linux-kernel - Re: [RFC] Restrict kernel spawning of threads to a specified set of cpus.

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Message-ID: <20130913144008.GF21832@twins.programming.kicks-ass.net>
Date:	Fri, 13 Sep 2013 16:40:08 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Gilad Ben-Yossef <gilad@...yossef.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Frysinger <vapier@...too.org>,
	linux-kernel@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC] Restrict kernel spawning of threads to a specified set of
 cpus.

On Fri, Sep 13, 2013 at 01:54:53PM +0000, Christoph Lameter wrote:
> 
> > > If we really want to solve that race, then may be we can think of a kernel_parameter
> >
> > No bloody kernel params. I'd much rather create a pointless kthread to
> > act as usermodehelper parent that people can set context on (move it
> > into cgroups, set affinity, whatever) so it automagically propagates to
> > all userspace helper thingies.
> >
> > Is there anything other than usermodehelper we need to be concerned
> > with? One that comes to mind would be unbound workqueue threads. Do we
> > want to share the parent with usermodehelpers or have these two classes
> > have different parents?
> 
> So you want to keep those silly racy move-all-threads-to-some-cpus scripts
> around? 

No, creating a parent for them closes the race. It should also makes it
lots easier to find the kids by using ppid.

> A kernel parameter would allow a clean bootup with threads
> starting out on the specific processors we want them to.

Blergh, no. A kernel should boot, a kernel should allow you to configure
things, a kernel should not be limited to boot time settings.

> Also there is even more work ahead to deal with things like kswapd,
> writeback threads, compaction and various other scanners that should also
> be restricted. Mostly one thread per node is sufficient. This is not
> simple to do from user space.

IIRC we have one kswapd per node, not sure about the others. And why is
this not simple from userspace? All these are long-running threads and
from a quick look they can have their affinity changed.


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