Skip to content

Commit

Permalink
psi: eliminate kthread_worker from psi trigger scheduling mechanism
Browse files Browse the repository at this point in the history 
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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200528195442.190116-1-surenb@google.com
  • Loading branch information
Suren Baghdasaryan authored and Peter Zijlstra committed Jun 15, 2020
1 parent f4291df commit 461daba
Show file tree
Hide file tree
Showing 2 changed files with 68 additions and 52 deletions.
7 changes: 4 additions & 3 deletions include/linux/psi_types.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand Down
113 changes: 64 additions & 49 deletions kernel/sched/psi.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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));
Expand All @@ -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)
Expand Down Expand Up @@ -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();
Expand Down Expand Up @@ -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)
{
Expand Down Expand Up @@ -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);
Expand All @@ -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;
Expand All @@ -1158,39 +1175,37 @@ 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);
}
}

mutex_unlock(&group->trigger_lock);

/*
* 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);
}
Expand Down

0 comments on commit 461daba

Please sign in to comment.