Messages in this thread | | | From | Morten Rasmussen <> | Subject | [PATCHv4 00/12] sched/fair: Migrate 'misfit' tasks on asymmetric capacity systems | Date | Wed, 4 Jul 2018 11:17:38 +0100 |
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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 tasks.
REQUESTS=1000 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 & done
for i in $LITTLES; do sysbench --max-requests=$REQUESTS --test=cpu run \ | grep "total time:" & done wait
Results: 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
Changelog: v4 - 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 SD_ASYM_CPUCAPACITY. - 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@renesas.com>
v3 - 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 migrations. - 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@renesas.com>
v2 - Removed redudant condition in static_key enablement. - Fixed logic flaw in patch #2 reported by Yi Yao <yi.yao@intel.com> - 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@renesas.com>
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 asymmetry sched/core: Disable SD_PREFER_SIBLING on asymmetric cpu capacity domains
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(-)
-- 2.7.4
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