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

Jeff Garzik
2007-04-10T08:34:09Z

Andrew Morton wrote:
> On Tue, 10 Apr 2007 03:05:56 -0400 Jeff Garzik <jeff@garzik.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