From: Johannes Weiner <hannes@cmpxchg.org>
To: Suren Baghdasaryan <surenb@google.com>
Cc: peterz@infradead.org, mingo@redhat.com, juri.lelli@redhat.com,
vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
bristot@redhat.com, matthias.bgg@gmail.com, minchan@google.com,
timmurray@google.com, yt.chang@mediatek.com,
wenju.xu@mediatek.com, jonathan.jmchen@mediatek.com,
linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org,
linux-mediatek@lists.infradead.org, kernel-team@android.com,
SH Chen <show-hong.chen@mediatek.com>
Subject: Re: [PATCH v4 1/1] psi: stop relying on timer_pending for poll_work rescheduling
Date: Thu, 8 Jul 2021 17:40:22 -0400 [thread overview]
Message-ID: <YOdwxh3487PeMHRX@cmpxchg.org> (raw)
In-Reply-To: <20210708203648.2399667-1-surenb@google.com>
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@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>
prev parent reply other threads:[~2021-07-08 21:40 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-07-08 20:36 [PATCH v4 1/1] psi: stop relying on timer_pending for poll_work rescheduling Suren Baghdasaryan
2021-07-08 21:40 ` Johannes Weiner [this message]
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