwise unmarked donor record.

Note that at this point in the series, CONFIG_SCHED_PROXY_EXEC still
depends on !CONFIG_SCHED_CLASS_EXT, so proxy execution and sched_ext
cannot be enabled together. The scheduling changes are therefore
preparatory. A later patch removes this restriction.

Co-developed-by: John Stultz <jstultz@google.com>
Signed-off-by: John Stultz <jstultz@google.com>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
---
 kernel/sched/ext/ext.c | 53 ++++++++++++++++++++++++++++--------------
 1 file changed, 36 insertions(+), 17 deletions(-)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 6b7efb19d2843..1946824f3b8a9 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -1149,17 +1149,24 @@ static void touch_core_sched_dispatch(struct rq *rq, struct task_struct *p)
 
 static void update_curr_scx(struct rq *rq)
 {
-	struct task_struct *curr = rq->curr;
+	struct task_struct *donor;
 	s64 delta_exec;
 
+	/*
+	 * update_curr_scx() is selected through rq->donor->sched_class, not
+	 * rq->curr->sched_class, so @donor is always an EXT task here. If an EXT
+	 * owner executes for a FAIR donor, FAIR's update_curr() runs instead.
+	 */
+	donor = rq->donor;
+
 	delta_exec = update_curr_common(rq);
 	if (unlikely(delta_exec <= 0))
 		return;
 
-	if (curr->scx.slice != SCX_SLICE_INF) {
-		curr->scx.slice -= min_t(u64, curr->scx.slice, delta_exec);
-		if (!curr->scx.slice)
-			touch_core_sched(rq, curr);
+	if (donor->scx.slice != SCX_SLICE_INF) {
+		donor->scx.slice -= min_t(u64, donor->scx.slice, delta_exec);
+		if (!donor->scx.slice)
+			touch_core_sched(rq, donor);
 	}
 
 	dl_server_update(&rq->ext_server, delta_exec);
@@ -1320,9 +1327,9 @@ static void local_dsq_post_enq(struct scx_sched *sch, struct scx_dispatch_q *dsq
 	if (rq->scx.flags & SCX_RQ_IN_BALANCE)
 		return;
 
-	if ((enq_flags & SCX_ENQ_PREEMPT) && p != rq->curr &&
-	    rq->curr->sched_class == &ext_sched_class) {
-		rq->curr->scx.slice = 0;
+	if ((enq_flags & SCX_ENQ_PREEMPT) && p != rq->donor &&
+	    rq->donor->sched_class == &ext_sched_class) {
+		rq->donor->scx.slice = 0;
 		resched_curr(rq);
 	}
 }
@@ -2470,7 +2477,8 @@ static void dispatch_to_local_dsq(struct scx_sched *sch, struct rq *rq,
 		}
 
 		/* if the destination CPU is idle, wake it up */
-		if (!fallback && sched_class_above(p->sched_class, dst_rq->curr->sched_class))
+		if (!fallback && sched_class_above(p->sched_class,
+						      dst_rq->donor->sched_class))
 			resched_curr(dst_rq);
 	}
 
@@ -2678,6 +2686,7 @@ static int balance_one(struct rq *rq, struct task_struct *prev)
 static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
 {
 	struct scx_sched *sch = scx_task_sched(p);
+	bool can_stop_tick;
 
 	if (p->scx.flags & SCX_TASK_QUEUED) {
 		/*
@@ -2703,6 +2712,7 @@ static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
 	}
 
 	clr_task_runnable(p, true);
+	can_stop_tick = p->scx.slice == SCX_SLICE_INF && !task_is_blocked(p);
 
 	/*
 	 * @p is getting newly scheduled or got kicked after someone updated its
@@ -2713,7 +2723,7 @@ static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
 	 * nohz. In the future, we might want to add a mechanism to update
 	 * load_avgs periodically on tick-stopped CPUs.
 	 */
-	if (p->scx.slice == SCX_SLICE_INF) {
+	if (can_stop_tick) {
 		if (!(rq->scx.flags & SCX_RQ_CAN_STOP_TICK)) {
 			/*
 			 * Bypass mode always assigns finite slices, so @p
@@ -2734,7 +2744,8 @@ static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
 
 		/*
 		 * @rq still references the outgoing scheduling context. A finite
-		 * slice is sufficient by itself to require the tick.
+		 * slice or a blocked proxy donor is sufficient by itself to require
+		 * the tick.
 		 */
 		if (tick_nohz_full_cpu(cpu_of(rq)))
 			tick_nohz_dep_set_cpu(cpu_of(rq), TICK_DEP_BIT_SCHED);
@@ -2907,7 +2918,7 @@ static struct task_struct *first_local_task(struct rq *rq)
 static struct task_struct *
 do_pick_task_scx(struct rq *rq, struct rq_flags *rf, bool force_scx)
 {
-	struct task_struct *prev = rq->curr;
+	struct task_struct *prev = rq->donor;
 	bool keep_prev;
 	struct task_struct *p;
 
@@ -4060,14 +4071,14 @@ static void run_deferred(struct rq *rq)
 #ifdef CONFIG_NO_HZ_FULL
 bool scx_can_stop_tick(struct rq *rq)
 {
-	struct task_struct *p = rq->curr;
+	struct task_struct *p = rq->donor;
 	struct scx_sched *sch = scx_task_sched(p);
 
 	if (p->sched_class != &ext_sched_class)
 		return true;
 
 	/*
-	 * @rq->curr may still reference an outgoing EXT task after it has been
+	 * @rq->donor may still reference an outgoing EXT task after it has been
 	 * dequeued. If no EXT tasks are accounted on @rq, ignore its stale
 	 * slice state. If another task is dispatched from a DSQ,
 	 * set_next_task_scx() will update the dependency for the incoming task.
@@ -4081,7 +4092,8 @@ bool scx_can_stop_tick(struct rq *rq)
 	/*
 	 * @rq can dispatch from different DSQs, so we can't tell whether it
 	 * needs the tick or not by looking at nr_running. Allow stopping ticks
-	 * iff the BPF scheduler indicated so. See set_next_task_scx().
+	 * iff set_next_task_scx() determined that the selected scheduling context
+	 * can run tickless.
 	 */
 	return rq->scx.flags & SCX_RQ_CAN_STOP_TICK;
 }
@@ -6047,6 +6059,9 @@ static void scx_dump_cpu(struct scx_sched *sch, struct seq_buf *s,
 	dump_line(&ns, "          curr=%s[%d] class=%ps",
 		  rq->curr->comm, rq->curr->pid,
 		  rq->curr->sched_class);
+	dump_line(&ns, "          donor=%s[%d] class=%ps",
+		  rq->donor->comm, rq->donor->pid,
+		  rq->donor->sched_class);
 	if (!cpumask_empty(rq->scx.cpus_to_kick))
 		dump_line(&ns, "  cpus_to_kick   : %*pb",
 			  cpumask_pr_args(rq->scx.cpus_to_kick));
@@ -6090,6 +6105,10 @@ static void scx_dump_cpu(struct scx_sched *sch, struct seq_buf *s,
 	if (rq->curr->sched_class == &ext_sched_class &&
 	    (dump_all_tasks || scx_task_on_sched(sch, rq->curr)))
 		scx_dump_task(sch, s, dctx, rq, rq->curr, '*');
+	if (rq->donor != rq->curr &&
+	    rq->donor->sched_class == &ext_sched_class &&
+	    (dump_all_tasks || scx_task_on_sched(sch, rq->donor)))
+		scx_dump_task(sch, s, dctx, rq, rq->donor, ' ');
 
 	list_for_each_entry(p, &rq->scx.runnable_list, scx.runnable_node)
 		if (dump_all_tasks || scx_task_on_sched(sch, p))
@@ -7518,7 +7537,7 @@ static bool kick_one_cpu(s32 cpu, struct rq *this_rq, unsigned long *ksyncs)
 	unsigned long flags;
 
 	raw_spin_rq_lock_irqsave(rq, flags);
-	cur_class = rq->curr->sched_class;
+	cur_class = rq->donor->sched_class;
 
 	/*
 	 * During CPU hotplug, a CPU may depend on kicking itself to make
@@ -7530,7 +7549,7 @@ static bool kick_one_cpu(s32 cpu, struct rq *this_rq, unsigned long *ksyncs)
 	    !sched_class_above(cur_class, &ext_sched_class)) {
 		if (cpumask_test_cpu(cpu, this_scx->cpus_to_preempt)) {
 			if (cur_class == &ext_sched_class)
-				rq->curr->scx.slice = 0;
+				rq->donor->scx.slice = 0;
 			cpumask_clear_cpu(cpu, this_scx->cpus_to_preempt);
 		}
 
-- 
2.55.0