Re: [PATCH] fs, proc: Introduce the /proc/<pid>/children entry v2

[Oleg Nesterov <oleg@redhat.com>]

On 12/08, Andrew Morton wrote:
>
> On Fri, 9 Dec 2011 01:28:53 +0400
> Cyrill Gorcunov <gorcunov@gmail.com> wrote:
>
> > On Thu, Dec 08, 2011 at 05:35:35PM +0100, Oleg Nesterov wrote:
> > ...
> > >
> > > However, ->children list is not rcu-safe, this means that even
> > > list_for_each() itself is not safe. Either you need tasklist or
> > > we can probably make it rcu-safe...
> > >
> >
> > Andrew, Oleg, does the below one look more less fine? Note the
> > tasklist_lock is back and it worries me a bit since I imagine
> > one could be endlessly reading some /proc/<pid>/children file
> > increasing contention over this lock on the whole system
> > (regardless the fact that it's take for read only).
>
> It is a potential problem, from the lock-hold point of view and
> also it can cause large scheduling latencies.  What's involved in
> making ->children an rcu-protected list?

At first glance, this doesn't look trivial... forget_original_parent()
abuses ->sibling.

But yes, it is not really nice to hold tasklist_lock here. May be
we can change this code so that every iteration records the reported
task_struct and then tries to continue. This means we should verify
that ->real_parent is still the same under tasklist, but at least
this way we do not hold it throughout.

> > From: Cyrill Gorcunov <gorcunov@openvz.org>
> > Subject: [PATCH] fs, proc: Introduce the /proc/<pid>/children entry v4
> >
> > There is no easy way to make a reverse parent->children chain
> > from arbitrary <pid> (while parent pid is provided in "PPid"
> > field of /proc/<pid>/status).
> >
> > So instead of walking over all pids in the system to figure out which
> > children a task have -- we add explicit /proc/<pid>/children entry,
> > because kernel already has this kind of information but it is not
> > yet exported. This is a first level children, not the whole process
> > tree, neither the process threads are identified with this interface.
>
> The changelog doesn't explain why we want the patch, so there's no
> reason to merge it!  Something to do with c/r, yes?
>
> If so, I guess the feature could/should be configurable.  Probably with
> a CONFIG_PROC_CHILDREN which is selected by CONFIG_CHECKPOINT_RESTORE.
> Which is all getting a bit over the top, but I suppose we must do it.

Heh. This is the rare case when I personally like the new feature ;)
I mean, it looks "obviously useful" to me. If nothing else, it can
help to debug the problems. Probably the tools like pstree can use it.

Personally I'd even prefer /proc/pid/children/ directory (like
/proc/pid/task), but I guess this needs much more complications.

Oleg.