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

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Message-ID: <20070406200116.GA169@tv-sign.ru>
Date:	Sat, 7 Apr 2007 00:01:16 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Robin Holt <holt@....com>, Chris Snook <csnook@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: init's children list is long and slows reaping children.

On 04/06, Ingo Molnar wrote:
> 
> * Oleg Nesterov <oleg@...sign.ru> wrote:
> 
> > > 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.
> 
> > 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.
> 
> this isnt a kernel-thread special case. The right solution IMO is to 
> first migrate wait4()'s ->children use over to a new p->exiting_children 
> list and then to gradually get rid of all remaining uses of p->children. 
> (the first patch of which i sent a few minutes ago)
> 
> that way wait4() will be sped up, and quite dramatically i believe. No 
> need to deal with kthreads here at all - those just wont ever show up in 
> the ->exiting_children list. Am i missing something?

Probably it is I who missed something :)

But why can't we do both changes? I think it is just ugly to use init to
reap the kernel thread. Ok, wait4() can find zombie quickly if we do the
->children split. But /sbin/init could be swapped out, we still need to
deliver SIGCHLD, etc.

And I personally agree with Linus, it is nice to hide the kernel threads
from /sbin/init (or whatever) completely.

No?

Oleg.

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