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: <CAJZ5v0i5thgrdGNQ+a1tuw=CGa8PAhHt19GoDpC8KLLjNVoQLQ@mail.gmail.com>
Date:   Thu, 13 Feb 2020 11:17:11 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Amit Kucheria <amit.kucheria@...aro.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 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. :-)

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!