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

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Message-ID: <58c222ab-d974-74ad-e922-fc7dcc7d5294@arm.com>
Date:   Thu, 30 Jan 2020 11:47:57 +0000
From:   Lukasz Luba <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
Subject: Re: [PATCH 0/1] drivers: devfreq: use DELAYED_WORK in DEVFREQ
 monitoring subsystem

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@....com wrote:
> 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(-)
>