[PATCHv4 00/12] sched/fair: Migrate 'misfit' tasks on asymmetric capacity systems

From: Morten Rasmussen
Date: Wed Jul 04 2018 - 06:21:19 EST

On asymmetric cpu capacity systems (e.g. Arm big.LITTLE) it is crucial
for performance that cpu intensive tasks are aggressively migrated to
high capacity cpus as soon as those become available. The capacity
awareness tweaks already in the wake-up path can't handle this as such
tasks might run or be runnable forever. If they happen to be placed on a
low capacity cpu from the beginning they are stuck there forever while
high capacity cpus may have become available in the meantime.

To address this issue this patch set introduces a new "misfit"
load-balancing scenario in periodic/nohz/newly idle balance which tweaks
the load-balance conditions to ignore load per capacity in certain
cases. Since misfit tasks are commonly running alone on a cpu, more
aggressive active load-balancing is needed too.

The fundamental idea of this patch set has been in Android kernels for a
long time and is absolutely essential for consistent performance on
asymmetric cpu capacity systems.

The patches have been tested on:
1. Arm Juno (r0): 2+4 Cortex A57/A53
2. Hikey960: 4+4 Cortex A73/A53

Test case:
Big cpus are always kept busy. Pin a shorter running sysbench tasks to
big cpus, while creating a longer running set of unpinned sysbench

BIGS="1 2"
LITTLES="0 3 4 5"

# Don't care about the score for those, just keep the bigs busy
for i in $BIGS; do
taskset -c $i sysbench --max-requests=$((REQUESTS / 4)) \
--test=cpu run &>/dev/null &

for i in $LITTLES; do
sysbench --max-requests=$REQUESTS --test=cpu run \
| grep "total time:" &


Single runs with completion time of each task
Juno (tip)
total time: 1.2608s
total time: 1.2995s
total time: 1.5954s
total time: 1.7463s

Juno (misfit)
total time: 1.2575s
total time: 1.3004s
total time: 1.5860s
total time: 1.5871s

Hikey960 (tip)
total time: 1.7431s
total time: 2.2914s
total time: 2.5976s
total time: 1.7280s

Hikey960 (misfit)
total time: 1.7866s
total time: 1.7513s
total time: 1.6918s
total time: 1.6965s

10 run summary (tracking longest running task for each run)
Juno Hikey960
avg max avg max
tip 1.7465 1.7469 2.5997 2.6131
misfit 1.6016 1.6192 1.8506 1.9666

- Added check for empty cpu_map in sd_init().
- Added patch to disable SD_ASYM_CPUCAPACITY for root_domains that don't
observe capacity asymmetry if the system as a whole is asymmetric.
- Added patch to disable SD_PREFER_SIBLING on the sched_domain level below
- Rebased against tip/sched/core.
- Fixed uninitialised variable introduced in update_sd_lb_stats.
- Added patch to do a slight variable initialisation cleanup in update_sd_lb_stats.
- Removed superfluous type changes for temp variables assigned to root_domain->overload.
- Reworded commit for the patch setting rq->rd->overload when misfit.
- v3 Tested-by: Gaku Inami <gaku.inami.xh@xxxxxxxxxxx>

- Fixed locking around static_key.
- Changed group per-cpu capacity comparison to be based on max rather
than min capacity.
- Added patch to prevent occasional pointless high->low capacity
- Changed type of group_misfit_task_load and misfit_task_load to
unsigned long.
- Changed fbq() to pick the cpu with highest misfit_task_load rather
than breaking when the first is found.
- Rebased against tip/sched/core.
- v2 Tested-by: Gaku Inami <gaku.inami.xh@xxxxxxxxxxx>

- Removed redudant condition in static_key enablement.
- Fixed logic flaw in patch #2 reported by Yi Yao <yi.yao@xxxxxxxxx>
- Dropped patch #4 as although the patch seems to make sense no benefit
has been proven.
- Dropped root_domain->overload renaming
- Changed type of root_domain->overload to int
- Wrapped accesses of rq->rd->overload with READ/WRITE_ONCE
- v1 Tested-by: Gaku Inami <gaku.inami.xh@xxxxxxxxxxx>

Chris Redpath (1):
sched/fair: Don't move tasks to lower capacity cpus unless necessary

Morten Rasmussen (6):
sched: Add static_key for asymmetric cpu capacity optimizations
sched/fair: Add group_misfit_task load-balance type
sched: Add sched_group per-cpu max capacity
sched/fair: Consider misfit tasks when load-balancing
sched/core: Disable SD_ASYM_CPUCAPACITY for root_domains without
sched/core: Disable SD_PREFER_SIBLING on asymmetric cpu capacity

Valentin Schneider (5):
sched/fair: Kick nohz balance if rq->misfit_task_load
sched/fair: Change prefer_sibling type to bool
sched: Change root_domain->overload type to int
sched: Wrap rq->rd->overload accesses with READ/WRITE_ONCE
sched/fair: Set rq->rd->overload when misfit

kernel/sched/fair.c | 161 +++++++++++++++++++++++++++++++++++++++++-------
kernel/sched/sched.h | 16 +++--
kernel/sched/topology.c | 53 ++++++++++++++--
3 files changed, 199 insertions(+), 31 deletions(-)