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

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Message-Id: <20070410003702.f8a49b75.akpm@linux-foundation.org>
Date:	Tue, 10 Apr 2007 00:37:02 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Jeff Garzik <jeff@...zik.org>
Cc:	Dave Jones <davej@...hat.com>, 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 Tue, 10 Apr 2007 03:05:56 -0400 Jeff Garzik <jeff@...zik.org> wrote:

> My main 
> worry with keventd is that we might get stuck behind an unrelated 
> process for an undefined length of time.

I don't think it has ever been demonstrated that keventd latency is
excessive, or a problem.  I guess we could instrument it and fix stuff
easily enough.

The main problem with keventd has been flush_scheduled_work() deadlocks: the
caller of flush_scheduled_work() wants to flush work item A, but holds some
lock which is also needed by unrelated work item B.  Most of the time, it
works.  But if item B happens to be queued the flush_scheduled_work() will
deadlock.

The fix is to flush-and-cancel just item A: if it's not running yet, cancel
it.  If it is running, wait until it has finished.  Oleg's

	void cancel_work_sync(struct work_struct *work)

is queued for 2.6.22 and should permit some kthread->keventd conversions
which would previously been deadlocky.

The thing to concentrate on here is the per-cpu threads, which is where the
proliferation appears to be coming from.  Conversions to
schedule_work()+cancel_work_sync() and conversions to
create_singlethread_workqueue().

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