Re: Alternatives to /sys/kernel/debug/wakeup_sources

["Rafael J. Wysocki" <rafael@kernel.org>]

On Mon, Jun 24, 2019 at 2:27 PM Joel Fernandes <joelaf@google.com> wrote:
>
> On Mon, Jun 24, 2019 at 3:37 AM Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> >
> > On Sun, Jun 23, 2019 at 06:48:43PM -0700, Tri Vo wrote:
> > > On Wed, Jun 19, 2019 at 1:35 AM Rafael J. Wysocki <rafael@kernel.org> wrote:
> > > >
> > > > On Wed, Jun 19, 2019 at 1:52 AM Joel Fernandes <joelaf@google.com> wrote:
> > > > >
> > > > > On Tue, Jun 18, 2019 at 7:15 PM Tri Vo <trong@android.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.
> >
> > Why? would you want that?
>
> This sounds like what Rafael suggested (quoted above), right?

I did, basically to avoid exposing the same information in two places
via different code paths.

I tend to forget about this limitation, sorry for the confusion.

That's not a big deal, though, the attributes under
/sys/devices/.../power/ just need to stay the way they are (for
backwards compatibility).