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

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Date:	Tue, 10 Apr 2007 04:33:57 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Andrew Morton <akpm@...ux-foundation.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.

Andrew Morton wrote:
> 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.

It's simple math, combined with user expectations.

On a 1-CPU or 2-CPU box, if you have three or more tasks, all of which 
are doing hardware reset tasks that could take 30-60 seconds (realistic 
for libata, SCSI and network drivers, at least), then OBVIOUSLY you have 
other tasks blocked for that length of time.

Since the cause of the latency is msleep() -- the entire reason why the 
driver wanted to use a kernel thread in the first place -- it would seem 
to me that the simple fix is to start a new thread, possibly exceeding 
the number of CPUs in the box.

> The main problem with keventd has been flush_scheduled_work() deadlocks: the

That's been a problem in the past, yes, but a minor one.

I'm talking about a key conceptual problem with keventd.

It is easy to see how an extra-long tg3 hardware reset might prevent a 
disk hotplug event from being processed for 30-60 seconds.  And as 
hardware gets more complex -- see the Intel IOP storage card which runs 
Linux -- the reset times get longer, too.

So the issue is /not/ deadlocks.

> The thing to concentrate on here is the per-cpu threads, which is where the
> proliferation appears to be coming from.

Strongly agreed.

	Jeff

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