[PATCH v7 16/23] sched: Add deactivated (sleeping) owner handling to find_proxy_task()

From: John Stultz
Date: Tue Dec 19 2023 - 19:23:40 EST

From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>

If the blocked_on chain resolves to a sleeping owner, deactivate
selected task, and enqueue it on the sleeping owner task.
Then re-activate it later when the owner is woken up.

NOTE: This has been particularly challenging to get working
properly, and some of the locking is particularly ackward. I'd
very much appreciate review and feedback for ways to simplify
this.

Cc: Joel Fernandes <joelaf@xxxxxxxxxx>
Cc: Qais Yousef <qyousef@xxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Juri Lelli <juri.lelli@xxxxxxxxxx>
Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Ben Segall <bsegall@xxxxxxxxxx>
Cc: Zimuzo Ezeozue <zezeozue@xxxxxxxxxx>
Cc: Youssef Esmat <youssefesmat@xxxxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxx>
Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
Cc: Metin Kaya <Metin.Kaya@xxxxxxx>
Cc: Xuewen Yan <xuewen.yan94@xxxxxxxxx>
Cc: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: kernel-team@xxxxxxxxxxx
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
Signed-off-by: Valentin Schneider <valentin.schneider@xxxxxxx>
Signed-off-by: Connor O'Brien <connoro@xxxxxxxxxx>
[jstultz: This was broken out from the larger proxy() patch]
Signed-off-by: John Stultz <jstultz@xxxxxxxxxx>
---
v5:
* Split out from larger proxy patch
v6:
* Major rework, replacing the single list head per task with
per-task list head and nodes, creating a tree structure so
we only wake up decendents of the task woken.
* Reworked the locking to take the task->pi_lock, so we can
avoid mid-chain wakeup races from try_to_wake_up() called by
the ww_mutex logic.
v7:
* Drop ununessary __nested lock annotation, as we already drop
the lock prior.
* Add comments on #else & #endif lines, and clearer function
names, and commit message tweaks as suggested by Metin Kaya
* Move activate_blocked_entities() call from ttwu_queue to
try_to_wake_up() to simplify locking. Thanks to questions from
Metin Kaya
* Fix irqsave/irqrestore usage now we call this outside where
the pi_lock is held
* Fix activate_blocked_entitites not preserving wake_cpu
* Fix for UP builds
---
include/linux/sched.h | 3 +
kernel/fork.c | 4 +
kernel/sched/core.c | 214 ++++++++++++++++++++++++++++++++++++++----
3 files changed, 202 insertions(+), 19 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 8020e224e057..6f982948a105 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1158,6 +1158,9 @@ struct task_struct {
enum blocked_on_state blocked_on_state;
struct mutex *blocked_on; /* lock we're blocked on */
struct task_struct *blocked_donor; /* task that is boosting this task */
+ struct list_head blocked_head; /* tasks blocked on this task */
+ struct list_head blocked_node; /* our entry on someone elses blocked_head */
+ struct task_struct *sleeping_owner; /* task our blocked_node is enqueued on */
raw_spinlock_t blocked_lock;

#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
diff --git a/kernel/fork.c b/kernel/fork.c
index 138fc23cad43..56f5e19c268e 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2460,6 +2460,10 @@ __latent_entropy struct task_struct *copy_process(
p->blocked_on_state = BO_RUNNABLE;
p->blocked_on = NULL; /* not blocked yet */
p->blocked_donor = NULL; /* nobody is boosting p yet */
+
+ INIT_LIST_HEAD(&p->blocked_head);
+ INIT_LIST_HEAD(&p->blocked_node);
+ p->sleeping_owner = NULL;
#ifdef CONFIG_BCACHE
p->sequential_io = 0;
p->sequential_io_avg = 0;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index e0afa228bc9d..0cd63bd0bdcd 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3785,6 +3785,133 @@ static inline void ttwu_do_wakeup(struct task_struct *p)
trace_sched_wakeup(p);
}

+#ifdef CONFIG_SCHED_PROXY_EXEC
+static void do_activate_task(struct rq *rq, struct task_struct *p, int en_flags)
+{
+ lockdep_assert_rq_held(rq);
+
+ if (!sched_proxy_exec()) {
+ activate_task(rq, p, en_flags);
+ return;
+ }
+
+ if (p->sleeping_owner) {
+ struct task_struct *owner = p->sleeping_owner;
+
+ raw_spin_lock(&owner->blocked_lock);
+ list_del_init(&p->blocked_node);
+ p->sleeping_owner = NULL;
+ raw_spin_unlock(&owner->blocked_lock);
+ }
+
+ /*
+ * By calling activate_task with blocked_lock held, we
+ * order against the find_proxy_task() blocked_task case
+ * such that no more blocked tasks will be enqueued on p
+ * once we release p->blocked_lock.
+ */
+ raw_spin_lock(&p->blocked_lock);
+ WARN_ON(task_cpu(p) != cpu_of(rq));
+ activate_task(rq, p, en_flags);
+ raw_spin_unlock(&p->blocked_lock);
+}
+
+#ifdef CONFIG_SMP
+static inline void proxy_set_task_cpu(struct task_struct *p, int cpu)
+{
+ unsigned int wake_cpu;
+
+ /* Preserve wake_cpu */
+ wake_cpu = p->wake_cpu;
+ __set_task_cpu(p, cpu);
+ p->wake_cpu = wake_cpu;
+}
+#else /* !CONFIG_SMP */
+static inline void proxy_set_task_cpu(struct task_struct *p, int cpu)
+{
+ __set_task_cpu(p, cpu);
+}
+#endif /* CONFIG_SMP */
+
+static void activate_blocked_entities(struct rq *target_rq,
+ struct task_struct *owner,
+ int wake_flags)
+{
+ unsigned long flags;
+ struct rq_flags rf;
+ int target_cpu = cpu_of(target_rq);
+ int en_flags = ENQUEUE_WAKEUP | ENQUEUE_NOCLOCK;
+
+ if (wake_flags & WF_MIGRATED)
+ en_flags |= ENQUEUE_MIGRATED;
+ /*
+ * A whole bunch of 'proxy' tasks back this blocked task, wake
+ * them all up to give this task its 'fair' share.
+ */
+ raw_spin_lock_irqsave(&owner->blocked_lock, flags);
+ while (!list_empty(&owner->blocked_head)) {
+ struct task_struct *pp;
+ unsigned int state;
+
+ pp = list_first_entry(&owner->blocked_head,
+ struct task_struct,
+ blocked_node);
+ BUG_ON(pp == owner);
+ list_del_init(&pp->blocked_node);
+ WARN_ON(!pp->sleeping_owner);
+ pp->sleeping_owner = NULL;
+ raw_spin_unlock_irqrestore(&owner->blocked_lock, flags);
+
+ raw_spin_lock_irqsave(&pp->pi_lock, flags);
+ state = READ_ONCE(pp->__state);
+ /* Avoid racing with ttwu */
+ if (state == TASK_WAKING) {
+ raw_spin_unlock_irqrestore(&pp->pi_lock, flags);
+ raw_spin_lock_irqsave(&owner->blocked_lock, flags);
+ continue;
+ }
+ if (READ_ONCE(pp->on_rq)) {
+ /*
+ * We raced with a non mutex handoff activation of pp.
+ * That activation will also take care of activating
+ * all of the tasks after pp in the blocked_entry list,
+ * so we're done here.
+ */
+ raw_spin_unlock_irqrestore(&pp->pi_lock, flags);
+ raw_spin_lock_irqsave(&owner->blocked_lock, flags);
+ continue;
+ }
+
+ proxy_set_task_cpu(pp, target_cpu);
+
+ rq_lock_irqsave(target_rq, &rf);
+ update_rq_clock(target_rq);
+ do_activate_task(target_rq, pp, en_flags);
+ resched_curr(target_rq);
+ rq_unlock_irqrestore(target_rq, &rf);
+ raw_spin_unlock_irqrestore(&pp->pi_lock, flags);
+
+ /* recurse - XXX This needs to be reworked to avoid recursing */
+ activate_blocked_entities(target_rq, pp, wake_flags);
+
+ raw_spin_lock_irqsave(&owner->blocked_lock, flags);
+ }
+ raw_spin_unlock_irqrestore(&owner->blocked_lock, flags);
+}
+#else /* !CONFIG_SCHED_PROXY_EXEC */
+static inline void do_activate_task(struct rq *rq, struct task_struct *p,
+ int en_flags)
+{
+ activate_task(rq, p, en_flags);
+}
+
+static inline void activate_blocked_entities(struct rq *target_rq,
+ struct task_struct *owner,
+ int wake_flags)
+{
+}
+#endif /* CONFIG_SCHED_PROXY_EXEC */
+
#ifdef CONFIG_SMP
static inline bool proxy_needs_return(struct rq *rq, struct task_struct *p)
{
@@ -3839,7 +3966,7 @@ ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags,
atomic_dec(&task_rq(p)->nr_iowait);
}

- activate_task(rq, p, en_flags);
+ do_activate_task(rq, p, en_flags);
wakeup_preempt(rq, p, wake_flags);

ttwu_do_wakeup(p);
@@ -3936,13 +4063,19 @@ void sched_ttwu_pending(void *arg)
update_rq_clock(rq);

llist_for_each_entry_safe(p, t, llist, wake_entry.llist) {
+ int wake_flags;
if (WARN_ON_ONCE(p->on_cpu))
smp_cond_load_acquire(&p->on_cpu, !VAL);

if (WARN_ON_ONCE(task_cpu(p) != cpu_of(rq)))
set_task_cpu(p, cpu_of(rq));

- ttwu_do_activate(rq, p, p->sched_remote_wakeup ? WF_MIGRATED : 0, &rf);
+ wake_flags = p->sched_remote_wakeup ? WF_MIGRATED : 0;
+ ttwu_do_activate(rq, p, wake_flags, &rf);
+ rq_unlock(rq, &rf);
+ activate_blocked_entities(rq, p, wake_flags);
+ rq_lock(rq, &rf);
+ update_rq_clock(rq);
}

/*
@@ -4423,6 +4556,7 @@ int try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
if (p->blocked_on_state == BO_WAKING)
p->blocked_on_state = BO_RUNNABLE;
raw_spin_unlock_irqrestore(&p->blocked_lock, flags);
+ activate_blocked_entities(cpu_rq(cpu), p, wake_flags);
out:
if (success)
ttwu_stat(p, task_cpu(p), wake_flags);
@@ -6663,19 +6797,6 @@ proxy_resched_idle(struct rq *rq, struct task_struct *next)
return rq->idle;
}

-static bool proxy_deactivate(struct rq *rq, struct task_struct *next)
-{
- unsigned long state = READ_ONCE(next->__state);
-
- /* Don't deactivate if the state has been changed to TASK_RUNNING */
- if (state == TASK_RUNNING)
- return false;
- if (!try_to_deactivate_task(rq, next, state, true))
- return false;
- proxy_resched_idle(rq, next);
- return true;
-}
-
#ifdef CONFIG_SMP
/*
* If the blocked-on relationship crosses CPUs, migrate @p to the
@@ -6761,6 +6882,31 @@ proxy_migrate_task(struct rq *rq, struct rq_flags *rf,
}
#endif /* CONFIG_SMP */

+static void proxy_enqueue_on_owner(struct rq *rq, struct task_struct *owner,
+ struct task_struct *next)
+{
+ /*
+ * ttwu_activate() will pick them up and place them on whatever rq
+ * @owner will run next.
+ */
+ if (!owner->on_rq) {
+ BUG_ON(!next->on_rq);
+ deactivate_task(rq, next, DEQUEUE_SLEEP);
+ if (task_current_selected(rq, next)) {
+ put_prev_task(rq, next);
+ rq_set_selected(rq, rq->idle);
+ }
+ /*
+ * ttwu_do_activate must not have a chance to activate p
+ * elsewhere before it's fully extricated from its old rq.
+ */
+ WARN_ON(next->sleeping_owner);
+ next->sleeping_owner = owner;
+ smp_mb();
+ list_add(&next->blocked_node, &owner->blocked_head);
+ }
+}
+
/*
* Find who @next (currently blocked on a mutex) can proxy for.
*
@@ -6866,10 +7012,40 @@ find_proxy_task(struct rq *rq, struct task_struct *next, struct rq_flags *rf)
}

if (!owner->on_rq) {
- /* XXX Don't handle blocked owners yet */
- if (!proxy_deactivate(rq, next))
- ret = next;
- goto out;
+ /*
+ * rq->curr must not be added to the blocked_head list or else
+ * ttwu_do_activate could enqueue it elsewhere before it switches
+ * out here. The approach to avoid this is the same as in the
+ * migrate_task case.
+ */
+ if (curr_in_chain) {
+ raw_spin_unlock(&p->blocked_lock);
+ raw_spin_unlock(&mutex->wait_lock);
+ return proxy_resched_idle(rq, next);
+ }
+
+ /*
+ * If !@owner->on_rq, holding @rq->lock will not pin the task,
+ * so we cannot drop @mutex->wait_lock until we're sure its a blocked
+ * task on this rq.
+ *
+ * We use @owner->blocked_lock to serialize against ttwu_activate().
+ * Either we see its new owner->on_rq or it will see our list_add().
+ */
+ if (owner != p) {
+ raw_spin_unlock(&p->blocked_lock);
+ raw_spin_lock(&owner->blocked_lock);
+ }
+
+ proxy_enqueue_on_owner(rq, owner, next);
+
+ if (task_current_selected(rq, next)) {
+ put_prev_task(rq, next);
+ rq_set_selected(rq, rq->idle);
+ }
+ raw_spin_unlock(&owner->blocked_lock);
+ raw_spin_unlock(&mutex->wait_lock);
+ return NULL; /* retry task selection */
}

if (owner == p) {
--
2.43.0.472.g3155946c3a-goog