Re: [PATCH 0/3] cgroup/misc: Add hwcap masks to the misc controller

[Chen Ridong <chenridong@huaweicloud.com>]

On 2025/12/5 14:39, Andrei Vagin wrote:
> On Thu, Dec 4, 2025 at 6:52 PM Chen Ridong <chenridong@huaweicloud.com> wrote:
>>
>>
>>
>> On 2025/12/5 8:58, Andrei Vagin wrote:
>>> This patch series introduces a mechanism to mask hardware capabilities
>>> (AT_HWCAP) reported to user-space processes via the misc cgroup
>>> controller.
>>>
>>> To support C/R operations (snapshots, live migration) in heterogeneous
>>> clusters, we must ensure that processes utilize CPU features available
>>> on all potential target nodes. To solve this, we need to advertise a
>>> common feature set across the cluster. This patchset allows users to
>>> configure a mask for AT_HWCAP, AT_HWCAP2. This ensures that applications
>>> within a container only detect and use features guaranteed to be
>>> available on all potential target hosts.
>>>
>>
>> Could you elaborate on how this mask mechanism would be used in practice?
>>
>> Based on my understanding of the implementation, the parent’s mask is effectively a subset of the
>> child’s mask, meaning the parent does not impose any additional restrictions on its children. This
>> behavior appears to differ from typical cgroup controllers, where children are further constrained
>> by their parent’s settings. This raises the question: is the cgroup model an appropriate fit for
>> this functionality?
> 
> Chen,
> 
> Thank you for the question. I think I was not clear enough in the
> description.
> 
> The misc.mask file works by masking out available features; any feature
> bit set in the mask will not be advertised to processes within that
> cgroup. When a child cgroup is created, its effective mask is  a
> combination of its own mask and its parent's effective mask. This means
> any feature masked by either the parent or the child will be hidden from
> processes in the child cgroup.
> 
> For example:
> - If a parent cgroup masks out feature A (mask=0b001), processes in it
>   won't see feature A.
> - If we create a child cgroup under it and set its mask to hide feature
>   B (mask=0b010), the effective mask for processes in the child cgroup
>   becomes 0b011. They will see neither feature A nor B.
> 

Let me ask some basic questions:

When is the misc.mask typically set? Is it only configured before starting a container (e.g., before
docker run), or can it be adjusted dynamically while processes are already running?

I'm concerned about a potential scenario: If a child process initially has access to a CPU feature,
but then its parent cgroup masks that feature out, could the child process remain unaware of this
change?

Specifically, if a process has already cached or relied on a CPU capability before the mask was
applied, would it continue to assume it has that capability, leading to potential issues if it
attempts to use instructions that are now masked out?

Does such a scenario exist in practice?

> This ensures that a feature hidden by a parent cannot be re-enabled by a
> child. A child can only impose further restrictions by masking out
> additional features. I think this behaviour is well aligned with the cgroup
> model.
> 
> Thanks,
> Andrei

-- 
Best regards,
Ridong