Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 3B9E021742
Date:   Thu, 18 Jan 2018 04:04:43 +0100
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Luiz Capitulino <lcapitulino@redhat.com>
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: <20180118030441.GB20310@lerouge>
References: <1515039937-367-1-git-send-email-frederic@kernel.org>
 <20180112141813.32dcc84d@redhat.com>
 <20180116154055.GA27042@lerouge>
 <20180116115211.7fd55c9a@redhat.com>
 <20180116225126.GA32665@lerouge>
 <20180117123801.11f41f99@redhat.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20180117123801.11f41f99@redhat.com>
User-Agent: Mutt/1.5.24 (2015-08-30)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Wed, Jan 17, 2018 at 12:38:01PM -0500, Luiz Capitulino wrote:
> 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.

Good point. Also I think you convinced me toward just applying that tick offload
on the existing nohz kernel parameter right away, that is, to both existing "nohz_full="
and "isolcpus=nohz".

After all that tick offload is an implementation detail.

Like you said if people complain about a regression, we can still fix it
with a new option. But eventually I doubt this will be needed.

I'll respin with that.

Thanks!