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Subject[PATCH v2 00/13] sched: Clean-ups and asymmetric cpu capacity support
Date
Hi,

The scheduler is currently not doing much to help performance on systems with
asymmetric compute capacities (read ARM big.LITTLE). This series improves the
situation with a few tweaks mainly to the task wake-up path that considers
compute capacity at wake-up and not just whether a cpu is idle for these
systems. This gives us consistent, and potentially higher, throughput in
partially utilized scenarios. SMP behaviour and performance should be
unaffected.

Test 0:
for i in `seq 1 10`; \
do sysbench --test=cpu --max-time=3 --num-threads=1 run; \
done \
| awk '{if ($4=="events:") {print $5; sum +=$5; runs +=1}} \
END {print "Average events: " sum/runs}'

Target: ARM TC2 (2xA15+3xA7)

(Higher is better)
tip: Average events: 146.9
patch: Average events: 217.9

Test 1:
perf stat --null --repeat 10 -- \
perf bench sched messaging -g 50 -l 5000

Target: Intel IVB-EP (2*10*2)

tip: 4.861970420 seconds time elapsed ( +- 1.39% )
patch: 4.886204224 seconds time elapsed ( +- 0.75% )

Target: ARM TC2 A7-only (3xA7) (-l 1000)

tip: 61.485682596 seconds time elapsed ( +- 0.07% )
patch: 62.667950130 seconds time elapsed ( +- 0.36% )

More analysis:

Statistics from mixed periodic task workload (rt-app) containing both
big and little task, single run on ARM TC2:

tu = Task utilization big/little
pcpu = Previous cpu big/little
tcpu = This (waker) cpu big/little
dl = New cpu is little
db = New cpu is big
sis = New cpu chosen by select_idle_sibling()
figc = New cpu chosen by find_idlest_*()
ww = wake_wide(task) count for figc wakeups
bw = sd_flag & SD_BALANCE_WAKE (non-fork/exec wake)
for figc wakeups

case tu pcpu tcpu dl db sis figc ww bw
1 l l l 122 68 28 162 161 161
2 l l b 11 4 0 15 15 15
3 l b l 0 252 8 244 244 244
4 l b b 36 1928 711 1253 1016 1016
5 b l l 5 19 0 24 22 24
6 b l b 5 1 0 6 0 6
7 b b l 0 31 0 31 31 31
8 b b b 1 194 109 86 59 59
--------------------------------------------------
180 2497 856 1821

Cases 1-4 + 8 are fine to be served by select_idle_sibling() as both
this_cpu and prev_cpu are suitable cpus for the task. However, as the
figc column reveals, those cases are often served by find_idlest_*()
anyway due to wake_wide() sending the wakeup that way when
SD_BALANCE_WAKE is set on the sched_domains.

Pulling in the wakee_flip patch (dropped in v2) from v1 shifts a
significant share of the wakeups to sis from figc:

case tu pcpu tcpu dl db sis figc ww bw
1 l l l 537 8 537 8 6 6
2 l l b 49 11 32 28 28 28
3 l b l 4 323 322 5 5 5
4 l b b 1 1910 1209 702 458 456
5 b l l 0 5 0 5 1 5
6 b l b 0 0 0 0 0 0
7 b b l 0 32 0 32 2 32
8 b b b 0 198 168 30 13 13
--------------------------------------------------
591 2487 2268 810

Notes:

Active migration of tasks away from small capacity cpus isn't addressed
in this set although it is necessary for consistent throughput in other
scenarios on asymmetric cpu capacity systems.

The infrastructure to enable capacity awareness for arm64 is not provided here
but will be based on Juri's DT bindings patch set [1]. A combined preview
branch is available [2].

[1] https://lkml.org/lkml/2016/6/15/291
[2] git://linux-arm.org/linux-power.git capacity_awareness_v2_arm64_v1

Patch 1-3: Generic fixes and clean-ups.
Patch 4-11: Improve capacity awareness.
Patch 11-12: Arch features for arm to enable asymmetric capacity support.

v2:

- Dropped patch ignoring wakee_flips for pid=0 for now as we can not
distinguish cpu time processing irqs from idle time.

- Dropped disabling WAKE_AFFINE as suggested by Vincent to allow more
scenarios to use fast-path (select_idle_sibling()). Asymmetric wake
conditions adjusted accordingly.

- Changed use of new SD_ASYM_CPUCAPACITY slightly. Now enables
SD_BALANCE_WAKE.

- Minor clean-ups and rebased to more recent tip/sched/core.

v1: https://lkml.org/lkml/2014/5/23/621

Dietmar Eggemann (1):
sched: Store maximum per-cpu capacity in root domain

Morten Rasmussen (12):
sched: Fix power to capacity renaming in comment
sched/fair: Consistent use of prev_cpu in wakeup path
sched/fair: Optimize find_idlest_cpu() when there is no choice
sched: Introduce SD_ASYM_CPUCAPACITY sched_domain topology flag
sched: Enable SD_BALANCE_WAKE for asymmetric capacity systems
sched/fair: Let asymmetric cpu configurations balance at wake-up
sched/fair: Compute task/cpu utilization at wake-up more correctly
sched/fair: Consider spare capacity in find_idlest_group()
sched: Add per-cpu max capacity to sched_group_capacity
sched/fair: Avoid pulling tasks from non-overloaded higher capacity
groups
arm: Set SD_ASYM_CPUCAPACITY for big.LITTLE platforms
arm: Update arch_scale_cpu_capacity() to reflect change to define

arch/arm/include/asm/topology.h | 5 +
arch/arm/kernel/topology.c | 25 ++++-
include/linux/sched.h | 3 +-
kernel/sched/core.c | 21 +++-
kernel/sched/fair.c | 212 +++++++++++++++++++++++++++++++++++-----
kernel/sched/sched.h | 5 +-
6 files changed, 241 insertions(+), 30 deletions(-)

--
1.9.1

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