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

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Message-ID: <aBuSmITszR9AdoyL@slm.duckdns.org>
Date: Wed, 7 May 2025 07:04:24 -1000
From: Tejun Heo <tj@...nel.org>
To: Changwoo Min <changwoo@...lia.com>
Cc: lukasz.luba@....com, rafael@...nel.org, len.brown@...el.com,
	pavel@...nel.org, christian.loehle@....com, kernel-dev@...lia.com,
	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] PM: EM: Add inotify support when the energy model is
 updated.

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@...lia.com>

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.

Thanks.

-- 
tejun