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Date:   Thu, 4 Aug 2022 12:56:36 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Chengming Zhou <zhouchengming@bytedance.com>
Cc:     Tejun Heo <tj@kernel.org>, surenb@google.com, mingo@redhat.com,
        peterz@infradead.org, corbet@lwn.net, akpm@linux-foundation.org,
        rdunlap@infradead.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, songmuchun@bytedance.com,
        cgroups@vger.kernel.org
Subject: Re: [PATCH 8/9] sched/psi: add kernel cmdline parameter
 psi_inner_cgroup
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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.