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.
