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Subject[PATCH 0/4] sched/fair: Improve fairness between cfs tasks
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When the system doesn't have enough cycles for all tasks, the scheduler
must ensure a fair split of those CPUs cycles between CFS tasks. The
fairness of some use cases can't be solved with a static distribution of
the tasks on the system and requires a periodic rebalancing of the system
but this dynamic behavior is not always optimal and the fair distribution
of the CPU's time is not always ensured.

The patchset improves the fairness by decreasing the constraint for
selecting migratable tasks with the number of failed load balance. This
change enables then to decrease the imbalance threshold because 1st LB
will try to migrate tasks that fully match the imbalance.

Some tests results:

- small 2 x 4 cores arm64 system

hackbench -l (256000/#grp) -g #grp

grp tip/sched/core +patchset improvement
1 1.420(+/- 11.72 %) 1.382(+/-10.50 %) 2.72 %
4 1.295(+/- 2.72 %) 1.218(+/- 2.97 %) 0.76 %
8 1.220(+/- 2.17 %) 1.218(+/- 1.60 %) 0.17 %
16 1.258(+/- 1.88 %) 1.250(+/- 1,78 %) 0.58 %


fairness tests: run always running rt-app threads
monitor the ratio between min/max work done by threads

v5.9-rc1 w/ patchset
9 threads avg 78.3% (+/- 6.60%) 91.20% (+/- 2.44%)
worst 68.6% 85.67%

11 threads avg 65.91% (+/- 8.26%) 91.34% (+/- 1.87%)
worst 53.52% 87.26%

- large 2 nodes x 28 cores x 4 threads arm64 system

The hackbench tests that I usually run as well as the sp.C.x and lu.C.x
tests with 224 threads have not shown any difference with a mix of less
than 0.5% of improvements or regressions.

Vincent Guittot (4):
sched/fair: relax constraint on task's load during load balance
sched/fair: reduce minimal imbalance threshold
sched/fair: minimize concurrent LBs between domain level
sched/fair: reduce busy load balance interval

kernel/sched/fair.c | 7 +++++--
kernel/sched/topology.c | 4 ++--
2 files changed, 7 insertions(+), 4 deletions(-)

--
2.17.1

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