[PATCH v3 1/6] cgroup: Allow registration and lookup of cgroup private data

[Tejun Heo]

Hello, Matt.

cc'ing Roman and Alexei.

On Tue, Mar 06, 2018 at 03:46:55PM -0800, Matt Roper wrote:
> There are cases where other parts of the kernel may wish to store data
> associated with individual cgroups without building a full cgroup
> controller.  Let's add interfaces to allow them to register and lookup
> this private data for individual cgroups.
> 
> A kernel system (e.g., a driver) that wishes to register private data
> for a cgroup will do so by subclassing the 'struct cgroup_priv'
> structure to describe the necessary data to store.  Before registering a
> private data structure to a cgroup, the caller should fill in the 'key'
> and 'free' fields of the base cgroup_priv structure.
> 
>  * 'key' should be a unique void* that will act as a key for future
>    privdata lookups/removals.  Note that this allows drivers to store
>    per-device private data for a cgroup by using a device pointer as a key.
> 
>  * 'free' should be a function pointer to a function that may be used
>    to destroy the private data.  This function will be called
>    automatically if the underlying cgroup is destroyed.

This feature turned out to have more users than I originally
anticipated and bpf also wants something like this to track network
states.  The requirements are pretty similar but not quite the same.
The extra requirements are...

* Lookup must be really cheap.  Probably using pointer hash or walking
  list isn't great, so maybe idr based lookup + RCU protected index
  table per cgroup?

* It should support both regular memory and percpu pointers.  Given
  that what cgroup does is pretty much cgroup:key -> pointer lookup,
  it's mostly about getting the interface right so that it's not too
  error-prone.

Sorry about moving the goalpost.

Thanks.

-- 
tejun