linux-kernel - Re: Alternatives to /sys/kernel/debug/wakeup

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Message-ID: <CANA+-vCBW=P=dpJGfcKTt7SoNKzWcpP5pwZHSDMU6MkwBKoC9A@mail.gmail.com>
Date:   Sun, 23 Jun 2019 18:48:43 -0700
From:   Tri Vo <trong@...roid.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Joel Fernandes <joelaf@...gle.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Sandeep Patil <sspatil@...roid.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Hridya Valsaraju <hridya@...gle.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: Alternatives to /sys/kernel/debug/wakeup_sources

On Wed, Jun 19, 2019 at 1:35 AM Rafael J. Wysocki <rafael@...nel.org> wrote:
>
> On Wed, Jun 19, 2019 at 1:52 AM Joel Fernandes <joelaf@...gle.com> wrote:
> >
> > On Tue, Jun 18, 2019 at 7:15 PM Tri Vo <trong@...roid.com> wrote:
> > [snip]
> > > > > > >
> > > > > > > Android userspace reading wakeup_sources is not ideal because:
> > > > > > > - Debugfs API is not stable, i.e. Android tools built on top of it are
> > > > > > > not guaranteed to be backward/forward compatible.
> > > > > > > - This file requires debugfs to be mounted, which itself is
> > > > > > > undesirable for security reasons.
> > > > > > >
> > > > > > > To address these problems, we want to contribute a way to expose these
> > > > > > > statistics that doesn't depend on debugfs.
> > > > > > >
> > > > > > > Some initial thoughts/questions: Should we expose the stats in sysfs?
> > > > > > > Or maybe implement eBPF-based solution? What do you think?
> > > > >
> > > > > We are going through Android's out-of-tree kernel dependencies along with
> > > > > userspace APIs that are not necessarily considered "stable and forever
> > > > > supported" upstream. The debugfs dependencies showed up on our radar as a
> > > > > result and so we are wondering if we should worry about changes in debugfs
> > > > > interface and hence the question(s) below.
> > > > >
> > > > > So, can we rely on /d/wakeup_sources to be considered a userspace API and
> > > > > hence maintained stable as we do for other /proc and /sys entries?
> > > > >
> > > > > If yes, then we will go ahead and add tests for this in LTP or
> > > > > somewhere else suitable.
> > > >
> > > > No, debugfs is not ABI.
> > > >
> > > > > If no, then we would love to hear suggestions for any changes that need to be
> > > > > made or we simply just move the debugfs entry into somewhere like
> > > > > /sys/power/ ?
> > > >
> > > > No, moving that entire file from debugfs into sysfs is not an option either.
> > > >
> > > > The statistics for the wakeup sources associated with devices are already there
> > > > under /sys/devices/.../power/ , but I guess you want all wakeup sources?
> > > >
> > > > That would require adding a kobject to struct wakeup_source and exposing
> > > > all of the statistics as separate attributes under it.  In which case it would be
> > > > good to replace the existing wakeup statistics under /sys/devices/.../power/
> > > > with symbolic links to the attributes under the wakeup_source kobject.
> > >
> > > Thanks for your input, Rafael! Your suggestion makes sense. I'll work
> > > on a patch for this.
> >
> > Does that entail making each wake up source, a new sysfs node under a
> > particular device, and then adding stats under that new node?
>
> Not under a device, because there are wakeup source objects without
> associated devices.
>
> It is conceivable to have a "wakeup_sources" directory under
> /sys/power/ and sysfs nodes for all wakeup sources in there.
>
> Then, instead of exposing wakeup statistics directly under
> /sys/devices/.../power/, there can be symbolic links from there to the
> new wakeup source nodes under "wakeup_sources" (so as to avoid
> exposing the same data in two different places in sysfs, which may be
> confusing).

This may be a dumb question. Is it appropriate to make symbolic links
in sysfs from one attribute to another attribute? For example,
/sys/devices/.../power/wakeup_count ->
/sys/power/wakeup_sources/.../wakeup_count.

I only see kobject to kobject symlinks around. And I don't think we
can make /sys/devices/.../power/ directory a symlink to where our new
wakeup stats will be, since the former contains attributes other than
wakeup ones.

Thanks!