[PATCH v6 1/1] psi: stop relying on timer_pending for poll_work rescheduling
Chengming Zhou
zhouchengming at bytedance.com
Fri Oct 28 20:00:38 PDT 2022
On 2022/10/29 03:45, Suren Baghdasaryan wrote:
> 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 at mediatek.com>
> Reported-by: Wenju Xu <wenju.xu at mediatek.com>
> Reported-by: Jonathan Chen <jonathan.jmchen at mediatek.com>
> Signed-off-by: Suren Baghdasaryan <surenb at google.com>
> Tested-by: SH Chen <show-hong.chen at mediatek.com>
> Acked-by: Johannes Weiner <hannes at cmpxchg.org>
Reviewed-by: Chengming Zhou <zhouchengming at bytedance.com>
Thanks.
> ---
> Changes since v5 posted at
> https://lore.kernel.org/all/20221026233839.1934419-1-surenb@google.com/
> - Fixed spelling in the comments, per Chengming Zhou
>
> include/linux/psi_types.h | 1 +
> kernel/sched/psi.c | 62 ++++++++++++++++++++++++++++++++-------
> 2 files changed, 53 insertions(+), 10 deletions(-)
>
> diff --git a/include/linux/psi_types.h b/include/linux/psi_types.h
> index 6e4372735068..14a1ebb74e11 100644
> --- a/include/linux/psi_types.h
> +++ b/include/linux/psi_types.h
> @@ -177,6 +177,7 @@ 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;
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index dbaeac915895..94c4c5143f23 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -189,6 +189,7 @@ 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);
> group->poll_min_period = U32_MAX;
> @@ -580,18 +581,17 @@ 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();
> @@ -603,12 +603,15 @@ 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;
>
> @@ -616,6 +619,43 @@ 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 immediate 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) {
> @@ -641,7 +681,8 @@ 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);
> @@ -802,7 +843,7 @@ 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);
> @@ -956,7 +997,7 @@ void psi_account_irqtime(struct task_struct *task, u32 delta)
> write_seqcount_end(&groupc->seq);
>
> if (group->poll_states & (1 << PSI_IRQ_FULL))
> - psi_schedule_poll_work(group, 1);
> + psi_schedule_poll_work(group, 1, false);
> } while ((group = group->parent));
> }
> #endif
> @@ -1342,6 +1383,7 @@ void psi_trigger_destroy(struct psi_trigger *t)
> * can no longer be found through group->poll_task.
> */
> kthread_stop(task_to_destroy);
> + atomic_set(&group->poll_scheduled, 0);
> }
> kfree(t);
> }
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