@@ -158,6 +158,7 @@ struct psi_group {
struct timer_list poll_timer;
wait_queue_head_t poll_wait;
atomic_t poll_wakeup;
+ atomic_t poll_scheduled;
/* Protects data used by the monitor */
struct mutex trigger_lock;
@@ -196,6 +196,7 @@ 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));
@@ -559,18 +560,17 @@ static u64 update_triggers(struct psi_group *group, u64 now)
return now + group->poll_min_period;
}
-/* Schedule polling if it's not already scheduled. */
-static void psi_schedule_poll_work(struct psi_group *group, unsigned long delay)
+/* Schedule polling if it's not already scheduled or forced. */
+static void psi_schedule_poll_work(struct psi_group *group, unsigned long delay,
+ bool force)
{
struct task_struct *task;
/*
- * 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.
+ * atomic_xchg should be called even when !force to provide a
+ * full memory barrier (see the comment inside psi_poll_work).
*/
- if (timer_pending(&group->poll_timer))
+ if (atomic_xchg(&group->poll_scheduled, 1) && !force)
return;
rcu_read_lock();
@@ -582,12 +582,15 @@ static void psi_schedule_poll_work(struct psi_group *group, unsigned long delay)
*/
if (likely(task))
mod_timer(&group->poll_timer, jiffies + delay);
+ else
+ atomic_set(&group->poll_scheduled, 0);
rcu_read_unlock();
}
static void psi_poll_work(struct psi_group *group)
{
+ bool force_reschedule = false;
u32 changed_states;
u64 now;
@@ -595,6 +598,43 @@ static void psi_poll_work(struct psi_group *group)
now = sched_clock();
+ if (now > group->polling_until) {
+ /*
+ * We are either about to start or might stop polling if no
+ * state change was recorded. Resetting poll_scheduled leaves
+ * a small window for psi_group_change to sneak in and schedule
+ * an immegiate poll_work before we get to rescheduling. One
+ * potential extra wakeup at the end of the polling window
+ * should be negligible and polling_next_update still keeps
+ * updates correctly on schedule.
+ */
+ atomic_set(&group->poll_scheduled, 0);
+ /*
+ * A task change can race with the poll worker that is supposed to
+ * report on it. To avoid missing events, ensure ordering between
+ * poll_scheduled and the task state accesses, such that if the poll
+ * worker misses the state update, the task change is guaranteed to
+ * reschedule the poll worker:
+ *
+ * poll worker:
+ * atomic_set(poll_scheduled, 0)
+ * smp_mb()
+ * LOAD states
+ *
+ * task change:
+ * STORE states
+ * if atomic_xchg(poll_scheduled, 1) == 0:
+ * schedule poll worker
+ *
+ * The atomic_xchg() implies a full barrier.
+ */
+ smp_mb();
+ } else {
+ /* Polling window is not over, keep rescheduling */
+ force_reschedule = true;
+ }
+
+
collect_percpu_times(group, PSI_POLL, &changed_states);
if (changed_states & group->poll_states) {
@@ -620,7 +660,8 @@ static void psi_poll_work(struct psi_group *group)
group->polling_next_update = update_triggers(group, now);
psi_schedule_poll_work(group,
- nsecs_to_jiffies(group->polling_next_update - now) + 1);
+ nsecs_to_jiffies(group->polling_next_update - now) + 1,
+ force_reschedule);
out:
mutex_unlock(&group->trigger_lock);
@@ -744,7 +785,7 @@ static void psi_group_change(struct psi_group *group, int cpu,
write_seqcount_end(&groupc->seq);
if (state_mask & group->poll_states)
- psi_schedule_poll_work(group, 1);
+ psi_schedule_poll_work(group, 1, false);
if (wake_clock && !delayed_work_pending(&group->avgs_work))
schedule_delayed_work(&group->avgs_work, PSI_FREQ);
@@ -1239,6 +1280,7 @@ static void psi_trigger_destroy(struct kref *ref)
* can no longer be found through group->poll_task.
*/
kthread_stop(task_to_destroy);
+ atomic_set(&group->poll_scheduled, 0);
}
kfree(t);
}
Psi polling mechanism is trying to minimize the number of wakeups to run psi_poll_work and is currently relying on timer_pending() to detect when this work is already scheduled. This provides a window of opportunity for psi_group_change to schedule an immediate psi_poll_work after poll_timer_fn got called but before psi_poll_work could reschedule itself. Below is the depiction of this entire window: poll_timer_fn wake_up_interruptible(&group->poll_wait); psi_poll_worker wait_event_interruptible(group->poll_wait, ...) psi_poll_work psi_schedule_poll_work if (timer_pending(&group->poll_timer)) return; ... mod_timer(&group->poll_timer, jiffies + delay); Prior to 461daba06bdc we used to rely on poll_scheduled atomic which was reset and set back inside psi_poll_work and therefore this race window was much smaller. The larger window causes increased number of wakeups and our partners report visible power regression of ~10mA after applying 461daba06bdc. Bring back the poll_scheduled atomic and make this race window even narrower by resetting poll_scheduled only when we reach polling expiration time. This does not completely eliminate the possibility of extra wakeups caused by a race with psi_group_change however it will limit it to the worst case scenario of one extra wakeup per every tracking window (0.5s in the worst case). This patch also ensures correct ordering between clearing poll_scheduled flag and obtaining changed_states using memory barrier. Correct ordering between updating changed_states and setting poll_scheduled is ensured by atomic_xchg operation. By tracing the number of immediate rescheduling attempts performed by psi_group_change and the number of these attempts being blocked due to psi monitor being already active, we can assess the effects of this change: Before the patch: Run#1 Run#2 Run#3 Immediate reschedules attempted: 684365 1385156 1261240 Immediate reschedules blocked: 682846 1381654 1258682 Immediate reschedules (delta): 1519 3502 2558 Immediate reschedules (% of attempted): 0.22% 0.25% 0.20% After the patch: Run#1 Run#2 Run#3 Immediate reschedules attempted: 882244 770298 426218 Immediate reschedules blocked: 881996 769796 426074 Immediate reschedules (delta): 248 502 144 Immediate reschedules (% of attempted): 0.03% 0.07% 0.03% The number of non-blocked immediate reschedules dropped from 0.22-0.25% to 0.03-0.07%. The drop is attributed to the decrease in the race window size and the fact that we allow this race only when psi monitors reach polling window expiration time. Fixes: 461daba06bdc ("psi: eliminate kthread_worker from psi trigger scheduling mechanism") Reported-by: Kathleen Chang <yt.chang@mediatek.com> Reported-by: Wenju Xu <wenju.xu@mediatek.com> Reported-by: Jonathan Chen <jonathan.jmchen@mediatek.com> Signed-off-by: Suren Baghdasaryan <surenb@google.com> Tested-by: SH Chen <show-hong.chen@mediatek.com> --- changes in v4: - Updated comments about memory ordering requirements per Johannes include/linux/psi_types.h | 1 + kernel/sched/psi.c | 60 +++++++++++++++++++++++++++++++++------ 2 files changed, 52 insertions(+), 9 deletions(-)