Re: init's children list is long and slows reaping children.

Oleg Nesterov
2007-04-06T19:36:34Z

On 04/06, Linus Torvalds wrote:
>
> On Fri, 6 Apr 2007, Oleg Nesterov wrote:
> >
> > Oops. I misread stop_machine(), it does kernel_thread(), not kthread_create().
> > So "stopmachine" threads are all re-parented to init when the caller exits.
> > I think it makes sense to set ->exit_state = -1 in stopmachine(), regadless
> > of any other changes.
>
> I agree that "->exit_state = -1" makes sense, but I disagree in that I
> don't think it *matters*.
>
> Most of the problematic kernel threads are long-running (as in "never
> exit"). Things like eventd, khelper, migration-threads etc will have init
> as their parent and never actually exit, so their ->exit_state doesn't
> matter. What matters is that they are on the children list, so when you
> have 1024 CPU's, and init has many thousand of these per-cpu threads as
> its children, then the *user* threads (that do exit) will cause problems!
>
> I'd almost prefer to just not add kernel threads to any parent process
> list *at*all*.

Yes sure, I didn't argue with that. However, "->exit_state = -1" does matter,
we can't detach process unless we make it auto-reap.

Perhaps,

--- t/kernel/exit.c~ 2007-04-06 23:31:31.000000000 +0400
+++ t/kernel/exit.c 2007-04-06 23:31:57.000000000 +0400
@@ -275,10 +275,7 @@ static void reparent_to_init(void)
remove_parent(current);
current->parent = child_reaper(current);
current->real_parent = child_reaper(current);
- add_parent(current);
-
- /* Set the exit signal to SIGCHLD so we signal init on exit */
- current->exit_signal = SIGCHLD;
+ current->exit_signal = -1;

if (!has_rt_policy(current) && (task_nice(current) < 0))
set_user_nice(current, 0);

is enough. init is still our parent (make ps happy), but it can't see us,
we are not on ->children list.

Off course, we also need to add preparent_to_init() to kthread() and
(say) stopmachine(). Or we can create kernel_thread_detached() and
modify callers to use it.

Oleg.