[PATCH v1 1/1] erofs: lazily initialize per-CPU workers and CPU hotplug hooks

[Gao Xiang]

On 2025/3/31 11:14, Sandeep Dhavale wrote:
> Hi Gao,
>> Do we really need to destroy workers on the last mount?
>> it could cause many unnecessary init/uninit cycles.
>>
>> Or your requirement is just to defer per-CPU workers to
>> the first mount?
>>
>> If your case is the latter, I guess you could just call
>> erofs_init_percpu_workers() in z_erofs_init_super().
>>
>>> +{
>>> +     if (atomic_dec_and_test(&erofs_mount_count))
>>
>> So in that case, we won't need erofs_mount_count anymore,
>> you could just add a pcpu_worker_initialized atomic bool
>> to control that.
>>
> Android devices go through suspend and resume cycles aggressively.
> 
> And currently long running traces showed that erofs_workers being
> created and destroyed without active erofs mount.
> Your suggestion is good and could work for devices which do not use
> erofs at all.

> But if erofs is used once (and unmounted later),> we will not destroy the percpu workers.

Is there a real use case in Android like this?  It
would be really useful to write down something in the
commit message.

> 
> Can you please expand a little bit more on your concern
>> it could cause many unnecessary init/uninit cycles.
> Did you mean on the cases where only one erofs fs
> is mounted at time? Just trying to see if there is a better
> way to address your concern.

My concern is that it could slow down the mount time (on
the single mount/unmount) if there are too many CPUs
(especially on the server side.. 96 CPUs or more...)

Or I guess if kworker CPU hotplug is not used at all
for Android if "suspend and resume" latency is really
important, could we just add a mode to always initialize
pcpu kworkers for all possible CPUs.

Thanks,
Gao Xiang