linux-kernel - Re: [PATCH v2 12/12] sched: Add ttwu

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Message-ID: <20250708210233.GG477119@noisy.programming.kicks-ass.net>
Date: Tue, 8 Jul 2025 23:02:33 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: mingo@...hat.com, juri.lelli@...hat.com, vincent.guittot@...aro.org,
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
	vschneid@...hat.com, clm@...a.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 12/12] sched: Add ttwu_queue support for delayed tasks

On Tue, Jul 08, 2025 at 02:44:56PM +0200, Dietmar Eggemann wrote:

> > +	/*
> > +	 * NOTE: unlike the regular try_to_wake_up() path, this runs both
> > +	 * select_task_rq() and ttwu_do_migrate() while holding rq->lock
> > +	 * rather than p->pi_lock.
> > +	 */
> > +	cpu = select_task_rq(p, p->wake_cpu, &wake_flags);
> 
> There are 'lockdep_assert_held(&p->pi_lock)'s in select_task_rq() and
> select_task_rq_fair() which should trigger IMHO? Can they be changed the
> same way like  __task_rq_lock()?

It needs a slightly different fix; notably the reason for these is the
stability of the cpumasks. For that holding either p->pi_lock or
rq->lock is sufficient.

Something a little like so...

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3557,13 +3557,13 @@ static int select_fallback_rq(int cpu, s
 	return dest_cpu;
 }
 
-/*
- * The caller (fork, wakeup) owns p->pi_lock, ->cpus_ptr is stable.
- */
 static inline
 int select_task_rq(struct task_struct *p, int cpu, int *wake_flags)
 {
-	lockdep_assert_held(&p->pi_lock);
+	/*
+	 * Ensure the sched_setaffinity() state is stable.
+	 */
+	lockdep_assert_sched_held(p);
 
 	if (p->nr_cpus_allowed > 1 && !is_migration_disabled(p)) {
 		cpu = p->sched_class->select_task_rq(p, cpu, *wake_flags);
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8499,7 +8499,7 @@ select_task_rq_fair(struct task_struct *
 	/*
 	 * required for stable ->cpus_allowed
 	 */
-	lockdep_assert_held(&p->pi_lock);
+	lockdep_assert_sched_held(p);
 	if (wake_flags & WF_TTWU) {
 		record_wakee(p);
 
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1500,6 +1500,12 @@ static inline void lockdep_assert_rq_hel
 	lockdep_assert_held(__rq_lockp(rq));
 }
 
+static inline void lockdep_assert_sched_held(struct task_struct *p)
+{
+	lockdep_assert(lockdep_is_held(&p->pi_lock) ||
+		       lockdep_is_held(__rq_lockp(task_rq(p))));
+}
+
 extern void raw_spin_rq_lock_nested(struct rq *rq, int subclass);
 extern bool raw_spin_rq_trylock(struct rq *rq);
 extern void raw_spin_rq_unlock(struct rq *rq);