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

Andrew Morton
2007-04-10T02:31:19Z

On Mon, 9 Apr 2007 21:59:12 -0400 Dave Jones <davej@redhat.com> wrote:

> On Mon, Apr 09, 2007 at 05:23:39PM -0700, Andrew Morton wrote:
>
> > 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.
>
> Perhaps too easy. We have a bunch of kthreads sitting around that afaict,
> are there 'just in case', not because they're actually in use.

This is fun.

> 10 ? S< 0:00 [khelper]

That one's needed to parent the call_usermodehelper() apps. I don't think
it does anything else. We used to use keventd for this but that had some
problem whcih I forget. (Who went and misnamed keventd to "events", too?
Nobody calls it "events", and with good reason)

> Why doesn't this get spawned when it needs to?
>
> 164 ? S< 0:00 [cqueue/0]
> 165 ? S< 0:00 [cqueue/1]
>
> I'm not even sure wth these are.

Me either.

> ...
>

: root 3 0.0 0.0 0 0 ? S 18:51 0:00 [watchdog/0]

That's the softlockup detector. Confusingly named to look like a, err,
watchdog. Could probably use keventd.

: root 5 0.0 0.0 0 0 ? S 18:51 0:00 [khelper]

That's there to parent the kthread_create()d threads. Could presumably use
khelper.

: root 152 0.0 0.0 0 0 ? S 18:51 0:00 [ata/0]

Does it need to be per-cpu?

: root 153 0.0 0.0 0 0 ? S 18:51 0:00 [ata_aux]

That's a single-threaded workqueue handler. Perhaps could use keventd.

: root 299 0.0 0.0 0 0 ? S 18:51 0:00 [scsi_eh_0]
: root 300 0.0 0.0 0 0 ? S 18:51 0:00 [scsi_eh_1]
: root 305 0.0 0.0 0 0 ? S 18:51 0:00 [scsi_eh_2]
: root 306 0.0 0.0 0 0 ? S 18:51 0:00 [scsi_eh_3]

This machine has one CPU, one sata disk and one DVD drive. The above is
hard to explain.

: root 319 0.0 0.0 0 0 ? S 18:51 0:00 [pccardd]

hm.

: root 331 0.0 0.0 0 0 ? S 18:51 0:00 [kpsmoused]

hm.

: root 337 0.0 0.0 0 0 ? S 18:51 0:00 [kedac]

hm. I didn't know that the Vaio had EDAC.

: root 1173 0.0 0.0 0 0 ? S 18:51 0:00 [khpsbpkt]

I can't even pronounce that.

: root 1354 0.0 0.0 0 0 ? S 18:51 0:00 [knodemgrd_0]

OK, I do have 1394 hardware, but it hasn't been used.

: root 1636 0.0 0.0 0 0 ? S 18:52 0:00 [kondemand/0]

I blame davej.

> > otoh, a lot of these inefficeincies are probably down in scruffy drivers
> > rather than in core or top-level code.
>
> You say scruffy, but most of the proliferation of kthreads comes
> from code written in the last few years. Compare the explosion of kthreads
> we see coming from 2.4 to 2.6. It's disturbing, and I don't see it
> slowing down at all.
>
> On the 2-way box I grabbed the above ps output from, I end up with 69 kthreads.
> It doesn't surprise me at all that bigger iron is starting to see issues.
>

Sure.

I don't think it's completely silly to object to all this. Sure, a kernel
thread is worth 4k in the best case, but I bet they have associated unused
resources and as we've seen, they can cause overhead.