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

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Message-ID: <4492c013-4e59-425a-859f-5f8b30fb922d@linux.alibaba.com>
Date: Mon, 31 Mar 2025 11:48:20 +0800
From: Gao Xiang <hsiangkao@...ux.alibaba.com>
To: Sandeep Dhavale <dhavale@...gle.com>
Cc: kernel-team@...roid.com, linux-kernel@...r.kernel.org,
 linux-erofs mailing list <linux-erofs@...ts.ozlabs.org>,
 Gao Xiang <xiang@...nel.org>, Chao Yu <chao@...nel.org>,
 Yue Hu <zbestahu@...il.com>, Jeffle Xu <jefflexu@...ux.alibaba.com>
Subject: Re: [PATCH v1 1/1] erofs: lazily initialize per-CPU workers and CPU
 hotplug hooks



On 2025/3/31 11:28, Gao Xiang wrote:
> 
> 
> 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.

Ok, ignore this part after more thinking.

Thanks,
Gao Xiang