Re: [PATCH v4 1/1] psi: stop relying on timer_pending for poll_work rescheduling

[Johannes Weiner]

On Thu, Jul 08, 2021 at 01:36:48PM -0700, 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>