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

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Message-ID: <m1ejmx76fk.fsf@ebiederm.dsl.xmission.com>
Date:	Fri, 06 Apr 2007 11:34:39 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Oleg Nesterov <oleg@...sign.ru>, Robin Holt <holt@....com>,
	Chris Snook <csnook@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org
Subject: Re: init's children list is long and slows reaping children.

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> 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*. 

Oleg is coming from a different case where it was found that exiting kernel
threads were causing problems for nash when nash was run as init in an
initramfs.  While I think that case is likely a user space bug because
nash should check the pid from waidpid before assuming the process it
was waiting for returned.

I do think you can construct valid embedded cases where you can prove
there that among the user space processes certain actions are safe
when they exit that a spare kernel space thread could invalidate.

It just happens that the exit signal of kernel threads just matters on
the other end of system size.

Eric
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