Re: [PATCH v2] PM: EM: Add inotify support when the energy model is updated.

[Changwoo Min]

Hi Tejun,

Thanks for the comments!

On 5/8/25 02:04, Tejun Heo wrote:
> Hello,
>
> On Wed, May 07, 2025 at 10:47:28AM +0900, Changwoo Min wrote:
> > The sched_ext schedulers [1] currently access the energy model through the
> > debugfs to make energy-aware scheduling decisions [2]. The userspace part
> > of a sched_ext scheduler feeds the necessary (post-processed) energy-model
> > information to the BPF part of the scheduler.
> >
> > However, there is a limitation in the current debugfs support of the energy
> > model. When the energy model is updated (em_dev_update_perf_domain), there
> > is no way for the userspace part to know such changes (besides polling the
> > debugfs files).
> >
> > Therefore, add inotify support (IN_MODIFY) when the energy model is updated.
> > With this inotify support, the directory of an updated performance domain
> > (e.g., /sys/kernel/debug/energy_model/cpu0) and its parent directory (e.g.,
> > /sys/kernel/debug/energy_model) are inotified. Therefore, a sched_ext
> > scheduler (or any userspace application) monitors the energy model change
> > in userspace using the regular inotify interface.
> >
> > Note that accessing the energy model information from userspace has many
> > advantages over other alternatives, especially adding new BPF kfuncs. The
> > userspace has much more freedom than the BPF code (e.g., using external
> > libraries and floating point arithmetics), which may be infeasible (if not
> > impossible) in the BPF/kernel code.
> >
> > [1] https://lwn.net/Articles/922405/
> > [2] https://github.com/sched-ext/scx/pull/1624
> >
> > Signed-off-by: Changwoo Min <changwoo@xxxxxxxxxx>
>
> FWIW, this looks simple enough and workable to me. Just a nit below:
>
> > +static void em_debug_update(struct device *dev)
> > +{
> > +	struct dentry *d;
> > +
> > +	d = debugfs_lookup(dev_name(dev), rootdir);
> > +	fsnotify_dentry(d, FS_MODIFY);
> > +}
>
> Would something like em_debug_notify_updated() or em_debug_updated() be
> better? em_debug_update() sounds like it's actively updating something.

I agree that em_debug_update() sounds misleading.
em_debug_notify_updated() delivers clear meaning, so I will change it as
you suggested.

Regards,
Changwoo Min

> Thanks.