This is the first part of the new chapter of the [linux insides](http://0xax.gitbooks.io/linux-insides/content/) book and as you may guess by part's name - this part will cover [control groups](https://en.wikipedia.org/wiki/Cgroups) or `cgroups` mechanism in the Linux kernel.
`Cgroups` are special mechanism provided by the Linux kernel which allows us to allocate kind of `resources` like processor time, number of processes per group, amount of memory per control group or combination of such resources for a process or set of processes. `Cgroups` are organized hierarchically and here this mechanism is similar to usual processes as they are hierarchical too and child `cgroups` inherit set of certain parameters from their parents. But actually they are not the same. The main differences between `cgroups` and normal processes that many different hierarchies of control groups may exist simultaneously in one time while normal process three is always single. This was not a casual step because each control group hierarchy is attached to set of control group `subsystems`.
`Cgroups` are special mechanism provided by the Linux kernel which allows us to allocate kind of `resources` like processor time, number of processes per group, amount of memory per control group or combination of such resources for a process or set of processes. `Cgroups` are organized hierarchically and here this mechanism is similar to usual processes as they are hierarchical too and child `cgroups` inherit set of certain parameters from their parents. But actually they are not the same. The main differences between `cgroups` and normal processes that many different hierarchies of control groups may exist simultaneously in one time while normal process tree is always single. This was not a casual step because each control group hierarchy is attached to set of control group `subsystems`.
One `control group subsystem` represents one kind of resources like a processor time or number of [pids](https://en.wikipedia.org/wiki/Process_identifier) or in other words number of processes for a `control group`. Linux kernel provides support for following twelve `control group subsystems`:
@ -105,7 +105,7 @@ Now let's go to the place where `cgroupfs` is mounted on our computer. As we jus
$ cd /sys/fs/cgroup
```
And now let's go to the `devices` subdirectory which represents kind of resouces that allows or denies access to devices by tasks in a `cgroup`:
And now let's go to the `devices` subdirectory which represents kind of resources that allows or denies access to devices by tasks in a `cgroup`:
```
# cd /devices
@ -137,7 +137,7 @@ For this moment we are interested in `tasks` and `devices.deny` files. The first
# echo "c 5:0 w" > devices.deny
```
Let's go step by step throug this line. The first `c` letter represents type of a device. In our case the `/dev/tty` is `char device`. We can verify this from output of `ls` command:
Let's go step by step through this line. The first `c` letter represents type of a device. In our case the `/dev/tty` is `char device`. We can verify this from output of `ls` command:
```
~$ ls -l /dev/tty
@ -275,7 +275,7 @@ struct cgroup_subsys {
}
```
Where for example `ccs_online` and `ccs_offline` callbacks are called after a cgroup successfully will complet all allocations and a cgroup will be before releasing respectively. The `early_init` flags marks subsystems which may/should be initialized early. The `id` and `name` fields represents unique identifier in the array of registered subsystems for a cgroup and `name` of a subsystem respectively. The last - `root` fields represents pointer to the root of of a cgroup hierarchy.
Where for example `ccs_online` and `ccs_offline` callbacks are called after a cgroup successfully will complete all allocations and a cgroup will be before releasing respectively. The `early_init` flags marks subsystems which may/should be initialized early. The `id` and `name` fields represents unique identifier in the array of registered subsystems for a cgroup and `name` of a subsystem respectively. The last - `root` fields represents pointer to the root of of a cgroup hierarchy.
Of course the `cgroup_subsys` structure bigger and has other fields, but it is enough for now. Now as we got to know important structures related to `cgroups` mechanism, let's return to the `cgroup_init_early` function. Main purpose of this function is to do early initialization of some subsystems. As you already may guess, these `early` subsystems should have `cgroup_subsys->early_init = 1`. Let's look what subsystems may be initialized early.
sachin
7 years ago