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

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Date:	Fri, 6 Apr 2007 19:47:50 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Robin Holt <holt@....com>, Chris Snook <csnook@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: init's children list is long and slows reaping children.

On 04/06, Eric W. Biederman wrote:
>
> Oleg Nesterov <oleg@...sign.ru> writes:
> 
> >> At first glance your patch looks reasonable.
> >> 
> >> Unfortunately it only applies to the rare thread that calls daemonize,
> >> and not also to kernel/kthread/kthread() which means it will miss many of
> >> our current kernel threads.
> >
> > Note that a thread created by kthread_create() has ->parent == "[kthread]",
> > not /sbin/init (unless it was created before core_initcall). So I don't really
> > understand how stop_machine() creates the threads parented to init.
> 
> Duh.  I was looking for that detail but the code was sufficiently
> abstracted that I missed it skimming through.  Now I see it thanks.
> That at least explains how a handful of threads created with
> kthread_create have init as their parent.

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.

> My gut feel says it makes sense to change reparent_to_init to 
> reparent_to_kthread.  Which will get the all of the kernel daemons
> under a single parent.  Further I think it makes sense to see if
> we can start kthread earlier. Having kthread have pid == 2 would be
> nice.  That way we can kill the conditional that is only used during
> early initialization (make the code simpler and more predictable).

Yes. But we can't create helper_wq ([kthread]) before init_workqueues().
We can call helper_init() from init_workqueues() before creating "events"
though.

Oleg.

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