Re: A Plumber’s Wish List for Linux

[Li Zefan]

Lennart Poettering wrote:
> On Sat, 22.10.11 12:21, Frederic Weisbecker (fweisbec@xxxxxxxxx) wrote:
> 
>> If you really need to stop any forks in a cgroup, then a cgroup core feature
>> handling that very single purpose would be better and more efficient.
> 
> We'd be happy with that and this is what we originally suggested actually.
> 
>> That said I'm not really sure why you're using cgroups in Systemd.
> 
> We want to reliably label processes in a hierarchial way, so that this
> is inherited by all child processes, cannot be overriden by unprivileged
> code (subject to some classic Unix access control handling) and get
> notifications when such a label stops referring to any process. We use
> that for sticking the service name on a process, so that all CGI
> processes of Apache are automatically assigned the same service as
> apache itself. And we want a notification when all of apache's processes
> die. And we also want to be able to kill Apache compeltely by killing
> all its processes.
> 
> cgroups provides us with all of that, though the last two items only in
> a suboptimal way: notification of cgroups running empty is ugly, since
> it is done by spawning a usermode helper (we'd prefer a netlink msg or
> so), and the process killing is a bit racy.
> 

How about using eventfd? You can create an eventfd for the specific "tasks"
file, and when the cgroup gets empty (no task in it), you'll get a notification.

It should be easy to implement, since cgroup already supports eventfd-based
API.
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