[PATCH 0/6] cpufreq: schedutil: fixes for flags updates

Patrick Bellasi
2017-03-02T15:49:26Z

The current version of schedutil has some issues related to the management
of update flags used by systems with frequency domains spawning multiple CPUs.

Each time a CPU utilisation update is issued by the scheduler a set of flags
are configured to define (mainly) which class is asking for a utilisation
update. These flags are then used by the frequency selection policy to
identify the OPP to choose.

In the current implementation, CPU flags are overridden each time the
scheduler calls schedutil for an update. Such a behaviour produces issues
in these scenarios, where we assume CPU1 and CPU2 share the same frequency
domain:
a) a RT task which executed on CPU1 can keep the domain at an high frequency
for a long period of time, even if there are no longer RT tasks on
CPUs in that domain
b) a FAIR task co-scheduled in the same CPU of a RT task can override the
flags configured by the RT task and potentially this can cause an
unwanted frequency drop

These misbehaviours have been verified using a set of simple rt-app based
synthetic workloads, running on a ARM's Juno board, and results are
available in this Notebook [1].

This series proposes a set of fixes for the aforementioned issues as well
as a small improvement to speedup the selection of the maximum frequency
when RT tasks enter a CPU.

This series is based on top of today's tip/sched/core and is public available
from this repository:

git://www.linux-arm.com/linux-pb eas/schedutil/flags_fixes

Cheers Patrick

[1] https://gist.github.com/d6a21b459a18091b2b058668a550010d

Patrick Bellasi (6):
cpufreq: schedutil: reset sg_cpus's flags at IDLE enter
cpufreq: schedutil: ignore the sugov kthread for frequencies
selections
cpufreq: schedutil: ensure max frequency while running RT/DL tasks
cpufreq: schedutil: relax rate-limiting while running RT/DL tasks
cpufreq: schedutil: avoid utilisation update when not necessary
sched/rt: fast switch to maximum frequency when RT tasks are scheduled

include/linux/sched.h | 1 +
kernel/sched/cpufreq_schedutil.c | 59 ++++++++++++++++++++++++++++++++++------
kernel/sched/idle_task.c | 4 +++
kernel/sched/rt.c | 15 ++++++++--
4 files changed, 68 insertions(+), 11 deletions(-)

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2.7.4