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Subject[PATCH 1/1] psi: eliminate kthread_worker from psi trigger scheduling mechanism
From
Each psi group requires a dedicated kthread_delayed_work and
kthread_worker. Since no other work can be performed using psi_group's
kthread_worker, the same result can be obtained using a task_struct and
a timer directly. This makes psi triggering simpler by removing lists
and locks involved with kthread_worker usage and eliminates the need for
poll_scheduled atomic use in the hot path.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
This patch is meant to address Peter's request in [1] to pull
kthread_queue_delayed_work() out from under rq->lock. This should also address
the lockdep warning about possibility of a circular dependency described in [2]

[1]: https://lore.kernel.org/lkml/20200428163125.GC16910@hirez.programming.kicks-ass.net/
[2]: https://lore.kernel.org/lkml/CAJuCfpG4NkhpQvZjgXZ_3gm6Hf1QgN_eUOQ8iX9Cv1k9whLwSQ@mail.gmail.com
---
include/linux/psi_types.h | 7 ++-
kernel/sched/psi.c | 113 +++++++++++++++++++++-----------------
2 files changed, 68 insertions(+), 52 deletions(-)

diff --git a/include/linux/psi_types.h b/include/linux/psi_types.h
index 4b7258495a04..b95f3211566a 100644
--- a/include/linux/psi_types.h
+++ b/include/linux/psi_types.h
@@ -153,9 +153,10 @@ struct psi_group {
unsigned long avg[NR_PSI_STATES - 1][3];

/* Monitor work control */
- atomic_t poll_scheduled;
- struct kthread_worker __rcu *poll_kworker;
- struct kthread_delayed_work poll_work;
+ struct task_struct __rcu *poll_task;
+ struct timer_list poll_timer;
+ wait_queue_head_t poll_wait;
+ atomic_t poll_wakeup;

/* Protects data used by the monitor */
struct mutex trigger_lock;
diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
index 8f45cdb6463b..e53b711bd643 100644
--- a/kernel/sched/psi.c
+++ b/kernel/sched/psi.c
@@ -190,7 +190,6 @@ static void group_init(struct psi_group *group)
INIT_DELAYED_WORK(&group->avgs_work, psi_avgs_work);
mutex_init(&group->avgs_lock);
/* Init trigger-related members */
- atomic_set(&group->poll_scheduled, 0);
mutex_init(&group->trigger_lock);
INIT_LIST_HEAD(&group->triggers);
memset(group->nr_triggers, 0, sizeof(group->nr_triggers));
@@ -199,7 +198,7 @@ static void group_init(struct psi_group *group)
memset(group->polling_total, 0, sizeof(group->polling_total));
group->polling_next_update = ULLONG_MAX;
group->polling_until = 0;
- rcu_assign_pointer(group->poll_kworker, NULL);
+ rcu_assign_pointer(group->poll_task, NULL);
}

void __init psi_init(void)
@@ -547,47 +546,38 @@ static u64 update_triggers(struct psi_group *group, u64 now)
return now + group->poll_min_period;
}

-/*
- * Schedule polling if it's not already scheduled. It's safe to call even from
- * hotpath because even though kthread_queue_delayed_work takes worker->lock
- * spinlock that spinlock is never contended due to poll_scheduled atomic
- * preventing such competition.
- */
+/* Schedule polling if it's not already scheduled. */
static void psi_schedule_poll_work(struct psi_group *group, unsigned long delay)
{
- struct kthread_worker *kworker;
+ struct task_struct *task;

- /* Do not reschedule if already scheduled */
- if (atomic_cmpxchg(&group->poll_scheduled, 0, 1) != 0)
+ /*
+ * Do not reschedule if already scheduled.
+ * Possible race with a timer scheduled after this check but before
+ * mod_timer below can be tolerated because group->polling_next_update
+ * will keep updates on schedule.
+ */
+ if (timer_pending(&group->poll_timer))
return;

rcu_read_lock();

- kworker = rcu_dereference(group->poll_kworker);
+ task = rcu_dereference(group->poll_task);
/*
* kworker might be NULL in case psi_trigger_destroy races with
* psi_task_change (hotpath) which can't use locks
*/
- if (likely(kworker))
- kthread_queue_delayed_work(kworker, &group->poll_work, delay);
- else
- atomic_set(&group->poll_scheduled, 0);
+ if (likely(task))
+ mod_timer(&group->poll_timer, jiffies + delay);

rcu_read_unlock();
}

-static void psi_poll_work(struct kthread_work *work)
+static void psi_poll_work(struct psi_group *group)
{
- struct kthread_delayed_work *dwork;
- struct psi_group *group;
u32 changed_states;
u64 now;

- dwork = container_of(work, struct kthread_delayed_work, work);
- group = container_of(dwork, struct psi_group, poll_work);
-
- atomic_set(&group->poll_scheduled, 0);
-
mutex_lock(&group->trigger_lock);

now = sched_clock();
@@ -623,6 +613,35 @@ static void psi_poll_work(struct kthread_work *work)
mutex_unlock(&group->trigger_lock);
}

+static int psi_poll_worker(void *data)
+{
+ struct psi_group *group = (struct psi_group *)data;
+ struct sched_param param = {
+ .sched_priority = 1,
+ };
+
+ sched_setscheduler_nocheck(current, SCHED_FIFO, &param);
+
+ while (true) {
+ wait_event_interruptible(group->poll_wait,
+ atomic_cmpxchg(&group->poll_wakeup, 1, 0) ||
+ kthread_should_stop());
+ if (kthread_should_stop())
+ break;
+
+ psi_poll_work(group);
+ }
+ return 0;
+}
+
+static void poll_timer_fn(struct timer_list *t)
+{
+ struct psi_group *group = from_timer(group, t, poll_timer);
+
+ atomic_set(&group->poll_wakeup, 1);
+ wake_up_interruptible(&group->poll_wait);
+}
+
static void record_times(struct psi_group_cpu *groupc, int cpu,
bool memstall_tick)
{
@@ -1099,22 +1118,20 @@ struct psi_trigger *psi_trigger_create(struct psi_group *group,

mutex_lock(&group->trigger_lock);

- if (!rcu_access_pointer(group->poll_kworker)) {
- struct sched_param param = {
- .sched_priority = 1,
- };
- struct kthread_worker *kworker;
+ if (!rcu_access_pointer(group->poll_task)) {
+ struct task_struct *task;

- kworker = kthread_create_worker(0, "psimon");
- if (IS_ERR(kworker)) {
+ task = kthread_create(psi_poll_worker, group, "psimon");
+ if (IS_ERR(task)) {
kfree(t);
mutex_unlock(&group->trigger_lock);
- return ERR_CAST(kworker);
+ return ERR_CAST(task);
}
- sched_setscheduler_nocheck(kworker->task, SCHED_FIFO, &param);
- kthread_init_delayed_work(&group->poll_work,
- psi_poll_work);
- rcu_assign_pointer(group->poll_kworker, kworker);
+ atomic_set(&group->poll_wakeup, 0);
+ init_waitqueue_head(&group->poll_wait);
+ wake_up_process(task);
+ timer_setup(&group->poll_timer, poll_timer_fn, 0);
+ rcu_assign_pointer(group->poll_task, task);
}

list_add(&t->node, &group->triggers);
@@ -1132,7 +1149,7 @@ static void psi_trigger_destroy(struct kref *ref)
{
struct psi_trigger *t = container_of(ref, struct psi_trigger, refcount);
struct psi_group *group = t->group;
- struct kthread_worker *kworker_to_destroy = NULL;
+ struct task_struct *task_to_destroy = NULL;

if (static_branch_likely(&psi_disabled))
return;
@@ -1158,13 +1175,13 @@ static void psi_trigger_destroy(struct kref *ref)
period = min(period, div_u64(tmp->win.size,
UPDATES_PER_WINDOW));
group->poll_min_period = period;
- /* Destroy poll_kworker when the last trigger is destroyed */
+ /* Destroy poll_task when the last trigger is destroyed */
if (group->poll_states == 0) {
group->polling_until = 0;
- kworker_to_destroy = rcu_dereference_protected(
- group->poll_kworker,
+ task_to_destroy = rcu_dereference_protected(
+ group->poll_task,
lockdep_is_held(&group->trigger_lock));
- rcu_assign_pointer(group->poll_kworker, NULL);
+ rcu_assign_pointer(group->poll_task, NULL);
}
}

@@ -1172,25 +1189,23 @@ static void psi_trigger_destroy(struct kref *ref)

/*
* Wait for both *trigger_ptr from psi_trigger_replace and
- * poll_kworker RCUs to complete their read-side critical sections
- * before destroying the trigger and optionally the poll_kworker
+ * poll_task RCUs to complete their read-side critical sections
+ * before destroying the trigger and optionally the poll_task
*/
synchronize_rcu();
/*
* Destroy the kworker after releasing trigger_lock to prevent a
* deadlock while waiting for psi_poll_work to acquire trigger_lock
*/
- if (kworker_to_destroy) {
+ if (task_to_destroy) {
/*
* After the RCU grace period has expired, the worker
- * can no longer be found through group->poll_kworker.
+ * can no longer be found through group->poll_task.
* But it might have been already scheduled before
* that - deschedule it cleanly before destroying it.
*/
- kthread_cancel_delayed_work_sync(&group->poll_work);
- atomic_set(&group->poll_scheduled, 0);
-
- kthread_destroy_worker(kworker_to_destroy);
+ del_timer_sync(&group->poll_timer);
+ kthread_stop(task_to_destroy);
}
kfree(t);
}
--
2.27.0.rc0.183.gde8f92d652-goog
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