Re: --freezer option to /bin/restart

["Serge E. Hallyn" <serue-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

Quoting Oren Laadan (orenl-eQaUEPhvms7ENvBUuze7eA@public.gmane.org):
> 
> 
> Serge E. Hallyn wrote:
> >Quoting Oren Laadan (orenl-eQaUEPhvms7ENvBUuze7eA@public.gmane.org):
> >>
> >>Sukadev Bhattiprolu wrote:
> >>>The help message for --freezer=CGROUP option says:
> >>>
> >>>	freeze tasks in freezer group CGROUP on success
> >>>
> >>>and if this option specifies an existing cgroup, then (and only
> >>>then) is the RESTART_FROZEN flag to sys_restart() set.
> >>>
> >>>Does the --freezer option have to specify an existing cgroup ?
> >>As it is now - yes;
> >>
> >>In theory I would like use -F (--freezer) without an  argument,
> >>to use RESTART_FROZEN without explicitly adding tasks to a
> >>target cgroup.
> >>
> >>The trick is that we need to put all the restarted tasks in the
> >>traget cgroup. The coordinator must remain outside that cgroup,
> >>or it will be frozen as well when restart succeeds.
> >>
> >>This means that restarted tasks (at least the root task) have
> >>to be explicitly added to the cgroup - which is why I require
> >>that the cgroup name be passed.
> >>
> >>This could be avoided if we could add 'restart' to a cgruop and
> >>then after spawning the entire tree, we could remove 'restart'
> >>from that cgroup. I don't think this is possible ?
> >
> >Why not?  You can certainly move to freezer:/restart.XYZ, spawn
> >tasks, then move back to freezer:/.  And since the tasks should
> >hang in sys_restart() until the coordinator calls sys_restart()
> >itself, there should be no race issues.  Do you mean that something
> >else won't work?
> 
> No, I just wasn't thinking right...
> 
> But even in this case, we need some more wisdom: we need to know
> where the coordinator starts (in which cgroup) before moving it
> to a new cgroup, so we can put it back in the original cgroup. Is
> there an API for that ?

Yup - cat /proc/self/cgroup, or I suspect the libcgroup API gives
you a friendlier interface (so you don't have to parse the file
yourself).

> >>>Is there a way to specify that the restarted application belong
> >>>to a new (yet-to-be created) cgroup AND to leave the application
> >>>FROZEN after restart ? Or would we need a new command line option
> >>>and field in 'struct app_restart_args' to specify this ?
> >>This is exactly what --freezer does. Well, alsmot - except for
> >>"yet-to-be created". Do you want 'restart' to create the cgroup
> >>if it doesn't exist ?
> >
> >I thought that was what Suka was asking for.
> 
> If he wants to add it to the 'restart' utility, then sure, with
> a proper argument.
> 
> I'm not sure that creating a control group fits into the mandate
> of a library call/api. Instead, I'd expect the caller to arrange
> for a cgroup to be created.
> 
> Oren.