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Date:   Wed, 17 Jan 2018 12:38:01 -0500
From:   Luiz Capitulino <lcapitulino@redhat.com>
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     Ingo Molnar <mingo@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Chris Metcalf <cmetcalf@mellanox.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Christoph Lameter <cl@linux.com>,
        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
        Wanpeng Li <kernellwp@gmail.com>,
        Mike Galbraith <efault@gmx.de>, Rik van Riel <riel@redhat.com>
Subject: Re: [GIT PULL] isolation: 1Hz residual tick offloading v3
Message-ID: <20180117123801.11f41f99@redhat.com>
In-Reply-To: <20180116225126.GA32665@lerouge>
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        <20180112141813.32dcc84d@redhat.com>
        <20180116154055.GA27042@lerouge>
        <20180116115211.7fd55c9a@redhat.com>
        <20180116225126.GA32665@lerouge>
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On Tue, 16 Jan 2018 23:51:29 +0100
Frederic Weisbecker <frederic@kernel.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@kernel.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.