[Devel] Re: --freezer option to /bin/restart

[Oren Laadan]

Serge E. Hallyn wrote:
> Quoting Oren Laadan (orenl at cs.columbia.edu):
>>
>> 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 ?

> 
>>> 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.

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