Re: PSI idle-shutoff

[Suren Baghdasaryan]

On Fri, Sep 16, 2022 at 10:45 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Wed, Sep 14, 2022 at 11:20 PM Pavan Kondeti
> <quic_pkondeti@xxxxxxxxxxx> wrote:
> >
> > On Tue, Sep 13, 2022 at 07:38:17PM +0530, Pavan Kondeti wrote:
> > > 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.
>
> Hi Pavan,
> Thanks for reporting the issue. IIRC [1] was meant to fix exactly this
> issue. At the time it was written I tested it and it seemed to work.
> Maybe I missed something or some other change introduced afterwards
> affected the shutoff logic. I'll take a closer look next week when I'm
> back at my computer and will consult with Johannes.

Sorry for the delay. I had some time to look into this and test psi
shutoff on my device and I think you are right. The patch I mentioned
prevents new psi_avgs_work from being scheduled when the only non-idle
task is psi_avgs_work itself, however the regular 2sec averaging work
will still go on. I think we could record the fact that the only
active task is psi_avgs_work in record_times() using a new
psi_group_cpu.state_mask flag and then prevent psi_avgs_work() from
rescheduling itself if that flag is set for all non-idle cpus. I'll
test this approach and will post a patch for review if that works.
Thanks,
Suren.

> Thanks,
> Suren.
>
> [1] 1b69ac6b40eb "psi: fix aggregation idle shut-off"
>
> > >
> > > Probably I am missing some thing here. Can you please clarify how we
> > > shut off re-arming the psi avg work?
> > >
> >
> > I have collected traces on an idle system (running android12-5.10 with minimal
> > user space). This is a older kernel, however the issue remain on latest kernel
> > as per code inspection.
> >
> > I have eliminated noise created by other work items. For example, vmstat_work.
> > This is a deferrable work but gets executed since this is queued on the same
> > CPU on which PSI work timer is queued. So I have increased
> > sysctl_stat_interval to 60 * HZ to supress this work.
> >
> > As we can see from the traces, CPU#7 comes out of idle only to execute PSI
> > work for every 2 seconds. The work is always re-armed from the psi_avgs_work()
> > as it finds PSI_NONIDLE condition. The non-idle time is essentially
> >
> > non_idle_time = (work_start_now - wakeup_now) + (sleep_prev - work_end_prev)
> >
> > The first term accounts the non-idle time since the task woken up (queued) to
> > the execution of the work item. It is around ~4 usec (54.119420 - 54.119416)
> >
> > The second term account for the previous update. ~2 usec (52.135424 -
> > 52.135422).
> >
> > PSI work needs to be run when there is some activity after the last update is done
> > i.e last time the work is run. Since we use non-deferrable timer, the other
> > deferrable timers gets woken up and they might queue work or wakeup other threads
> > and creates activity which inturn makes PSI work to be scheduled.
> >
> > PSI work can't just be made deferrable work. Because, it is a system level
> > work and if the CPU on which it is queued is idle for longer duration but the
> > other CPUs are active, we miss PSI updates. What we probably need is a global
> > deferrable timers [1] i.e this timer should not be bound to any CPU but
> > run when any of the CPU comes out of idle. As long as one CPU is busy, we keep
> > running the PSI but if the whole system is idle, we never wakeup.
> >
> >           <idle>-0     [007]    52.135402: cpu_idle:             state=4294967295 cpu_id=7
> >           <idle>-0     [007]    52.135415: workqueue_activate_work: work struct 0xffffffc011bd5010
> >           <idle>-0     [007]    52.135417: sched_wakeup:         comm=kworker/7:3 pid=196 prio=120 target_cpu=007
> >           <idle>-0     [007]    52.135421: sched_switch:         prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/7:3 next_pid=196 next_prio=120
> >      kworker/7:3-196   [007]    52.135421: workqueue_execute_start: work struct 0xffffffc011bd5010: function psi_avgs_work
> >      kworker/7:3-196   [007]    52.135422: timer_start:          timer=0xffffffc011bd5040 function=delayed_work_timer_fn expires=4294905814 [timeout=494] cpu=7 idx=123 flags=D|P|I
> >      kworker/7:3-196   [007]    52.135422: workqueue_execute_end: work struct 0xffffffc011bd5010: function psi_avgs_work
> >      kworker/7:3-196   [007]    52.135424: sched_switch:         prev_comm=kworker/7:3 prev_pid=196 prev_prio=120 prev_state=I ==> next_comm=swapper/7 next_pid=0 next_prio=120
> >           <idle>-0     [007]    52.135428: cpu_idle:             state=0 cpu_id=7
> >
> >           <system is idle and gets woken up after 2 seconds due to PSI work>
> >
> >           <idle>-0     [007]    54.119402: cpu_idle:             state=4294967295 cpu_id=7
> >           <idle>-0     [007]    54.119414: workqueue_activate_work: work struct 0xffffffc011bd5010
> >           <idle>-0     [007]    54.119416: sched_wakeup:         comm=kworker/7:3 pid=196 prio=120 target_cpu=007
> >           <idle>-0     [007]    54.119420: sched_switch:         prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=R ==> next_comm=kworker/7:3 next_pid=196 next_prio=120
> >      kworker/7:3-196   [007]    54.119420: workqueue_execute_start: work struct 0xffffffc011bd5010: function psi_avgs_work
> >      kworker/7:3-196   [007]    54.119421: timer_start:          timer=0xffffffc011bd5040 function=delayed_work_timer_fn expires=4294906315 [timeout=499] cpu=7 idx=122 flags=D|P|I
> >      kworker/7:3-196   [007]    54.119422: workqueue_execute_end: work struct 0xffffffc011bd5010: function psi_avgs_work
> >
> > [1]
> > https://lore.kernel.org/lkml/1430188744-24737-1-git-send-email-joonwoop@xxxxxxxxxxxxxx/
> >
> > Thanks,
> > Pavan