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

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Date:   Tue, 16 Jan 2018 11:52:11 -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 16:41:00 +0100
Frederic Weisbecker <frederic@...nel.org> wrote:

> On Fri, Jan 12, 2018 at 02:18:13PM -0500, Luiz Capitulino wrote:
> > On Thu,  4 Jan 2018 05:25:32 +0100
> > Frederic Weisbecker <frederic@...nel.org> wrote:
> >   
> > > Ingo,
> > > 
> > > Please pull the sched/0hz branch that can be found at:
> > > 
> > > git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
> > > 	sched/0hz
> > > 
> > > HEAD: 9e932b2cc707209febd130978a5eb9f4a943a3f4
> > > 
> > > --
> > > Now that scheduler_tick() has become resilient towards the absence of
> > > ticks, current->sched_class->task_tick() is the last piece that needs
> > > at least 1Hz tick to keep scheduler stats alive.
> > > 
> > > This patchset adds a flag to the isolcpus boot option to offload the
> > > residual 1Hz tick. This way the nohz_full CPUs don't have anymore tick
> > > (assuming nothing else requires it) as their residual 1Hz tick is
> > > offloaded to the housekeepers.
> > > 
> > > For quick testing, say on CPUs 1-7:
> > > 
> > > 	"isolcpus=nohz_offload,domain,1-7"  
> > 
> > Sorry for being very late to this series, but I've a few comments to
> > make (one right now and others in individual patches).
> > 
> > Why are extending isolcpus= given that it's a deprecated interface?
> > Some people have already moved away from isolcpus= now, but with this
> > new feature they will be forced back to using it.  
> 
> I tried to remove isolcpus or at least change the way it works so that its
> effects are reversible (ie: affine the init task instead of isolating domains)
> but that got nacked due to the behaviour's expectations for userspace.
> 
> That's when I realized that kernel parameters are like userspace ABIs,
> they can't be removed easily whether we deprecate them or not.
> 
> Also I needed to be able to control the various isolation features, and
> nohz_full is the wrong place to do that as nohz_full is really just an
> isolation feature like the others, nohz_full= should really just imply
> full dynticks and not watchdog, workqueue or tilegx NAPI isolation...

Yeah, I completely agree with that.

> 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.

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

 2. Where we add the control for the tick offload feature

I think item 1 is too complex to solve right now. IMHO, this series
should focus on item 2. And regarding item 2, I think we have two
choices to make:

 1. Make tick offload a first class citizen by making it default to
    nohz_full=. If there are regressions, we handle them

 2. Add a new option to nohz_full=, like nohz_full=tick_offload

As an avid user of nohz_full I'm dying to see option 1 happening,
but I'm not totally sure what the consequences can be.

Another idea is to add CONFIG_NOHZ_TICK_OFFLOAD as an experimental
feature.

> The complain about isolcpus is the immutable result. I'm thinking about
> making it modifiable to cpuset but I only see two possible solutions:
> 
> - Make the root cpuset modifiable
> - Create a directory called "isolcpus" visible on the first cpuset mount
>   and move all processes there.

So, if we move the control of the tick offload to nohz_full= itself,
we can completely ditch any isolcpus= change in this series.

I think this should give you a great relief :)

> > What about just adding the new functionality to nohz_full=? That is,
> > no new options, just make the tick go away since this has always been
> > what nohz_full= was intended to do?  
> 
> We can, or have isolcpus=nohz to do it, as both do almost the same.
> 
> But I'm afraid about the overhead for people used to nohz_full= once
> they upgrade their kernels and see those workqueues once per second.
> 
> We can still affine those workqueues (in fact the whole unbound workqueue
> mask) outside the nohz_full range. Still current users may be surprised
> about that new overhead on housekeeping CPUs...
>