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

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Message-ID: <98552c3b-4cd5-4dfe-8439-e8cc0cd99e39@igalia.com>
Date: Thu, 8 May 2025 15:03:13 +0900
From: Changwoo Min <changwoo@...lia.com>
To: Tejun Heo <tj@...nel.org>
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.

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@...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.

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.
>