Re: A Plumber’s Wish List for Linux

[Frederic Weisbecker <fweisbec@gmail.com>]

On Thu, Oct 20, 2011 at 01:31:11AM +0200, Lennart Poettering wrote:
> On Wed, 19.10.11 16:09, Paul Menage (paul@paulmenage.org) wrote:
> 
> > On Wed, Oct 19, 2011 at 4:03 PM, Lennart Poettering
> > <mzxreary@0pointer.de> wrote:
> > >
> > > For our systemd usecase a cgroup.signal file would not be useful. This
> > > is because we actually kill all members of the service's cgroup plus the
> > > main process of the service, which is usually also in the service's
> > > cgroup but sometimes isn't (for example: when the user logs in, the
> > > whole /sbin/login process ends up in the user's session cgroup, and is
> > > removed from the original service cgroup). Since we want to avoid
> > > killing the main service process twice in the case where it isn't in the
> > > servce cgroup we'd hence prefer to have some fork throttling logic in
> > > place, so that we can kill members flexibly in accordance with these
> > > rules.
> > 
> > By fork-throttling, do you just mean "0 or unlimited", or would you
> > actually want some kind of rate-limited throttling? If the former,
> > than I agree with Frederick that his task counter should solve that
> > problem.
> 
> Given that shutting down some services might involve forking off a few
> things (think: a shell script handling shutdown which forks off a couple
> of shell utilities) we'd want something that is between "from now on no
> forking at all" and "unlimited forking". This could be done in many
> different ways: we'd be happy if we could do time-based rate limiting,
> but we'd also be fine with defining a certain budget of additional forks
> a cgroup can do (i.e. "from now on you can do 50 more forks, then you'll
> get EPERM).

Thinking more about it, you shouldn't use the task counter subsystem for
Systemd. This is a subsystem that may bring some significant overhead
(ie: walk through the entire hierarchy every fork and exit). Doesn't
sound like something suitable for an init process.

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.

That said I'm not really sure why you're using cgroups in Systemd.