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psi: eliminate kthread_worker from psi trigger scheduling mechanism

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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
This commit is contained in:
Suren Baghdasaryan 2020-05-28 12:54:42 -07:00 committed by Peter Zijlstra
parent f4291df103
commit 461daba06b
2 changed files with 68 additions and 52 deletions
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7
include/linux/psi_types.h
View File

@ -153,9 +153,10 @@ struct psi_group {
unsigned long avg[NR_PSI_STATES - 1][3]; unsigned long avg[NR_PSI_STATES - 1][3];
/* Monitor work control */ /* Monitor work control */
atomic_t poll_scheduled; struct task_struct __rcu *poll_task;
struct kthread_worker __rcu *poll_kworker; struct timer_list poll_timer;
struct kthread_delayed_work poll_work; wait_queue_head_t poll_wait;
atomic_t poll_wakeup;
/* Protects data used by the monitor */ /* Protects data used by the monitor */
struct mutex trigger_lock; struct mutex trigger_lock;

113
kernel/sched/psi.c
View File

@ -190,7 +190,6 @@ static void group_init(struct psi_group *group)
INIT_DELAYED_WORK(&group->avgs_work, psi_avgs_work); INIT_DELAYED_WORK(&group->avgs_work, psi_avgs_work);
mutex_init(&group->avgs_lock); mutex_init(&group->avgs_lock);
/* Init trigger-related members */ /* Init trigger-related members */
atomic_set(&group->poll_scheduled, 0);
mutex_init(&group->trigger_lock); mutex_init(&group->trigger_lock);
INIT_LIST_HEAD(&group->triggers); INIT_LIST_HEAD(&group->triggers);
memset(group->nr_triggers, 0, sizeof(group->nr_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)); memset(group->polling_total, 0, sizeof(group->polling_total));
group->polling_next_update = ULLONG_MAX; group->polling_next_update = ULLONG_MAX;
group->polling_until = 0; group->polling_until = 0;
rcu_assign_pointer(group->poll_kworker, NULL); rcu_assign_pointer(group->poll_task, NULL);
} }
void __init psi_init(void) 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; return now + group->poll_min_period;
} }
/* /* Schedule polling if it's not already scheduled. */
* 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.
*/
static void psi_schedule_poll_work(struct psi_group *group, unsigned long delay) 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; return;
rcu_read_lock(); 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 * kworker might be NULL in case psi_trigger_destroy races with
* psi_task_change (hotpath) which can't use locks * psi_task_change (hotpath) which can't use locks
*/ */
if (likely(kworker)) if (likely(task))
kthread_queue_delayed_work(kworker, &group->poll_work, delay); mod_timer(&group->poll_timer, jiffies + delay);
else
atomic_set(&group->poll_scheduled, 0);
rcu_read_unlock(); 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; u32 changed_states;
u64 now; 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); mutex_lock(&group->trigger_lock);
now = sched_clock(); now = sched_clock();
@ -623,6 +613,35 @@ out:
mutex_unlock(&group->trigger_lock); 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, static void record_times(struct psi_group_cpu *groupc, int cpu,
bool memstall_tick) bool memstall_tick)
{ {
@ -1099,22 +1118,20 @@ struct psi_trigger *psi_trigger_create(struct psi_group *group,
mutex_lock(&group->trigger_lock); mutex_lock(&group->trigger_lock);
if (!rcu_access_pointer(group->poll_kworker)) { if (!rcu_access_pointer(group->poll_task)) {
struct sched_param param = { struct task_struct *task;
.sched_priority = 1,
};
struct kthread_worker *kworker;
kworker = kthread_create_worker(0, "psimon"); task = kthread_create(psi_poll_worker, group, "psimon");
if (IS_ERR(kworker)) { if (IS_ERR(task)) {
kfree(t); kfree(t);
mutex_unlock(&group->trigger_lock); mutex_unlock(&group->trigger_lock);
return ERR_CAST(kworker); return ERR_CAST(task);
} }
sched_setscheduler_nocheck(kworker->task, SCHED_FIFO, &param); atomic_set(&group->poll_wakeup, 0);
kthread_init_delayed_work(&group->poll_work, init_waitqueue_head(&group->poll_wait);
psi_poll_work); wake_up_process(task);
rcu_assign_pointer(group->poll_kworker, kworker); timer_setup(&group->poll_timer, poll_timer_fn, 0);
rcu_assign_pointer(group->poll_task, task);
} }
list_add(&t->node, &group->triggers); 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_trigger *t = container_of(ref, struct psi_trigger, refcount);
struct psi_group *group = t->group; 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)) if (static_branch_likely(&psi_disabled))
return; return;
@ -1158,13 +1175,13 @@ static void psi_trigger_destroy(struct kref *ref)
period = min(period, div_u64(tmp->win.size, period = min(period, div_u64(tmp->win.size,
UPDATES_PER_WINDOW)); UPDATES_PER_WINDOW));
group->poll_min_period = period; 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) { if (group->poll_states == 0) {
group->polling_until = 0; group->polling_until = 0;
kworker_to_destroy = rcu_dereference_protected( task_to_destroy = rcu_dereference_protected(
group->poll_kworker, group->poll_task,
lockdep_is_held(&group->trigger_lock)); 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 * Wait for both *trigger_ptr from psi_trigger_replace and
* poll_kworker RCUs to complete their read-side critical sections * poll_task RCUs to complete their read-side critical sections
* before destroying the trigger and optionally the poll_kworker * before destroying the trigger and optionally the poll_task
*/ */
synchronize_rcu(); synchronize_rcu();
/* /*
* Destroy the kworker after releasing trigger_lock to prevent a * Destroy the kworker after releasing trigger_lock to prevent a
* deadlock while waiting for psi_poll_work to acquire trigger_lock * 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 * 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 * But it might have been already scheduled before
* that - deschedule it cleanly before destroying it. * that - deschedule it cleanly before destroying it.
*/ */
kthread_cancel_delayed_work_sync(&group->poll_work); del_timer_sync(&group->poll_timer);
atomic_set(&group->poll_scheduled, 0); kthread_stop(task_to_destroy);
kthread_destroy_worker(kworker_to_destroy);
} }
kfree(t); kfree(t);
} }