[Devel] Re: pspace child_reaper

[Cedric Le Goater]

Hello,

Roman Kagan wrote:

[ ... ]

>> As for the per-container init process, the alternative to always
>> enforcing a separate init process for every container is to allow an
>> option of making the process which did the pidspace unshare (or is it
>> the parent of that process) masquerade as (pidspace=new_container, pid=1).
> 
> There's no point enforcing a separate 'init' process in every container.
> The root of the process tree in a namespace has to be the child reaper
> for that namespace meaning that
> 
> - it is immune to signals, ptracing, etc. from within the pidspace
> - every process in the pidspace is reparented to it once that process'
>   parent dies
> - when it dies the whole pidspace is termiated

That's how i feel also.

The key point here is that the process becoming the init of that pidspace
is immune to sigchlg : ignores them or garbage collects them or handles EINTR.

If we feel confortable with the above, let's bring back this question to a
 user space issue : the process doing an unshare of this pidspace must
handle the sigchld one way or the other.

> These are the standard properties of pid == 1 in UNIX.  If it happens to
> be (or execs) /sbin/init then indeed it'll sit in the background
> spawning the usual user processes when necessary, but it doesn't have to
> be.  E.g. I've just run an FC5 machine with init=/usr/bin/python which
> is how your application container would probably look like (the result
> of 'import os; os.system("ps axf")' in python prompt):
> 
>   PID TTY      STAT   TIME COMMAND
>     1 ?        S      0:00 /usr/bin/python
>     2 ?        SN     0:00 [ksoftirqd/0]
>     3 ?        S      0:00 [watchdog/0]
>     4 ?        S<     0:00 [events/0]
>     5 ?        S<     0:00 [khelper]
>     6 ?        S<     0:00 [kthread]
>     8 ?        S<     0:00  \_ [kblockd/0]
>     9 ?        S<     0:00  \_ [kacpid]
>    67 ?        S<     0:00  \_ [khubd]
>   122 ?        S      0:00  \_ [pdflush]
>   123 ?        S      0:00  \_ [pdflush]
>   125 ?        S<     0:00  \_ [aio/0]
>   212 ?        S<     0:00  \_ [kseriod]
>   282 ?        S<     0:00  \_ [kpsmoused]
>   303 ?        S<     0:00  \_ [scsi_eh_0]
>   124 ?        S      0:00 [kswapd0]
>   290 ?        Ss     0:00 /bin/nash /init
>   317 ?        S      0:00 [kjournald]
>   329 ?        R      0:00 sh -c ps axf
>   330 ?        R      0:00  \_ ps axf

yes

> so there's no fundamental difference between "system containers" and
> "application containers".

your example uses python which has a wait() loop sitting somewhere because
it needs to know how to handle processes, like any shell command
interpreter. but yes, it's something like this, with a process 1 knowing
how to handle sigchld.

thanks,

C.

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