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

[Phil Auld <pauld@redhat.com>]

Hi Vincent,

On Mon, Oct 21, 2019 at 10:44:20AM +0200 Vincent Guittot wrote:
> 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


Our kernel perf tests show good results across the board for v4. 

The issue we hit on the 8-node system is fixed. Thanks!

As we didn't see the fairness issue I don't expect the results to be
that different on v4a (with the followup patch) but those tests are
queued up now and we'll see what they look like. 

Numbers for my specific testcase (the cgroup imbalance) are basically 
the same as I posted for v3 (plus the better 8-node numbers). I.e. this
series solves that issue. 


Cheers,
Phil


> 
> >
> > 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

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