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

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Message-Id: <20070409172339.48d661d6.akpm@linux-foundation.org>
Date:	Mon, 9 Apr 2007 17:23:39 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jeff Garzik <jeff@...zik.org>
Cc:	Robin Holt <holt@....com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Jack Steiner <steiner@...ricas.sgi.com>
Subject: Re: init's children list is long and slows reaping children.

On Fri, 06 Apr 2007 18:38:40 -0400
Jeff Garzik <jeff@...zik.org> wrote:

> Robin Holt wrote:
> > We have been testing a new larger configuration and we are seeing a very
> > large scan time of init's tsk->children list.  In the cases we are seeing,
> > there are numerous kernel processes created for each cpu (ie: events/0
> > ... events/<big number>, xfslogd/0 ... xfslogd/<big number>).  These are
> > all on the list ahead of the processes we are currently trying to reap.
> 
> What about attacking the explosion of kernel threads?
> 
> As CPU counts increase, the number of per-CPU kernel threads gets really 
> ridiculous.
> 
> I would rather change the implementation under the hood to start per-CPU 
> threads on demand, similar to a thread-pool implementation.
> 
> Boxes with $BigNum CPUs probably won't ever use half of those threads.

I suspect there are quite a few kernel threads which don't really need to
be threads at all: the code would quite happily work if it was changed to
use keventd, via schedule_work() and friends.  But kernel threads are
somewhat easier to code for.

I also suspect that there are a number of workqueue threads which
could/should have used create_singlethread_workqueue().  Often this is
because the developer just didn't think to do it.

otoh, a lot of these inefficeincies are probably down in scruffy drivers
rather than in core or top-level code.

<I also wonder where all these parented-by-init,
presumably-not-using-kthread kernel threads are coming from>

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