[RFC PATCH v3 0/6] sched: packing small tasks

From: Vincent Guittot
Date: Fri Mar 22 2013 - 08:29:38 EST


This patchset takes advantage of the new per-task load tracking that is
available in the kernel for packing the small tasks in as few as possible
CPU/Cluster/Core. The main goal of packing small tasks is to reduce the power
consumption in the low load use cases by minimizing the number of power domain
that are enabled. The packing is done in 2 steps:

The 1st step looks for the best place to pack tasks in a system according to
its topology and it defines a pack buddy CPU for each CPU if there is one
available. We define the best CPU during the build of the sched_domain instead
of evaluating it at runtime because it can be difficult to define a stable
buddy CPU in a low CPU load situation. The policy for defining a buddy CPU is
that we pack at all levels inside a node where a group of CPU can be power
gated independently from others. For describing this capability, a new flag
has been introduced SD_SHARE_POWERDOMAIN that is used to indicate whether the
groups of CPUs of a scheduling domain are sharing their power state. By
default, this flag has been set in all sched_domain in order to keep unchanged
the current behavior of the scheduler and only ARM platform clears the
SD_SHARE_POWERDOMAIN flag for MC and CPU level.

In a 2nd step, the scheduler checks the load average of a task which wakes up
as well as the load average of the buddy CPU and it can decide to migrate the
light tasks on a not busy buddy. This check is done during the wake up because
small tasks tend to wake up between periodic load balance and asynchronously
to each other which prevents the default mechanism to catch and migrate them
efficiently. A light task is defined by a runnable_avg_sum that is less than
20% of the runnable_avg_period. In fact, the former condition encloses 2 ones:
The average CPU load of the task must be less than 20% and the task must have
been runnable less than 10ms when it woke up last time in order to be
electable for the packing migration. So, a task than runs 1 ms each 5ms will
be considered as a small task but a task that runs 50 ms with a period of
500ms, will not.
Then, the business of the buddy CPU depends of the load average for the rq and
the number of running tasks. A CPU with a load average greater than 50% will
be considered as busy CPU whatever the number of running tasks is and this
threshold will be reduced by the number of running tasks in order to not
increase too much the wake up latency of a task. When the buddy CPU is busy,
the scheduler falls back to default CFS policy.

Change since V2:

- Migrate only a task that wakes up
- Change the light tasks threshold to 20%
- Change the loaded CPU threshold to not pull tasks if the current number of
running tasks is null but the load average is already greater than 50%
- Fix the algorithm for selecting the buddy CPU.

Change since V1:

Patch 2/6
- Change the flag name which was not clear. The new name is
- Create an architecture dependent function to tune the sched_domain flags
Patch 3/6
- Fix issues in the algorithm that looks for the best buddy CPU
- Use pr_debug instead of pr_info
- Fix for uniprocessor
Patch 4/6
- Remove the use of usage_avg_sum which has not been merged
Patch 5/6
- Change the way the coherency of runnable_avg_sum and runnable_avg_period is
Patch 6/6
- Use the arch dependent function to set/clear SD_SHARE_POWERDOMAIN for ARM

New results for v3:

This series has been tested with hackbench on ARM platform and the results
don't show any performance regression

Hackbench 3.9-rc2 +patches
Mean Time (10 tests): 2.048 2.015
stdev : 0.047 0.068

Previous results for V2:

This series has been tested with MP3 play back on ARM platform:
TC2 HMP (dual CA-15 and 3xCA-7 cluster).

The measurements have been done on an Ubuntu image during 60 seconds of
playback and the result has been normalized to 100.

| CA15 | CA7 | total |
default | 81 | 97 | 178 |
pack | 13 | 100 | 113 |

Previous results for V1:

The patch-set has been tested on ARM platforms: quad CA-9 SMP and TC2 HMP
(dual CA-15 and 3xCA-7 cluster). For ARM platform, the results have
demonstrated that it's worth packing small tasks at all topology levels.

The performance tests have been done on both platforms with sysbench. The
results don't show any performance regressions. These results are aligned with
the policy which uses the normal behavior with heavy use cases.

test: sysbench --test=cpu --num-threads=N --max-requests=R run

Results below is the average duration of 3 tests on the quad CA-9.
default is the current scheduler behavior (pack buddy CPU is -1)
pack is the scheduler with the pack mechanism

| default | pack |
N=8; R=200 | 3.1999 | 3.1921 |
N=8; R=2000 | 31.4939 | 31.4844 |
N=12; R=200 | 3.2043 | 3.2084 |
N=12; R=2000 | 31.4897 | 31.4831 |
N=16; R=200 | 3.1774 | 3.1824 |
N=16; R=2000 | 31.4899 | 31.4897 |

The power consumption tests have been done only on TC2 platform which has got
accessible power lines and I have used cyclictest to simulate small tasks. The
tests show some power consumption improvements.

test: cyclictest -t 8 -q -e 1000000 -D 20 & cyclictest -t 8 -q -e 1000000 -D 20

The measurements have been done during 16 seconds and the result has been
normalized to 100

| CA15 | CA7 | total |
default | 100 | 40 | 140 |
pack | <1 | 45 | <46 |

The A15 cluster is less power efficient than the A7 cluster but if we assume
that the tasks is well spread on both clusters, we can guest estimate that the
power consumption on a dual cluster of CA7 would have been for a default

| CA7 | CA7 | total |
default | 40 | 40 | 80 |

Vincent Guittot (6):
Revert "sched: Introduce temporary FAIR_GROUP_SCHED dependency for
sched: add a new SD_SHARE_POWERDOMAIN flag for sched_domain
sched: pack small tasks
sched: secure access to other CPU statistics
sched: pack the idle load balance

arch/arm/kernel/topology.c | 9 +++
arch/ia64/include/asm/topology.h | 1 +
arch/tile/include/asm/topology.h | 1 +
include/linux/sched.h | 9 +--
include/linux/topology.h | 4 +
kernel/sched/core.c | 14 ++--
kernel/sched/fair.c | 149 +++++++++++++++++++++++++++++++++++---
kernel/sched/sched.h | 14 ++--
8 files changed, 169 insertions(+), 32 deletions(-)


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