Re: [PATCH 0/4] sched/fair: Improve fairness between cfs tasks

[Valentin Schneider <valentin.schneider@arm.com>]

Hi Vincent,

On 14/09/20 11:03, Vincent Guittot wrote:
> 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.
>

Few nitpicks from my end, but no major objections - this looks mostly
sane to me.

> 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(-)