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

Metin Kaya
2023-12-22T08:33:47Z

On 20/12/2023 12:18 am, John Stultz wrote:
> From: Peter Zijlstra <peterz@infradead.org>
>
> 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

awkward?

> very much appreciate review and feedback for ways to simplify
> this.
>
> Cc: Joel Fernandes <joelaf@google.com>
> Cc: Qais Yousef <qyousef@google.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Valentin Schneider <vschneid@redhat.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ben Segall <bsegall@google.com>
> Cc: Zimuzo Ezeozue <zezeozue@google.com>
> Cc: Youssef Esmat <youssefesmat@google.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Waiman Long <longman@redhat.com>
> Cc: Boqun Feng <boqun.feng@gmail.com>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Metin Kaya <Metin.Kaya@arm.com>
> Cc: Xuewen Yan <xuewen.yan94@gmail.com>
> Cc: K Prateek Nayak <kprateek.nayak@amd.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: kernel-team@android.com
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> Signed-off-by: Connor O'Brien <connoro@google.com>
> [jstultz: This was broken out from the larger proxy() patch]
> Signed-off-by: John Stultz <jstultz@google.com>
> ---
> 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.

descendants

> * 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

unnecessary

> 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;

As we discussed off-list, there are now 7 fields introduced in
task_struct because of Proxy Execution support. I recommended to move
them into a container struct to save task_struct from a slight pollution
(similarly initialization of these fields in copy_process() can be moved
into a separate function), but you mentioned fair points about different
locks for different fields.

I wonder what other folks think about this minor concern.

>
> #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);

I'm unsure if it's a big deal, but IRQs are disabled here and
activate_blocked_entities() disables them again.

> + 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))

Now we can drop the last argument (deactivate_cond) of
try_to_deactivate_task() since it can be determined via
!task_is_blocked(p) I think. IOW:

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 30dfb6f14f2b..5b3b4ebca65d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6866,11 +6866,11 @@ pick_next_task(struct rq *rq, struct task_struct
*prev, struct rq_flags *rf)
#endif

static bool try_to_deactivate_task(struct rq *rq, struct task_struct *p,
- unsigned long task_state, bool deactivate_cond)
+ unsigned long task_state)
{
if (signal_pending_state(task_state, p)) {
WRITE_ONCE(p->__state, TASK_RUNNING);
- } else if (deactivate_cond) {
+ } else if (!task_is_blocked(p)) {
p->sched_contributes_to_load =
(task_state & TASK_UNINTERRUPTIBLE) &&
!(task_state & TASK_NOLOAD) &&
@@ -7329,8 +7329,7 @@ static void __sched notrace __schedule(unsigned
int sched_mode)
*/
prev_state = READ_ONCE(prev->__state);
if (!(sched_mode & SM_MASK_PREEMPT) && prev_state) {
- try_to_deactivate_task(rq, prev, prev_state,
- !task_is_blocked(prev));
+ try_to_deactivate_task(rq, prev, prev_state);
switch_count = &prev->nvcsw;
}

> - 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) {