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

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Message-ID: <20180118030441.GB20310@lerouge>
Date:   Thu, 18 Jan 2018 04:04:43 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Luiz Capitulino <lcapitulino@...hat.com>
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 Wed, Jan 17, 2018 at 12:38:01PM -0500, Luiz Capitulino wrote:
> 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.

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!