linux-kernel - Re: [GIT PULL] isolation: 1Hz residual tick offloading v3

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Date:   Wed, 17 Jan 2018 12:38:01 -0500
From:   Luiz Capitulino <lcapitulino@...hat.com>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Lameter <cl@...ux.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Mike Galbraith <efault@....de>, Rik van Riel <riel@...hat.com>
Subject: Re: [GIT PULL] isolation: 1Hz residual tick offloading v3

On Tue, 16 Jan 2018 23:51:29 +0100
Frederic Weisbecker <frederic@...nel.org> wrote:

> On Tue, Jan 16, 2018 at 11:52:11AM -0500, Luiz Capitulino wrote:
> > On Tue, 16 Jan 2018 16:41:00 +0100
> > Frederic Weisbecker <frederic@...nel.org> wrote:  
> > > So isolcpus= is now the place where we control the isolation features
> > > and nohz is one of them.  
> > 
> > That's the part I'm not very sure about. We've been advising users to
> > move away from isolcpus= when possible, but this very wanted nohz_offload
> > feature will force everyone back to using isolcpus= again.  
> 
> Note "isolcpus=nohz" only implies nohz. You need to add "domain" to get
> the behaviour that you've been advising users against. We are simply
> reusing a kernel parameter that was abandoned to now control the isolation
> features that were disorganized and opaque behind nohz.
> 
> > 
> > I have the impression this series is trying to solve two problems:
> > 
> >  1. How (and where) we control the various isolation features in the
> >     kernel  
> 
> No, that has already been done in the previous merge window. We have a
> dedicated isolation subsystem now (kernel/sched/isolation.c) and
> an interface to control all these isolation features that were abusively implied
> by nohz. The initial plan was to introduce "cpu_isolation=" but it looked too much like
> "isolcpus=". Then in fact, why not using "isolcpus=" and give it a second life.
> And there we are.

OK, I get it now. But then series has to un-deprecate isolcpus= otherwise
it doesn't make sense to use it.