add cgroups notes

docs/linux/containers/cgroups.md

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+# Linux Control Groups v2 ([`cgroups`][cgroups-v2-kernel-docs])
+
+<!-- WIP Notes -->
+- allow to implement metering and limiting on the resources used by processes
+
+- each subsystem has a hierarchy: separate hierarchies for CPU, memory, block I/O, ...
+- hierarchies are independent (the trees for memory, cpu can be different)
+- each process is *in* a "node" in each hierarchy
+- each hierarchy starts with 1 node (the root)
+- each node is a group of processes sharing the same resources
+
+---
+
+`cgroups` are a mechanism to *organize processes hierarchically* and *distribute system resources* along the hierarchy in a controlled and configurable manner.
+
+`cgroups` are largely composed of two parts: the *core* and the *controllers*.
+
+The **cgroup core** is primarily responsible for hierarchically organizing processes.   
+A **cgroup controller** is usually responsible for distributing a specific type of system resource along the hierarchy although there are utility controllers which serve purposes other than resource distribution. 
+
+cgroups form a tree structure and every process in the system belongs to *one and only one* cgroup. All threads of a process belong to the same cgroup.
+
+On creation, all processes are put in the cgroup that the parent process belongs to at the time. A process can be migrated to another cgroup. Migration of a process doesn't affect already existing descendant processes.
+
+Following certain structural constraints, controllers may be enabled or disabled selectively on a cgroup.  
+All controller behaviors are hierarchical - if a controller is enabled on a cgroup, it affects all
+processes which belong to the cgroups consisting the inclusive sub-hierarchy of the cgroup.  
+When a controller is enabled on a nested cgroup, it always restricts the resource distribution further. The
+restrictions set closer to the root in the hierarchy can not be overridden from further away.
+
+## Mounting
+
+Unlike v1, cgroups v2 has a single hierarchy that can be mounted with:
+
+```sh
+mount -t cgroup2 none <mount-point>
+```
+
+> **Note**: The cgroup filesystem has the magic number `0x63677270` ("cgrp")
+
+All controllers which support v2 and are not bound to a v1 hierarchy are automatically bound to the v2 hierarchy and show up at the root.  
+Controllers which are not in active use in the v2 hierarchy can be bound to other hierarchies.  
+This allows mixing v2 hierarchy with the legacy v1 multiple hierarchies in a fully backward compatible way.
+
+[cgroups-v2-kernel-docs]: https://www.kernel.org/doc/Documentation/cgroup-v2.txt "cgroups kernel docs"