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

[Chen Ridong <chenridong@huaweicloud.com>]

On 2026/1/29 17:23, Michal Koutný wrote:
> On Thu, Jan 29, 2026 at 06:31:33AM +0000, Chen Ridong <chenridong@huaweicloud.com> wrote:
>> From: Chen Ridong <chenridong@huawei.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