linux-kernel - Re: [RFC PATCH 12/12] housekeeping: Reimplement isolcpus on housekeeping

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Date:   Mon, 28 Aug 2017 15:31:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Frederic Weisbecker <fweisbec@...il.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        Christoph Lameter <cl@...ux.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Ingo Molnar <mingo@...nel.org>, Mike Galbraith <efault@....de>,
        Rik van Riel <riel@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>
Subject: Re: [RFC PATCH 12/12] housekeeping: Reimplement isolcpus on
 housekeeping

On Mon, Aug 28, 2017 at 03:23:06PM +0200, Frederic Weisbecker wrote:
> On Mon, Aug 28, 2017 at 12:09:57PM +0200, Peter Zijlstra wrote:
> > On Wed, Aug 23, 2017 at 03:51:11AM +0200, Frederic Weisbecker wrote:
> > > We want to centralize the isolation features on the housekeeping
> > > subsystem and scheduler isolation is a significant part of it.
> > > 
> > > While at it, this is a proposition for a reimplementation of isolcpus=
> > > that doesn't involve  scheduler domain isolation. Therefore this
> > > brings a behaviour change: all user tasks inherit init/1 affinity which
> > > avoid the isolcpus= range. But if a task later overrides its affinity
> > > which turns out to intersect an isolated CPU, load balancing may occur
> > > on it.
> > > 
> > > OTOH such a reimplementation that doesn't shortcut scheduler internals
> > > makes a better candidate for an interface extension to cpuset.
> > 
> > Not sure we can do this. It'll break users that rely on the no
> > scheduling thing, that's a well documented part of isolcpus.
> 
> That was my worry :-s  That NULL domain was probably a design mistake and
> I fear we now have to maintain it.

I'm fairly sure that was very intentional. If you want to isolate stuff
you don't want load-balancing. You get the same NULL domain with cpusets
if you disable balancing for a set of CPUs.

Now, I completely hate the isolcpus feature and wish is a speedy death,
but replacing it with something sensible is difficult because cgroups
:-(