Re: [PATCH] sched: Consider task_struct::saved_state in wait_task_inactive().

[Peter Zijlstra <peterz@infradead.org>]

On Fri, May 26, 2023 at 05:13:35PM +0200, Sebastian Andrzej Siewior wrote:
> On 2023-05-26 10:05:43 [+0200], Peter Zijlstra wrote:
> > New day, new chances... How's this? Code-gen doesn't look totally
> > insane, but then, making sense of an optimizing compiler's output is
> > always a wee challenge.
> 
> Noticed it too late but looks good. Tested, works.

Excellent; full patch below. Will go stick in tip/sched/core soonish.

---
Subject: sched: Consider task_struct::saved_state in wait_task_inactive()
From: Peter Zijlstra <peterz@infradead.org>
Date: Wed May 31 16:39:07 CEST 2023

With the introduction of task_struct::saved_state in commit
5f220be21418 ("sched/wakeup: Prepare for RT sleeping spin/rwlocks")
matching the task state has gotten more complicated. That same commit
changed try_to_wake_up() to consider both states, but
wait_task_inactive() has been neglected.

Sebastian noted that the wait_task_inactive() usage in
ptrace_check_attach() can misbehave when ptrace_stop() is blocked on
the tasklist_lock after it sets TASK_TRACED.

Therefore extract a common helper from ttwu_state_match() and use that
to teach wait_task_inactive() about the PREEMPT_RT locks.

Originally-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/sched/core.c |   59 ++++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 48 insertions(+), 11 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3341,6 +3341,39 @@ int migrate_swap(struct task_struct *cur
 }
 #endif /* CONFIG_NUMA_BALANCING */
 
+static __always_inline
+int __task_state_match(struct task_struct *p, unsigned int state)
+{
+	if (READ_ONCE(p->__state) & state)
+		return 1;
+
+#ifdef CONFIG_PREEMPT_RT
+	if (READ_ONCE(p->saved_state) & state)
+		return -1;
+#endif
+	return 0;
+}
+
+static __always_inline
+int task_state_match(struct task_struct *p, unsigned int state)
+{
+#ifdef CONFIG_PREEMPT_RT
+	int match;
+
+	/*
+	 * Serialize against current_save_and_set_rtlock_wait_state() and
+	 * current_restore_rtlock_saved_state().
+	 */
+	raw_spin_lock_irq(&p->pi_lock);
+	match = __task_state_match(p, state);
+	raw_spin_unlock_irq(&p->pi_lock);
+
+	return match;
+#else
+	return __task_state_match(p, state);
+#endif
+}
+
 /*
  * wait_task_inactive - wait for a thread to unschedule.
  *
@@ -3359,7 +3392,7 @@ int migrate_swap(struct task_struct *cur
  */
 unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state)
 {
-	int running, queued;
+	int running, queued, match;
 	struct rq_flags rf;
 	unsigned long ncsw;
 	struct rq *rq;
@@ -3385,7 +3418,7 @@ unsigned long wait_task_inactive(struct
 		 * is actually now running somewhere else!
 		 */
 		while (task_on_cpu(rq, p)) {
-			if (!(READ_ONCE(p->__state) & match_state))
+			if (!task_state_match(p, match_state))
 				return 0;
 			cpu_relax();
 		}
@@ -3400,8 +3433,15 @@ unsigned long wait_task_inactive(struct
 		running = task_on_cpu(rq, p);
 		queued = task_on_rq_queued(p);
 		ncsw = 0;
-		if (READ_ONCE(p->__state) & match_state)
+		if ((match = __task_state_match(p, match_state))) {
+			/*
+			 * When matching on p->saved_state, consider this task
+			 * still queued so it will wait.
+			 */
+			if (match < 0)
+				queued = 1;
 			ncsw = p->nvcsw | LONG_MIN; /* sets MSB */
+		}
 		task_rq_unlock(rq, p, &rf);
 
 		/*
@@ -4003,15 +4043,14 @@ static void ttwu_queue(struct task_struc
 static __always_inline
 bool ttwu_state_match(struct task_struct *p, unsigned int state, int *success)
 {
+	int match;
+
 	if (IS_ENABLED(CONFIG_DEBUG_PREEMPT)) {
 		WARN_ON_ONCE((state & TASK_RTLOCK_WAIT) &&
 			     state != TASK_RTLOCK_WAIT);
 	}
 
-	if (READ_ONCE(p->__state) & state) {
-		*success = 1;
-		return true;
-	}
+	*success = !!(match = __task_state_match(p, state));
 
 #ifdef CONFIG_PREEMPT_RT
 	/*
@@ -4027,12 +4066,10 @@ bool ttwu_state_match(struct task_struct
 	 * p::saved_state to TASK_RUNNING so any further tests will
 	 * not result in false positives vs. @success
 	 */
-	if (p->saved_state & state) {
+	if (match < 0)
 		p->saved_state = TASK_RUNNING;
-		*success = 1;
-	}
 #endif
-	return false;
+	return match > 0;
 }
 
 /*