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

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Message-ID: <330276be-f9cb-4263-85f5-20fe2e10cf72@linux.alibaba.com>
Date: Mon, 31 Mar 2025 11:28:14 +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 11:14, 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