Re: [PATCH 8/9] sched/psi: add kernel cmdline parameter psi_inner_cgroup

Johannes Weiner
2022-08-04T16:56:49Z

On Thu, Aug 04, 2022 at 09:51:31PM +0800, Chengming Zhou wrote:
> On 2022/8/4 03:22, Johannes Weiner wrote:
> > On Wed, Aug 03, 2022 at 07:58:27AM -1000, Tejun Heo wrote:
> >> Hello,
> >>
> >> On Wed, Aug 03, 2022 at 08:17:22PM +0800, Chengming Zhou wrote:
> >>>> Assuming the above isn't wrong, if we can figure out how we can re-enable
> >>>> it, which is more difficult as the counters need to be resynchronized with
> >>>> the current state, that'd be ideal. Then, we can just allow each cgroup to
> >>>> enable / disable PSI reporting dynamically as they see fit.
> >>>
> >>> This method is more fine-grained but more difficult like you said above.
> >>> I think it may meet most needs to disable PSI stats in intermediate cgroups?
> >>
> >> So, I'm not necessarily against implementing something easier but we at
> >> least wanna get the interface right, so that if we decide to do the full
> >> thing later we can easily expand on the existing interface. ie. let's please
> >> not be too hacky. I don't think it'd be that difficult to implement
> >> per-cgroup disable-only operation that we can later expand to allow
> >> re-enabling, right?
> >
> > It should be relatively straight-forward to disable and re-enable
> > state aggregation, time tracking, averaging on a per-cgroup level, if
> > we can live with losing history from while it was disabled. I.e. the
> > avgs will restart from 0, total= will have gaps - should be okay, IMO.
> >
> > Where it gets trickier is also stopping the tracking of task counts in
> > a cgroup. For re-enabling afterwards, we'd have to freeze scheduler
> > and cgroup state and find all tasks of interest across all CPUs for
> > the given cgroup to recreate the counts. I'm not quite sure whether
> > that's feasible, and if so, whether it's worth the savings.
> >
> > It might be good to benchmark the two disabling steps independently.
> > Maybe stopping aggregation while keeping task counts is good enough,
> > and we can commit to a disable/re-enable interface from the start.
> >
> > Or maybe it's all in the cachelines and iteration, and stopping the
> > aggregation while still writing task counts isn't saving much. In that
> > case we'd have to look closer at reconstructing task counts, to see if
> > later re-enabling is actually a practical option or whether a one-off
> > kill switch is more realistic.
> >
> > Chengming, can you experiment with disabling: record_times(), the
> > test_state() loop and state_mask construction, and the averaging
> > worker - while keeping the groupc->tasks updates?
>
> Hello,
>
> I did this experiment today with disabling record_times(), test_state()
> loop and averaging worker, while only keeping groupc->tasks[] updates,
> the results look promising.
>
> mmtests/config-scheduler-perfpipe on Intel Xeon Platinum with 3 levels of cgroup:
>
> perfpipe
> tip tip patched
> psi=off psi=on only groupc->tasks[]
> Min Time 7.99 ( 0.00%) 8.86 ( -10.95%) 8.31 ( -4.08%)
> 1st-qrtle Time 8.11 ( 0.00%) 8.94 ( -10.22%) 8.39 ( -3.46%)
> 2nd-qrtle Time 8.17 ( 0.00%) 9.02 ( -10.42%) 8.44 ( -3.37%)
> 3rd-qrtle Time 8.20 ( 0.00%) 9.08 ( -10.72%) 8.48 ( -3.43%)
> Max-1 Time 7.99 ( 0.00%) 8.86 ( -10.95%) 8.31 ( -4.08%)
> Max-5 Time 7.99 ( 0.00%) 8.86 ( -10.95%) 8.31 ( -4.08%)
> Max-10 Time 8.09 ( 0.00%) 8.89 ( -9.96%) 8.35 ( -3.22%)
> Max-90 Time 8.31 ( 0.00%) 9.13 ( -9.90%) 8.55 ( -2.95%)
> Max-95 Time 8.32 ( 0.00%) 9.14 ( -9.88%) 8.55 ( -2.81%)
> Max-99 Time 8.39 ( 0.00%) 9.26 ( -10.30%) 8.57 ( -2.09%)
> Max Time 8.56 ( 0.00%) 9.26 ( -8.23%) 8.72 ( -1.90%)
> Amean Time 8.19 ( 0.00%) 9.03 * -10.26%* 8.45 * -3.27%*

Fantastic!

> Tejun suggested using a bitmap in task to remember whether the task is accounted
> at a given level or not, which I think also is a very good idea, but I haven't
> clearly figure out how to do it.
>
> The above performance test result looks good to me, so I think we can implement this
> per-cgroup "cgroup.psi" interface to disable/re-enable PSI stats from the start,
> and we can change to a better implementation if needed later?

Yes, that sounds good to me.