Messages in this thread | | | From | Suren Baghdasaryan <> | Date | Fri, 28 Oct 2022 12:49:06 -0700 | Subject | Re: [PATCH v5 1/1] psi: stop relying on timer_pending for poll_work rescheduling |
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On Fri, Oct 28, 2022 at 9:44 AM Suren Baghdasaryan <surenb@google.com> wrote: > > On Fri, Oct 28, 2022 at 2:59 AM Chengming Zhou > <zhouchengming@bytedance.com> wrote: > > > > Hello, > > > > On 2022/10/27 07:38, 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@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> > > > Acked-by: Johannes Weiner <hannes@cmpxchg.org> > > > --- > > > Changes since v4 posted at > > > https://lore.kernel.org/all/20221010225744.101629-1-surenb@google.com/ > > > - Added missing parameter in psi_schedule_poll_work() call used only when > > > CONFIG_IRQ_TIME_ACCOUNTING is enabled, reported by kernel test robot. > > > > > > 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..19d05b5c8a55 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 immegiate poll_work before we get to rescheduling. One > > > > "immegiate" should be "immediate"? > > Ack. I'll post a new version with this fix.
Fixed version is posted as v6 at: https://lore.kernel.org/all/20221028194541.813985-1-surenb@google.com/ Thanks, Suren.
> > > > > > > > + * 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; > > > + } > > > > Maybe we don't need force_reschedule special case? If this poller > > need to reschedule and see force_reschedule set by task change, > > then it doesn't re-arm poll_timer. > > IIRC (it has been a year since we discussed this patch), > force_reschedule is used to skip extra operations (resetting > poll_scheduled atomic + the memory barrier) when we know beforehand > that we need to reschedule the work regardless of the `changed_states` > result. We need to reschedule unconditionally because we are still > within the polling window. > Thanks, > Suren. > > > > > Or if we want this poller to override the timeout value of poll_timer, > > we can always use force==true here? > > > > Thanks. > > > > > + > > > + > > > 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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