Re: [PATCH v4 00/10] sched/fair: rework the CFS load balance

[Vincent Guittot <vincent.guittot@linaro.org>]

On Mon, 21 Oct 2019 at 09:50, Ingo Molnar <mingo@kernel.org> wrote:
>
>
> * Vincent Guittot <vincent.guittot@linaro.org> wrote:
>
> > Several wrong task placement have been raised with the current load
> > balance algorithm but their fixes are not always straight forward and
> > end up with using biased values to force migrations. A cleanup and rework
> > of the load balance will help to handle such UCs and enable to fine grain
> > the behavior of the scheduler for other cases.
> >
> > Patch 1 has already been sent separately and only consolidate asym policy
> > in one place and help the review of the changes in load_balance.
> >
> > Patch 2 renames the sum of h_nr_running in stats.
> >
> > Patch 3 removes meaningless imbalance computation to make review of
> > patch 4 easier.
> >
> > Patch 4 reworks load_balance algorithm and fixes some wrong task placement
> > but try to stay conservative.
> >
> > Patch 5 add the sum of nr_running to monitor non cfs tasks and take that
> > into account when pulling tasks.
> >
> > Patch 6 replaces runnable_load by load now that the signal is only used
> > when overloaded.
> >
> > Patch 7 improves the spread of tasks at the 1st scheduling level.
> >
> > Patch 8 uses utilization instead of load in all steps of misfit task
> > path.
> >
> > Patch 9 replaces runnable_load_avg by load_avg in the wake up path.
> >
> > Patch 10 optimizes find_idlest_group() that was using both runnable_load
> > and load. This has not been squashed with previous patch to ease the
> > review.
> >
> > Patch 11 reworks find_idlest_group() to follow the same steps as
> > find_busiest_group()
> >
> > Some benchmarks results based on 8 iterations of each tests:
> > - small arm64 dual quad cores system
> >
> >            tip/sched/core        w/ this patchset    improvement
> > schedpipe      53125 +/-0.18%        53443 +/-0.52%   (+0.60%)
> >
> > hackbench -l (2560/#grp) -g #grp
> >  1 groups      1.579 +/-29.16%       1.410 +/-13.46% (+10.70%)
> >  4 groups      1.269 +/-9.69%        1.205 +/-3.27%   (+5.00%)
> >  8 groups      1.117 +/-1.51%        1.123 +/-1.27%   (+4.57%)
> > 16 groups      1.176 +/-1.76%        1.164 +/-2.42%   (+1.07%)
> >
> > Unixbench shell8
> >   1 test     1963.48 +/-0.36%       1902.88 +/-0.73%    (-3.09%)
> > 224 tests    2427.60 +/-0.20%       2469.80 +/-0.42%  (1.74%)
> >
> > - large arm64 2 nodes / 224 cores system
> >
> >            tip/sched/core        w/ this patchset    improvement
> > schedpipe     124084 +/-1.36%       124445 +/-0.67%   (+0.29%)
> >
> > hackbench -l (256000/#grp) -g #grp
> >   1 groups    15.305 +/-1.50%       14.001 +/-1.99%   (+8.52%)
> >   4 groups     5.959 +/-0.70%        5.542 +/-3.76%   (+6.99%)
> >  16 groups     3.120 +/-1.72%        3.253 +/-0.61%   (-4.92%)
> >  32 groups     2.911 +/-0.88%        2.837 +/-1.16%   (+2.54%)
> >  64 groups     2.805 +/-1.90%        2.716 +/-1.18%   (+3.17%)
> > 128 groups     3.166 +/-7.71%        3.891 +/-6.77%   (+5.82%)
> > 256 groups     3.655 +/-10.09%       3.185 +/-6.65%  (+12.87%)
> >
> > dbench
> >   1 groups   328.176 +/-0.29%      330.217 +/-0.32%   (+0.62%)
> >   4 groups   930.739 +/-0.50%      957.173 +/-0.66%   (+2.84%)
> >  16 groups  1928.292 +/-0.36%     1978.234 +/-0.88%   (+0.92%)
> >  32 groups  2369.348 +/-1.72%     2454.020 +/-0.90%   (+3.57%)
> >  64 groups  2583.880 +/-3.39%     2618.860 +/-0.84%   (+1.35%)
> > 128 groups  2256.406 +/-10.67%    2392.498 +/-2.13%   (+6.03%)
> > 256 groups  1257.546 +/-3.81%     1674.684 +/-4.97%  (+33.17%)
> >
> > Unixbench shell8
> >   1 test     6944.16 +/-0.02     6605.82 +/-0.11      (-4.87%)
> > 224 tests   13499.02 +/-0.14    13637.94 +/-0.47%     (+1.03%)
> > lkp reported a -10% regression on shell8 (1 test) for v3 that
> > seems that is partially recovered on my platform with v4.
> >
> > tip/sched/core sha1:
> >   commit 563c4f85f9f0 ("Merge branch 'sched/rt' into sched/core, to pick up -rt changes")
> >
> > Changes since v3:
> > - small typo and variable ordering fixes
> > - add some acked/reviewed tag
> > - set 1 instead of load for migrate_misfit
> > - use nr_h_running instead of load for asym_packing
> > - update the optimization of find_idlest_group() and put back somes
> >  conditions when comparing load
> > - rework find_idlest_group() to match find_busiest_group() behavior
> >
> > Changes since v2:
> > - fix typo and reorder code
> > - some minor code fixes
> > - optimize the find_idles_group()
> >
> > Not covered in this patchset:
> > - Better detection of overloaded and fully busy state, especially for cases
> >   when nr_running > nr CPUs.
> >
> > Vincent Guittot (11):
> >   sched/fair: clean up asym packing
> >   sched/fair: rename sum_nr_running to sum_h_nr_running
> >   sched/fair: remove meaningless imbalance calculation
> >   sched/fair: rework load_balance
> >   sched/fair: use rq->nr_running when balancing load
> >   sched/fair: use load instead of runnable load in load_balance
> >   sched/fair: evenly spread tasks when not overloaded
> >   sched/fair: use utilization to select misfit task
> >   sched/fair: use load instead of runnable load in wakeup path
> >   sched/fair: optimize find_idlest_group
> >   sched/fair: rework find_idlest_group
> >
> >  kernel/sched/fair.c | 1181 +++++++++++++++++++++++++++++----------------------
> >  1 file changed, 682 insertions(+), 499 deletions(-)
>
> Thanks, that's an excellent series!
>
> I've queued it up in sched/core with a handful of readability edits to
> comments and changelogs.

Thanks

>
> There are some upstreaming caveats though, I expect this series to be a
> performance regression magnet:
>
>  - load_balance() and wake-up changes invariably are such: some workloads
>    only work/scale well by accident, and if we touch the logic it might
>    flip over into a less advantageous scheduling pattern.
>
>  - In particular the changes from balancing and waking on runnable load
>    to full load that includes blocking *will* shift IO-intensive
>    workloads that you tests don't fully capture I believe. You also made
>    idle balancing more aggressive in essence - which might reduce cache
>    locality for some workloads.
>
> A full run on Mel Gorman's magic scalability test-suite would be super
> useful ...
>
> Anyway, please be on the lookout for such performance regression reports.

Yes I monitor the regressions on the mailing list

>
> Also, we seem to have grown a fair amount of these TODO entries:
>
>   kernel/sched/fair.c: * XXX borrowed from update_sg_lb_stats
>   kernel/sched/fair.c: * XXX: only do this for the part of runnable > running ?
>   kernel/sched/fair.c:     * XXX illustrate
>   kernel/sched/fair.c:    } else if (sd_flag & SD_BALANCE_WAKE) { /* XXX always ? */
>   kernel/sched/fair.c: * can also include other factors [XXX].
>   kernel/sched/fair.c: * [XXX expand on:
>   kernel/sched/fair.c: * [XXX more?]
>   kernel/sched/fair.c: * [XXX write more on how we solve this.. _after_ merging pjt's patches that
>   kernel/sched/fair.c:             * XXX for now avg_load is not computed and always 0 so we
>   kernel/sched/fair.c:            /* XXX broken for overlapping NUMA groups */
>

I will have a look :-)

> :-)
>
> Thanks,
>
>         Ingo