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Date:   Tue, 28 Jun 2022 11:02:40 -0400
From:   "Alex Xu (Hello71)" <alex_y_xu@yahoo.ca>
Subject: CONFIG_ANDROID (was: rcu_sched detected expedited stalls in amdgpu
 after suspend)
To:     paulmck@kernel.org, rcu@vger.kernel.org, urezki@gmail.com,
        uladzislau.rezki@sony.com,
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        Theodore Ts'o <tytso@mit.edu>
Cc:     alexander.deucher@amd.com, christian.koenig@amd.com,
        Xinhui.Pan@amd.com, amd-gfx@lists.freedesktop.org
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Excerpts from Paul E. McKenney's message of June 28, 2022 12:12 am:
> On Mon, Jun 27, 2022 at 09:50:53PM -0400, Alex Xu (Hello71) wrote:
>> Ah, I see. I have selected the default value for=20
>> CONFIG_RCU_EXP_CPU_STALL_TIMEOUT, but that is 20 if ANDROID. I am not=20
>> using Android; I'm not sure there exist Android devices with AMD GPUs.=20
>> However, I have set CONFIG_ANDROID=3Dy in order to use=20
>> ANDROID_BINDER_IPC=3Dm for emulation.
>>=20
>> In general, I think CONFIG_ANDROID is not a reliable method for=20
>> detecting if the kernel is for an Android device; for example, Fedora=20
>> sets CONFIG_ANDROID, but (AFAIK) its kernel is not intended for use with=
=20
>> Android userspace.
>>=20
>> On the other hand, it's not clear to me why the value 20 should be for=20
>> Android only anyways. If, as you say in=20
>> https://lore.kernel.org/lkml/20220216195508.GM4285@paulmck-ThinkPad-P17-=
Gen-1/,
>> it is related to the size of the system, perhaps some other heuristic=20
>> would be more appropriate.
>=20
> It is related to the fact that quite a few Android guys want these
> 20-millisecond short-timeout expedited RCU CPU stall warnings, but no one
> else does.  Not yet anyway.
>=20
> And let's face it, the intent and purpose of CONFIG_ANDROID=3Dy is extrem=
ely
> straightforward and unmistakeable.  So perhaps people not running Android
> devices but wanting a little bit of the Android functionality should do
> something other than setting CONFIG_ANDROID=3Dy in their .config files.  =
Me,
> I am surprised that it took this long for something like this to bite you=
.
>=20
> But just out of curiosity, what would you suggest instead?

Both Debian and Fedora set CONFIG_ANDROID, specifically for binder. If=20
major distro vendors are consistently making this "mistake", then=20
perhaps the problem is elsewhere.

In my own opinion, assuming that binderfs means Android vendor is not a=20
good assumption. The ANDROID help says:

> Enable support for various drivers needed on the Android platform

It doesn't say "Enable only if building an Android device", or "Enable=20
only if you are Google". Isn't the traditional Linux philosophy a=20
collection of pieces to be assembled, without gratuitous hidden=20
dependencies? For example, [0] removes the unnecessary Android=20
dependency, it doesn't block the whole thing with "depends on ANDROID".

It seems to me that the proper way to set some configuration for Android=20
kernels is or should be to ask the Android kernel config maintainers,=20
not to set it based on an upstream kernel option. There is, after all,=20
no CONFIG_FEDORA or CONFIG_UBUNTU or CONFIG_HANNAH_MONTANA.

WireGuard and random also use CONFIG_ANDROID in a similar "proxy" way as=20
rcu, there to see if suspends are "frequent". This seems dubious for the=20
same reasons.

I wonder if it might be time to retire CONFIG_ANDROID: the only=20
remaining driver covered is binder, which originates from Android but=20
is no longer used exclusively on Android systems. Like ufs-qcom, binder=20
is no longer used exclusively on Android devices; it is also used for=20
Android device emulators, which might be used on Android-like mobile=20
devices, or might not.

My understanding is that both Android and upstream kernel developers=20
intend to add no more Android-specific drivers, so binder should be the=20
only one covered for the foreseeable future.

> For that matter, why the private reply?

Mail client issues, not intentional. Lists re-added, plus Android,=20
WireGuard, and random.

Thanks,
Alex.

[0] https://lore.kernel.org/all/20220321151853.24138-1-krzk@kernel.org/