linux-kernel - Re: [PATCH] exit: PR_SET_ANCHOR for marking processes as reapers for child processes

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Date:	Thu, 23 Dec 2010 16:44:59 +0100
From:	Lennart Poettering <mzxreary@...inter.de>
To:	Scott James Remnant <scott@...split.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] exit: PR_SET_ANCHOR for marking processes as reapers
 for  child processes

On Tue, 21.12.10 12:05, Scott James Remnant (scott@...split.com) wrote:

> > PID namespaces primarily provide an independent PID numbering scheme for
> > a subset of processes, i.e. so that identical may PIDs refer to different
> > processes depending on the namespace they are running in. As a side
> > effect this also provides init-like behaviour for processes that aren't
> > the original PID 1 of the operating system. For systemd we are only
> > interested in this side effect, but are not interested at all in the
> > renumbering of processes, and in fact would even really dislike if it
> > happened. That's why PR_SET_ANCHOR is useful: it gives us init-like
> > behaviour without renaming all processes.
> >
> Right, but I don't get why you need this behavior to supervise either
> system or user processes.  You already get all the functionality you
> need to track processes via either cgroups or the proc connector (or a
> combination of both).

Well, we want a clean way to get access to the full siginfo_t of the
SIGCHLD for the main process of a service. the proc connector is awful
and cgroups does not pass siginfo_t's back to userspace, hence the
cleanest way to get this done properly and beautifully is to make the
session systemd a mini-init via PR_SET_ANCHOR, because then the per-user
systemd's and the per-system systemd can use the exact same code to
handle process managment.

> So is this really just about making ps look pretty, as Kay says?

That's a side effect, but for me it's mostly about getting a simple way
to get the SIGCHLDs, focussed on the children of the session manager and
with minimal wakeups.

Lennart

-- 
Lennart Poettering - Red Hat, Inc.
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