linux-kernel - Re: [PATCH -next] cgroup: increase maximum subsystem count from 16 to 32

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Message-ID: <3a12eb16-3a91-4278-9dfd-6c6f424e7f9f@huaweicloud.com>
Date: Thu, 29 Jan 2026 17:51:33 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Michal Koutný <mkoutny@...e.com>
Cc: tj@...nel.org, hannes@...xchg.org, rostedt@...dmis.org,
 mhiramat@...nel.org, mathieu.desnoyers@...icios.com, inwardvessel@...il.com,
 shakeel.butt@...ux.dev, cgroups@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
 lujialin4@...wei.com
Subject: Re: [PATCH -next] cgroup: increase maximum subsystem count from 16 to
 32



On 2026/1/29 17:23, Michal Koutný wrote:
> On Thu, Jan 29, 2026 at 06:31:33AM +0000, Chen Ridong <chenridong@...weicloud.com> wrote:
>> From: Chen Ridong <chenridong@...wei.com>
>>
>> The current cgroup subsystem limit of 16 is insufficient, as the number of
>> subsystems has already reached this maximum.
> 
> Indeed. But some of them are legacy (and some novel). Do you really need
> one kernel image with every subsys config enabled?
> 

We compiled with 'make allmodconfig'.

>> Attempting to add new subsystems beyond this limit results in boot
>> failures.
> 
> That sounds like BUILD_BUG_ON(CGROUP_SUBSYS_COUNT > 16) doesn't trigger
> during build for you. Is the macro broken?
> 

The BUILD_BUG_ON(CGROUP_SUBSYS_COUNT > 16) macro worked correctly. However, I
only modified the code to allow compilation to pass, and the system subsequently
failed to boot.

>> This patch increases the maximum number of supported cgroup subsystems from
>> 16 to 32, providing adequate headroom for future subsystem additions.
> 
> It may be needed one day but I'd suggest binding this change with
> introduction of actual new controller.
> >
> (As we have some CONFIG_*_V1 options that default to N, I'm thinking
> about switching config's default to N as well (like:
> CONFIG_CGROUP_CPUACCT CONFIG_CGROUP_DEVICE CONFIG_CGROUP_FREEZER
> CONFIG_CGROUP_DEBGU), arch/x86/configs/x86_64_defconfig is not exactly
> pinnacle of freshness :-/)
> 
> 

Can I propose increasing the maximum number now? If we switch certain configs to
default N and then a new subsystem is added later, the default configuration may
work fine, but it will become a problem under allmodconfig — which some users
actually rely on.

Besides, this shouldn't be a major change, right?

-- 
Best regards,
Ridong