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

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Date:	Tue, 10 Apr 2007 17:06:50 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Oleg Nesterov <oleg@...sign.ru>, Robin Holt <holt@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Chris Snook <csnook@...hat.com>, linux-kernel@...r.kernel.org,
	Jack Steiner <steiner@...ricas.sgi.com>
Subject: Re: init's children list is long and slows reaping children.


* Eric W. Biederman <ebiederm@...ssion.com> wrote:

> > on a second thought: the p->children list is needed for the whole 
> > child/parent task tree, which is needed for sys_getppid().
> 
> Yes, something Oleg said made me realize that.
> 
> As long as the reparent isn't to complex it isn't required that we 
> have exactly one list .
> 
> > The question is, does anything require us to reparent to within the 
> > same thread group?
> 
> I think my head is finally on straight about this question.
> 
> Currently there is the silly linux specific parent death signal 
> (pdeath_signal).  Oleg's memory was a better than mine on this score.
> 
> However there is no indication that the parent death signal being sent 
> when a thread leader dies is actually correct, or even interesting. It 
> probably should only be sent when getppid changes.
> 
> So with pdeath_signal fixed that is nothing that requires us to 
> reparent within the same thread group.
> 
> I'm trying to remember what the story is now.  There is a nasty race 
> somewhere with reparenting, a threaded parent setting SIGCHLD to 
> SIGIGN, and non-default signals that results in an zombie that no one 
> can wait for and reap.  It requires being reparented twice to trigger.
> 
> Anyway it is a real mess and if we can remove the stupid multi-headed 
> child lists things would become much simpler and the problem could not 
> occur.
> 
> Plus the code would become much simpler...
> 
> utrace appears to have removed the ptrace_children list and the 
> special cases that entailed.

so ... is anyone pursuing this? This would allow us to make sys_wait4() 
faster and more scalable: no tasklist_lock bouncing for example.

	Ingo
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