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

From: Peter Zijlstra <peterz@infradead.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: linux-kernel@vger.kernel.org, Ben Segall <bsegall@google.com>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>, Mel Gorman <mgorman@suse.de>,
	Steven Rostedt <rostedt@goodmis.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Valentin Schneider <vschneid@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>
Subject: Re: [PATCH] sched: Consider task_struct::saved_state in wait_task_inactive().
Date: Thu, 25 May 2023 18:52:44 +0200	[thread overview]
Message-ID: <20230525165244.GV83892@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <Y++UzubyNavLKFDP@linutronix.de>

On Fri, Feb 17, 2023 at 03:53:02PM +0100, Sebastian Andrzej Siewior wrote:

> +static __always_inline bool state_mismatch(struct task_struct *p, unsigned int match_state)
> +{
> +	unsigned long flags;
> +	bool mismatch;
> +
> +	raw_spin_lock_irqsave(&p->pi_lock, flags);
> +	if (READ_ONCE(p->__state) & match_state)
> +		mismatch = false;
> +	else if (READ_ONCE(p->saved_state) & match_state)
> +		mismatch = false;
> +	else
> +		mismatch = true;
> +
> +	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> +	return mismatch;
> +}
> +static __always_inline bool state_match(struct task_struct *p, unsigned int match_state,
> +					bool *wait)
> +{
> +	if (READ_ONCE(p->__state) & match_state)
> +		return true;
> +	if (READ_ONCE(p->saved_state) & match_state) {
> +		*wait = true;
> +		return true;
> +	}
> +	return false;
> +}
> +#else
> +static __always_inline bool state_mismatch(struct task_struct *p, unsigned int match_state)
> +{
> +	return !(READ_ONCE(p->__state) & match_state);
> +}
> +static __always_inline bool state_match(struct task_struct *p, unsigned int match_state,
> +					bool *wait)
> +{
> +	return (READ_ONCE(p->__state) & match_state);
> +}
> +#endif
> +
>  /*
>   * wait_task_inactive - wait for a thread to unschedule.
>   *

Urgh...

I've ended up with the below.. I've tried folding it with
ttwu_state_match() but every attempt so far makes it an unholy mess.

Now, if only we had proper lock guard then we could drop another few
lines, but alas.

---
 kernel/sched/core.c | 35 +++++++++++++++++++++++++++++++++--
 1 file changed, 33 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index a68d1276bab0..5a106629a98d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3341,6 +3341,37 @@ int migrate_swap(struct task_struct *cur, struct task_struct *p,
 }
 #endif /* CONFIG_NUMA_BALANCING */
 
+static __always_inline
+bool __wti_state_match(struct task_struct *p, unsigned int state, int *queued)
+{
+	if (READ_ONCE(p->__state) & state)
+		return true;
+
+#ifdef CONFIG_PREEMPT_RT
+	if (READ_ONCE(p->saved_state) & state) {
+		if (queued)
+			*queued = 1;
+		return true;
+	}
+#endif
+	return false;
+}
+
+static __always_inline bool wti_state_match(struct task_struct *p, unsigned int state)
+{
+#ifdef CONFIG_PREEMPT_RT
+	bool match;
+
+	raw_spin_lock_irq(&p->pi_lock);
+	match = __wti_state_match(p, state, NULL);
+	raw_spin_unlock_irq(&p->pi_lock);
+
+	return match;
+#else
+	return __wti_state_match(p, state, NULL);
+#endif
+}
+
 /*
  * wait_task_inactive - wait for a thread to unschedule.
  *
@@ -3385,7 +3416,7 @@ unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state
 		 * is actually now running somewhere else!
 		 */
 		while (task_on_cpu(rq, p)) {
-			if (!(READ_ONCE(p->__state) & match_state))
+			if (!wti_state_match(p, match_state))
 				return 0;
 			cpu_relax();
 		}
@@ -3400,7 +3431,7 @@ unsigned long wait_task_inactive(struct task_struct *p, unsigned int match_state
 		running = task_on_cpu(rq, p);
 		queued = task_on_rq_queued(p);
 		ncsw = 0;
-		if (READ_ONCE(p->__state) & match_state)
+		if (__wti_state_match(p, match_state, &queued))
 			ncsw = p->nvcsw | LONG_MIN; /* sets MSB */
 		task_rq_unlock(rq, p, &rf);
 
