linux-kernel - Re: [PATCH 00/28] PM: QoS: Get rid of unuseful code and rework CPU latency QoS interface

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Message-ID: <CAP245DUGieaBgN6NCOpjbZtWsX5Bext1S4NqySYXK_H6Go5FgA@mail.gmail.com>
Date:   Thu, 13 Feb 2020 16:19:56 +0530
From:   Amit Kucheria <amit.kucheria@...aro.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 00/28] PM: QoS: Get rid of unuseful code and rework CPU
 latency QoS interface

On Thu, Feb 13, 2020 at 3:47 PM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Thu, Feb 13, 2020 at 8:10 AM Amit Kucheria <amit.kucheria@...aro.org> wrote:
> >
> > On Wed, Feb 12, 2020 at 5:09 AM Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> > >
> > > Hi All,
> > >
> > > This series of patches is based on the observation that after commit
> > > c3082a674f46 ("PM: QoS: Get rid of unused flags") the only global PM QoS class
> > > in use is PM_QOS_CPU_DMA_LATENCY, but there is still a significant amount of
> > > code dedicated to the handling of global PM QoS classes in general.  That code
> > > takes up space and adds overhead in vain, so it is better to get rid of it.
> > >
> > > Moreover, with that unuseful code removed, the interface for adding QoS
> > > requests for CPU latency becomes inelegant and confusing, so it is better to
> > > clean it up.
> > >
> > > Patches [01/28-12/28] do the first part described above, which also includes
> > > some assorted cleanups of the core PM QoS code that doesn't go away.
> > >
> > > Patches [13/28-25/28] rework the CPU latency QoS interface (in the classic
> > > "define stubs, migrate users, change the API proper" manner), patches
> > > [26-27/28] update the general comments and documentation to match the code
> > > after the previous changes and the last one makes the CPU latency QoS depend
> > > on CPU_IDLE (because cpuidle is the only user of its target value today).
> > >
> > > The majority of the patches in this series don't change the functionality of
> > > the code at all (at least not intentionally).
> > >
> > > Please refer to the changelogs of individual patches for details.
> >
> > Hi Rafael,
> >
> > Nice cleanup to the code and docs.
> >
> > I've reviewed the series, and briefly tested it by setting latencies
> > from userspace. Can we not remove the debugfs interface? It is a quick
> > way to check the global cpu latency clamp on the system from userspace
> > without setting up tracepoints or writing a program to read
> > /dev/cpu_dma_latency.
>
> Come on.
>
> What about in Python?
>
> #!/usr/bin/env python
> import numpy as np
>
> if __name__ == '__main__':
>     f = open("/dev/cpu_dma_latency", "r")
>     print(np.fromfile(f, dtype=np.int32, count=1))
>     f.close()
>
> And probably you can do it in at least 20 different ways. :-)

Indeed, I can, just not as straightforward as "cat /debugfs/filename"
when you don't have python or perl in your buildroot initramfs.

Some hexdump/od acrobatics will yield the value, I guess.

> Also note that "echo the_debugfs_thing" does the equivalent, but the
> conversion takes place in the kernel.  Is it really a good idea to
> carry the whole debugfs interface because of that one conversion?
>
> > Except for patch 01/28 removing the debugfs interface, please feel to add my
> >
> > Reviewed-by: Amit Kucheria <amit.kucheria@...aro.org>
> > Tested-by: Amit Kucheria <amit.kucheria@...aro.org>
>
> Thanks!