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

From: Rafael J. Wysocki
Date: Tue Jun 18 2019 - 17:28:10 EST

On Tuesday, June 18, 2019 10:17:16 PM CEST Sandeep Patil wrote:
> Hi Rafael, Viresh etc.
> On Tue, Jun 11, 2019 at 10:31:16AM -0700, Tri Vo wrote:
> > On Tue, Jun 4, 2019 at 5:23 PM Tri Vo <trong@xxxxxxxxxxx> wrote:
> > >
> > > Hello Rafael,
> > >
> > > Currently, Android reads wakeup sources statistics from
> > > /sys/kernel/debug/wakeup_sources in production environment. This
> > > information is used, for example, to report which wake lock prevents
> > > the device from suspending.
> Android's usage of the 'wakeup_sources' from debugfs can is linked at[1].
> Basically, android's battery stats implementation to plot history for suspend
> blocking wakeup sources over device's boot cycle. This is used both for power
> specific bug reporting but also is one of the stats that will be used towards
> attributing the battery consumption to specific processes over the period of
> time.
> Android depended on the out-of-tree /proc/wakelocks before and now relies on
> wakeup_sources debugfs entry heavily for the aforementioned use cases.
> > >
> > > 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.

> As a side effect, if the entry moves out of debugfs, Android can run without
> mounting debugfs in production that I assume is a good thing.

And really Android developers might have thought about this a bit earlier.