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

[Andrew Morton <akpm@linux-foundation.org>]

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?

> ---
> 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.

Also, neither the changelog not the documentation mention the
loss-of-data problem which might occur if/when the lock is dropped.

The code now appears to be kinda-duplicating functionality which the
seq_file library provides.  Shouldn't this have been
changelogged/commented?  If it was, I wouldn't need to ask the next
question.

Why is it kinda-duplicating seq_file functionality?  Can we strengthen
the seq_file code so this is unnecessary?