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

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Message-ID: <m1mz1gxky3.fsf@ebiederm.dsl.xmission.com>
Date:	Tue, 10 Apr 2007 10:17:56 -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:
>
>> > 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?
>
> all of it :) Everything you mentioned makes sense quite a bit. The 
> thread signal handling of do_wait was added in a pretty arbitrary 
> fashion so i doubt there are strong requirements in that area. Apps 
> might have grown to get used to it meanwhile though, so we've got to do 
> it carefully.

I'm looking at.  If only because there is a reasonable chance doing this
will fix the races with a threaded init.

However I just found something nasty.  The wait __WNOTHREAD flag.

And my quick search seems to find at least one user space applications
that uses it, and it is widely documented so I suspect there are
others :(

I played with moving the lists into signal_struct, and short of
architecture specific users of task->children all I had to touch
were:

 include/linux/init_task.h |    2 +-
 include/linux/sched.h     |    5 +-
 kernel/exit.c             |  159 +++++++++++++++++++++------------------------
 kernel/fork.c             |    2 +-
 mm/oom_kill.c             |    4 +-

So it should be relatively easy to change this child lists around...

Eric
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