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Date:	Thu, 01 Jul 2010 15:07:26 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
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, 2010-07-01 at 15:50 +0300, Michael S. Tsirkin wrote:
> On Thu, Jul 01, 2010 at 02:32:43PM +0200, Peter Zijlstra wrote:
> > On Thu, 2010-07-01 at 14:55 +0300, Michael S. Tsirkin wrote:
> > 
> > > >  - why can't it set the kernel thread's affinity too?
> > > 
> > > It can. However: the threads are started internally by the driver
> > > when qemu does an ioctl.  What we want to do is give it a sensible
> > > default affinity. management tool can later tweak it if it wants to.
> > 
> > So have that ioctl return the tid of that new fancy thread and then set
> > its affinity, stuff it in cgroup, whatever you fancy.
> > 
> > > >  - what happens if someone changes the tasks' affinity?
> > > 
> > > We would normally create a cgroup including all internal
> > > tasks, making it easy to find and change affinity for
> > > them all if necessary. 
> > 
> > And to stuff them in a cgroup you also need the tid, at which point it
> > might as well set the affinity from userspace, right?
> 
> We also put it in a cgroup transparently. I think that it's actually
> important to do it on thread creation: if we don't, malicious userspace
> can create large amount of work exceeding the cgroup limits.
> 
> And the same applies so the affinity, right? If the qemu process
> is limited to a set of CPUs, isn't it important to make
> the kernel thread that does work our behalf limited to the same
> set of CPUs?

I'm not sure we have anything like this, but I wouldn't think so, if a
driver creates a kthread and manages to inject tons of work its not
typically limited to whatever limits apply to the task that supplied the
work.

Take the encryption threads for example, those don't run in the context
of whoever provides the data to be encrypted (file,net whatever) and
thus the task responsible could consume heaps more resources than when
it would have to do the encryption itself.

That's how kthreads work.
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