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.