Re: [PATCH v5 1/1] psi: stop relying on timer_pending for poll_work rescheduling

[Suren Baghdasaryan]

On Fri, Oct 28, 2022 at 2:59 AM Chengming Zhou
<zhouchengming@xxxxxxxxxxxxx> 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@xxxxxxxxxxxx>
> > Reported-by: Wenju Xu <wenju.xu@xxxxxxxxxxxx>
> > Reported-by: Jonathan Chen <jonathan.jmchen@xxxxxxxxxxxx>
> > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
> > Tested-by: SH Chen <show-hong.chen@xxxxxxxxxxxx>
> > Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> > ---
> > Changes since v4 posted at
> > https://lore.kernel.org/all/20221010225744.101629-1-surenb@xxxxxxxxxx/
> > - 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.

>
>
> > +              * 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);
> >  }