linux-kernel - Re: [PATCH 1/3] sched/isolation: Add HK_FLAG_SCHED to nohz

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Message-ID: <Ztr3aP7GId1yoDKx@pavilion.home>
Date: Fri, 6 Sep 2024 14:36:56 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Waiman Long <longman@...hat.com>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] sched/isolation: Add HK_FLAG_SCHED to nohz_full

Le Wed, Sep 04, 2024 at 03:04:45PM +0200, Peter Zijlstra a écrit :
> On Wed, Sep 04, 2024 at 02:44:26PM +0200, Frederic Weisbecker wrote:
> > Le Tue, Sep 03, 2024 at 09:23:53PM -0400, Waiman Long a écrit :
> > > > After discussing with Peter lately, the rules should be:
> > > > 
> > > > 1) If a nohz_full CPU is part of a multi-CPU domain, then it should
> > > >     be part of load balancing. Peter even says that nohz_full should be
> > > >     forbidden in this case, because the tick plays a role in the
> > > >     load balancing.
> > > 
> > > My understand is that most users will use nohz_full together with isolcpus.
> > > So nohz_full CPUs are also isolated and not in a sched domain. There may
> > > still be user setting nohz_full without isolcpus though, but that should be
> > > relatively rare.
> > 
> > Apparently there are users wanting to use isolation along with automatic
> > containers deployments such as kubernetes, which doesn't seem to work
> > well with isolcpus...
> 
> I've been proposing to get rid of isolcpus for at least the last 15
> years or so. There just isn't a good reason to ever use it. We were
> close and then the whole NOHZ_FULL thing came along.
> 
> You can create single CPU partitions using cpusets dynamically.

I'm not sure we could have removed isolcpus= even back then.
It has always been widely used and we would have broke someone's box.

> 
> > > Anyway, all these nohz_full/kernel_nose setting will only apply to CPUs in
> > > isolated cpuset partitions which will not be in a sched domain.
> > > 
> > > > 
> > > > 2) Otherwise, if CPU is not part of a domain or it is the only CPU of all its
> > > >     domains, then it can be out of the load balancing machinery.
> > > I am aware that a single-cpu domain is the same as being isolated with no
> > > load balancing.
> > 
> > By the way is it possible to have a single-cpu domain (sorry I'm a noob here)
> > or do such CPU always end up on a null domain?
> 
> IIRC they always end up with the null domain; but its been a while. It
> simply doesn't make much sense to have a 1 cpu domain. The way the
> topology code works is by always building the full domain tree, and then
> throwing away all levels that do not contribute, and in the 1 cpu case,
> that would be all of them.
> 
> Look for 'degenerate' in kernel/sched/topology.c.

Ok.

> 
> > > > 
> > > > I'm a bit scared about rule 1) because I know there are existing users of
> > > > nohz_full on multi-CPU domains... So I feel a bit trapped.
> > > 
> > > As stated before, this is not a common use case.
> > 
> > Not sure and anyway it's not a forbidden usecase. But this is anyway outside
> > the scope of this patchset.
> 
> Most crucially, it is a completely broken setup. It doesn't actually
> work well.
> 
> Taking it away will force people to fix their broken. That's a good
> thing, no?

I'm all for it but isn't the rule not to break userspace?

> 
> > > The isolcpus boot option is deprecated, as stated in kernel-parameters.txt.
> > 
> > We should undeprecate it, apparently it's still widely used. Perhaps by people
> > who can't afford to use cpusets/cgroups.
> 
> What is the actual problem with using cpusets? At the very least the
> whole nohz_full thing needs to be moved into cpusets so it isn't a fixed
> boot time thing anymore.

Sure that's the plan.

> 
> > > My plan is to deprecate nohz_full as well once we are able to make dynamic
> > > CPU isolation via cpuset works almost as good as isolcpus + nohz_full.
> > 
> > You can't really deprecate such a kernel boot option unfortunately. Believe me
> > I wish we could.
> 
> Why not? As I said, the only thing that's kept it around, and worse,
> made it more popular again, is this nohz_full nonsense. That never
> should've used isolcpus, but that's not something we can do anything
> about now.
> 
> Rigid, boot time only things are teh suck.

I know...

Thanks.