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

[Changwoo Min <changwoo@igalia.com>]

Hi Lukasz and Rafael,

Thank you for the pointers and guidance.

On 5/9/25 19:55, Lukasz Luba wrote:
> Hi Changwoo,
> 
> On 5/7/25 02:47, 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@igalia.com>
>> ---
>>
>> ChangeLog v1 -> v2:
>>    - Change em_debug_update() to only inotify the directory of an updated
>>      performance domain (and its parent directory).
>>    - Move the em_debug_update() call outside of the mutex lock.
>>    - Update the commit message to clarify its motivation and what will be
>>      inotified when updated.
>>
>>   kernel/power/energy_model.c | 12 ++++++++++++
>>   1 file changed, 12 insertions(+)
>>
> 
> I have discussed that with Rafael and we have similar view.
> The EM debugfs is not the right interface for this purpose.
> 
> A better design and mechanism for your purpose would be the netlink
> notification. It is present in the kernel in thermal framework
> and e.g. is used by Intel HFI
> - drivers/thermal/intel/intel_hfi.c
> - drivers/thermal/thermal_netlink.c
> It's able to send to the user space the information from FW about
> the CPUs' efficiency changes, which is similar to this EM modification.


I have considered netlink before. However, I chose the debugfs-inotify
path since it requires fewer changes.

However, if the netlink interface is better for this purpose (I agree
*debugfs* is not ideal), sure let's go with that direction.

> 
> Would you be interested in writing similar mechanism in the EM fwk?

Sure, I will work on it and send another patch set.

> 
> Regards,
> Lukasz
> 
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