[PATCH 4/6] sched/proxy: Switch proxy to use p->is_blocked

[Peter Zijlstra]

Rather than gate the proxy paths with p->blocked_on, use p->is_blocked.

This opens up the state: '->is_blocked && !->blocked_on' for future use.

Notably, only proxy and delayed tasks can be ->on_rq && ->is_blocked, and it is
guaranteed that sched_class::pick_task() will never return a delayed task.
Therefore any task returned from pick_next_task() that has ->is_blocked set,
must be a proxy task.

XXX: ttwu_runnable(): AFAICT this results in all delayed tasks getting blocked
and send down the long wakeup-path -- and while there were some plans there
[*], that was especially careful to not take all those locks.

[*] https://lore.kernel.org/r/20250702114924.091581796@xxxxxxxxxxxxx

Suggested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 kernel/sched/core.c |   12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3764,7 +3764,7 @@ static inline void proxy_reset_donor(str
  */
 static inline bool proxy_needs_return(struct rq *rq, struct task_struct *p)
 {
-	if (!task_is_blocked(p))
+	if (!p->is_blocked)
 		return false;
 
 	scoped_guard(raw_spinlock, &p->blocked_lock) {
@@ -6850,14 +6850,14 @@ find_proxy_task(struct rq *rq, struct ta
 	bool curr_in_chain = false;
 	int this_cpu = cpu_of(rq);
 	struct task_struct *p;
-	struct mutex *mutex;
 	int owner_cpu;
 
 	/* Follow blocked_on chain. */
-	for (p = donor; (mutex = p->blocked_on); p = owner) {
+	for (p = donor; p->is_blocked; p = owner) {
 		/* if its PROXY_WAKING, do return migration or run if current */
-		if (mutex == PROXY_WAKING) {
-			clear_task_blocked_on(p, PROXY_WAKING);
+		struct mutex *mutex = p->blocked_on;
+		if (!mutex || mutex == PROXY_WAKING) {
+			clear_task_blocked_on(p, mutex);
 			if (task_current(rq, p)) {
 				p->is_blocked = 0;
 				return p;
@@ -7128,7 +7128,7 @@ static void __sched notrace __schedule(i
 
 		rq_set_donor(rq, next);
 		next->blocked_donor = NULL;
-		if (unlikely(next->is_blocked && next->blocked_on)) {
+		if (unlikely(next->is_blocked)) {
 			next = find_proxy_task(rq, next, &rf);
 			if (!next) {
 				zap_balance_callbacks(rq);