linux-kernel - [RFC 1/1] sched: Remove unreliable wake_cpu check in proxy_needs

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Message-ID: <20250407134654.92841-1-hupu.gm@gmail.com>
Date: Mon,  7 Apr 2025 21:46:54 +0800
From: hupu <hupu.gm@...il.com>
To: jstultz@...gle.com,
	linux-kernel@...r.kernel.org
Cc: juri.lelli@...hat.com,
	peterz@...radead.org,
	vschneid@...hat.com,
	mingo@...hat.com,
	vincent.guittot@...aro.org,
	dietmar.eggemann@....com,
	rostedt@...dmis.org,
	bsegall@...gle.com,
	mgorman@...e.de,
	hupu@...nssion.com,
	hupu <hupu.gm@...il.com>
Subject: [RFC 1/1] sched: Remove unreliable wake_cpu check in proxy_needs_return

The (p->wake_cpu != cpu_of(rq)) check in proxy_needs_return() is unsafe
during early wakeup phase. When called via ttwu_runnable() path:

|-- try_to_wake_up
    |-- ttwu_runnable
        |-- proxy_needs_return    //we are here
    |-- select_task_rq
    |-- set_task_cpu              //set p->wake_cpu here
    |-- ttwu_queue

The p->wake_cpu at this point reflects the CPU where donor last ran before
blocking, not the target migration CPU. During blocking period:
1. CPU affinity may have been changed by other threads
2. Proxy migrations might have altered the effective wake_cpu
3. set_task_cpu() hasn't updated wake_cpu yet in this code path

This makes the wake_cpu vs current CPU comparison meaningless and potentially
dangerous. Rely on find_proxy_task()'s later migration logic to handle CPU
placement based on up-to-date affinity and scheduler state.

Signed-off-by: hupu <hupu.gm@...il.com>
---
 kernel/sched/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 3c4ef4c71cfd..ca4ca739eb85 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4047,7 +4047,7 @@ static inline bool proxy_needs_return(struct rq *rq, struct task_struct *p)

 	raw_spin_lock(&p->blocked_lock);
 	if (__get_task_blocked_on(p) && p->blocked_on_state & BO_NEEDS_RETURN) {
-		if (!task_current(rq, p) && (p->wake_cpu != cpu_of(rq))) {
+		if (!task_current(rq, p)) {
 			if (task_current_donor(rq, p)) {
 				put_prev_task(rq, p);
 				rq_set_donor(rq, rq->idle);
-- 
2.47.0