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

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Message-ID: <ZthWKgK9B_AUqSs1@localhost.localdomain>
Date: Wed, 4 Sep 2024 14:44:26 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Waiman Long <longman@...hat.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	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 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...

> 
> 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?

If it is possible, it's interesting to notice that such CPU can become ilb
(as opposed to null domain).

> > 
> > 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.

> 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.

> 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.

Thanks.

> Cheers,
> Longman
>