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

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Message-ID: <20130912151102.GB23839@somewhere>
Date:	Thu, 12 Sep 2013 17:11:04 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	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>
Subject: Re: [RFC] Restrict kernel spawning of threads to a specified set of
 cpus.

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.

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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