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

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Message-ID: <20130912185107.GV3966@linux.vnet.ibm.com>
Date:	Thu, 12 Sep 2013 11:51:07 -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 08:35:05PM +0200, Frederic Weisbecker wrote:
> On Thu, Sep 12, 2013 at 08:39:22AM -0700, Paul E. McKenney wrote:
> > 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?
> 
> Just offline the CPUs you want to isolate, affine your kthreads and re-online
> the CPUs.
> 
> If you're lucky enough to have 1024 CPUs, a winter night should be enough ;-)

Running at RT prio 99 to reduce the probability of respawns?  ;-)

							Thanx, Paul

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