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

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Date:	Thu, 12 Sep 2013 08:39:22 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Christoph Lameter <cl@...ux.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
Subject: Re: [RFC] Restrict kernel spawning of threads to a specified set of
 cpus.

On Thu, Sep 12, 2013 at 05:11:04PM +0200, Frederic Weisbecker wrote:
> On Thu, Sep 12, 2013 at 02:52:56PM +0000, Christoph Lameter wrote:
> > On Thu, 12 Sep 2013, Frederic Weisbecker wrote:
> > 
> > > > > Ok but you can change the affinity of a kthread from userspace, as
> > > > > long as you define a cpu set that is among that kthread's cpus allowed.
> > > >
> > > > Ok but at that point kthread has already spawned a lot of kernel threads.
> > > >
> > > > The same is true for init and kmod.
> > > >
> > >
> > > Ok but then we just need to set the affinity of all these kthreads.
> > > A simple lookup on /proc/[0-9]+/ should do the trick.
> > 
> > Yea but the kernel option makes it easy. No extras needed. Kernel brings
> > it up user space cleanly configured and ready to go.
> 
> Ok but really that's just two lines of bash. I really wish we don't complicate
> core kernel code for that.

OK, I will bite...  How do you handle the case where you have collected
all the kthreads, one of the kthreads spawns another kthread, then you
set affinity on the collected kthreads, which does not include the newly
spawned one?

							Thanx, Paul

> I think we all agree that the big issue here is that CPU isolation requires to set up
> a fragmented set of features and it's not at all obvious to do it correctly: full dynticks,
> rcu nocbs, kthreads affinity, timer_list, hrtimers, workqueues, IPIs, etc...
> 
> So IMHO what is missing is a reliable userspace tool that can handle all that: do
> the checks on pre-requirements, handle the kthreads and even user task affinity, tweak
> some sysctl stuffs to turn off features that generate noise, etc...
> 
> > This also allows us to cleanup kernel uses of cpumasks in such a way that
> > proper thread placement for various other uses (reclaim f.e. kswpad) is
> > possible.
> 
> Same here, a central tool should be able to solve that.
> 

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