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

[Vincent Guittot]

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(-)

-- 
2.7.4