linux-kernel - [PATCH 0/1] drivers: devfreq: use DELAYED

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Date:   Mon, 27 Jan 2020 15:17:30 +0000
From:   lukasz.luba@....com
To:     myungjoo.ham@...sung.com, kyungmin.park@...sung.com,
        cw00.choi@...sung.com, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     b.zolnierkie@...sung.com, lukasz.luba@....com
Subject: [PATCH 0/1] drivers: devfreq: use DELAYED_WORK in DEVFREQ monitoring subsystem

From: Lukasz Luba <lukasz.luba@....com>

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(-)

-- 
2.17.1