Re: [PATCH 0/1] drivers: devfreq: use DELAYED_WORK in DEVFREQ monitoring subsystem

From: Lukasz Luba
Date: Thu Jan 30 2020 - 06:48:04 EST


Hi Chanwoo, MyungJoo,

Gentle ping. The issue is not only in the devfreq itself,
but also it affects thermal. The devfreq cooling rely on
busy_time and total_time updated by the devfreq monitoring
(in simple_ondemand).
Thermal uses DELAYED_WORK and is more reliable, but uses stale
data from devfreq_dev_stats. It is especially visible when
you have cgroup spanning one cluster. Android uses cgroups
heavily. You can make easily this setup using 'taskset',
run some benchmarks and observe 'devfreq_monitor' traces and
timestamps, i.e. for your exynos-bus.

The patch is really non-invasive and simple. It can be a good starting
point for testing and proposing other solutions.

Regards,
Lukasz

On 1/27/20 3:17 PM, lukasz.luba@xxxxxxx wrote:
From: Lukasz Luba <lukasz.luba@xxxxxxx>

Hi all,

This patch is a continuation of my previous work for fixing DEVFREQ monitoring
subsystem [1]. The issue is around DEFERRABLE_WORK, which uses TIMER_DEFERRABLE
under the hood which will work normally when the system is busy, but will not
cause a CPU to come out of idle and serve the DEVFREQ monitoring requests.

This is especially important in the SMP systems with many CPUs, when the load
balance tries to keep some CPUs idle. The next service request could not be
triggered when the CPU went idle in the meantime.

The DELAYED_WORK is going to be triggered even on an idle CPU. This will allow
to call the DEVFREQ monitoring in reliable intervals. Some of the drivers might
use internal counters to monitor their load, when the DEVFREQ work is not
triggered in a predictable way, these counters might overflow leaving the
device in undefined state.

To observe the difference, the trace output might be used, i.e.

echo 1 > /sys/kernel/debug/tracing/events/devfreq/enable
#your test starts here, i.e. 'sleep 5' or 'dd ' or 'gfxbench'
echo 0 > /sys/kernel/debug/tracing/events/devfreq/enable
cat /sys/kernel/debug/tracing/trace

When there are some registered devfreq drivers, you should see the traces
'devfreq_moniotor' triggered in reliable intervals.

The patch set is based on Chanwoo's devfreq repository and branch
'devfreq-next' [2].

Regards,
Lukasz Luba

[1] https://lkml.org/lkml/2019/2/12/1179
[2] https://git.kernel.org/pub/scm/linux/kernel/git/chanwoo/linux.git/log/?h=devfreq-next


Lukasz Luba (1):
drivers: devfreq: add DELAYED_WORK to monitoring subsystem

drivers/devfreq/Kconfig | 19 +++++++++++++++++++
drivers/devfreq/devfreq.c | 6 +++++-
2 files changed, 24 insertions(+), 1 deletion(-)