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

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Date:	Tue, 10 Apr 2007 09:22:16 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Ingo Molnar <mingo@...e.hu>
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.

Ingo Molnar <mingo@...e.hu> writes:

> * 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.

which part?

Eric
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