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Date:	Thu, 1 Jul 2010 16:39:57 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Sridhar Samudrala <sri@...ibm.com>,
	Tejun Heo <tj@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
	netdev <netdev@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dmitri Vorobiev <dmitri.vorobiev@...ial.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH repost] sched: export sched_set/getaffinity to modules

On Thu, Jul 01, 2010 at 03:30:24PM +0200, Peter Zijlstra wrote:
> On Thu, 2010-07-01 at 16:08 +0300, Michael S. Tsirkin wrote:
> > On Thu, Jul 01, 2010 at 02:46:35PM +0200, Peter Zijlstra wrote:
> > > On Thu, 2010-07-01 at 14:34 +0200, Peter Zijlstra wrote:
> > > > On Thu, 2010-07-01 at 15:23 +0300, Michael S. Tsirkin wrote:
> > > > > 
> > > > > The patch using this is here:
> > > > > http://www.mail-archive.com/kvm@vger.kernel.org/msg35411.html
> > > > > 
> > > > > It simply copies the affinity from the parent when thread is created.
> > > > 
> > > > Sounds like policy, not something the kernel should do..
> > > 
> > > The alternative would be using clone() instead of thread_create() and
> > > inherit everything from the creating task.
> > > Inheriting from kthreadd and then undoing some aspects just sounds
> > > like daft policy that really ought to be in userspace.
> > 
> > Yes, that's basically what this patchset is trying to do:
> > create a workqueue inheriting everything from the creating task.
> > Sridhar started with an API to do exactly this:
> > http://linux.derkeiler.com/Mailing-Lists/Kernel/2010-05/msg07478.html
> > 
> > Then we switched to raw kthread to avoid stepping on cwq toes.
> > Maybe it makes sense to add kthread_clone (in addition to
> > kthread_create) that would do what you suggest?
> > If yes, any hints on an implementation?
> 
> I think that's called kernel_thread() see
> kernel/kthread.c:create_kthread().
> 
> Doing the whole kthreadd dance and then copying bits and pieces back
> sounds very fragile, so yeah, something like that should work.


Thanks!
Sridhar, Tejun, have the time to look into this approach?

> The other issue to consider is the thread group status of these things,
> I think it would be best if these threads were still considered part of
> the process that spawned them so that they would die nicely when the
> process gets whacked.

The proposed patch kills the thread when the fd is closed,
so I think this already works without making it part of the process.

> At which point one could wonder if the kthread interface makes any
> sense, why not let userspace fork tasks and let them call into the
> kernel to perform work...

One thing I wanted to avoid is letting userspace know
just how many threads are there. We are using a single one
now, but we used to have threads per-cpu, and we might
switch to a thread per virtqueue in the future.
IMO all this should ideally be transparent to userspace.

-- 
MST
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