Re: PSI idle-shutoff

From: Hillf Danton <hdanton@sina.com>
To: Suren Baghdasaryan <surenb@google.com>
Cc: Pavan Kondeti <quic_pkondeti@quicinc.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	quic_charante@quicinc.com
Subject: Re: PSI idle-shutoff
Date: Tue, 11 Oct 2022 19:38:18 +0800	[thread overview]
Message-ID: <20221011113818.340-1-hdanton@sina.com> (raw)
In-Reply-To: <CAJuCfpGN4TooWJuvoh9RZM96hW4BmuKjwnuT2pM4rGPLcGEinQ@mail.gmail.com>

On 10 Oct 2022 14:16:26 -0700 Suren Baghdasaryan <surenb@google.com>
> On Mon, Oct 10, 2022 at 3:57 AM Hillf Danton <hdanton@sina.com> wrote:
> > On 13 Sep 2022 19:38:17 +0530 Pavan Kondeti <quic_pkondeti@quicinc.com>
> > > Hi
> > >
> > > The fact that psi_avgs_work()->collect_percpu_times()->get_recent_times()
> > > run from a kworker thread, PSI_NONIDLE condition would be observed as
> > > there is a RUNNING task. So we would always end up re-arming the work.
> > >
> > > If the work is re-armed from the psi_avgs_work() it self, the backing off
> > > logic in psi_task_change() (will be moved to psi_task_switch soon) can't
> > > help. The work is already scheduled. so we don't do anything there.
> > >
> > > Probably I am missing some thing here. Can you please clarify how we
> > > shut off re-arming the psi avg work?
> >
> > Instead of open coding schedule_delayed_work() in bid to check if timer
> > hits the idle task (see delayed_work_timer_fn()), the idle task is tracked
> > in psi_task_switch() and checked by kworker to see if it preempted the idle
> > task.
> >
> > Only for thoughts now.
> >
> > Hillf
> >
> > +++ b/kernel/sched/psi.c
> > @@ -412,6 +412,8 @@ static u64 update_averages(struct psi_gr
> >         return avg_next_update;
> >  }
> >
> > +static DEFINE_PER_CPU(int, prev_task_is_idle);
> > +
> >  static void psi_avgs_work(struct work_struct *work)
> >  {
> >         struct delayed_work *dwork;
> > @@ -439,7 +441,7 @@ static void psi_avgs_work(struct work_st
> >         if (now >= group->avg_next_update)
> >                 group->avg_next_update = update_averages(group, now);
> >
> > -       if (nonidle) {
> > +       if (nonidle && 0 == per_cpu(prev_task_is_idle, raw_smp_processor_id())) {
> 
> This condition would be incorrect if nonidle was set by a cpu other
> than raw_smp_processor_id() and
> prev_task_is_idle[raw_smp_processor_id()] == 0.

Thanks for taking a look.

> IOW, if some activity happens on a non-current cpu, we would fail to
> reschedule psi_avgs_work for it.

Given activities on remote CPUs, can you specify what prevents psi_avgs_work
from being scheduled on remote CPUs if for example the local CPU has been
idle for a second?

> This can be fixed in collect_percpu_times() by
> considering prev_task_is_idle for all other CPUs as well. However
> Chengming's approach seems simpler to me TBH and does not require an
> additional per-cpu variable.

Good ideas are always welcome.