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

[Oleg Nesterov <oleg@tv-sign.ru>]

On 04/06, Oleg Nesterov wrote:
> 
> 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.

OK, this doesn't work. A multi-threaded init may do execve(). Can we forbid
exec() from non-main thread? init is special anyway. We can simplify de_thread()
in this case, and kill tasklist_lock around zap_other_threads().

So, we can re-parent a kernel thread to swapper. In that case it doesn't matter
if we put task on ->children list or not.

User-visible change. Acceptable?

> 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.

It would be very nice to introduce CLONE_KERNEL_THREAD instead, then

	--- kernel/fork.c~	2007-04-07 20:11:14.000000000 +0400
	+++ kernel/fork.c	2007-04-07 23:40:35.000000000 +0400
	@@ -1159,7 +1159,8 @@ static struct task_struct *copy_process(
		p->parent_exec_id = p->self_exec_id;
	 
		/* ok, now we should be set up.. */
	-	p->exit_signal = (clone_flags & CLONE_THREAD) ? -1 : (clone_flags & CSIGNAL);
	+	p->exit_signal = (clone_flags & (CLONE_THREAD|CLONE_KERNEL_THREAD))
	+					? -1 : (clone_flags & CSIGNAL);
		p->pdeath_signal = 0;
		p->exit_state = 0;
	 
	@@ -1196,6 +1197,8 @@ static struct task_struct *copy_process(
		/* CLONE_PARENT re-uses the old parent */
		if (clone_flags & (CLONE_PARENT|CLONE_THREAD))
			p->parent = current->parent;
	+	else if (unlikely(clone_flags & CLONE_KERNEL_THREAD))
	+		p->parent = &init_task;
		else
			p->parent = current;

That is all, very simple. However, in that case we should introduce

	#define SYS_CLONE_MASK	(~CLONE_KERNEL_THREAD)

and change every sys_clone() implementation to filter out non-SYS_CLONE_MASK
flags. This is trivial (and imho useful), but

	arch/sparc/kernel/process.c
	arch/sparc64/kernel/process.c
	arch/m68knommu/kernel/process.c
	arch/m68k/kernel/process.c
	arch/alpha/kernel/entry.S
	arch/h8300/kernel/process.c
	arch/v850/kernel/process.c
	arch/frv/kernel/kernel_thread.S
	arch/frv/kernel/kernel_thread.S
	arch/powerpc/kernel/misc_32.S
	arch/powerpc/kernel/misc_64.S

implement kthread_create() via sys_clone(). This means that without additional
effort from mainteners these arches can't take advantage of CLONE_KERNEL_THREAD.

Dear CC list, any advice?

Oleg.