linux-kernel - Re: [PATCH v4] time/tick-sched: idle load balancing when nohz

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Date: Thu, 16 May 2024 16:23:31 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Yun Levi <ppbuk5246@...il.com>, Joel Fernandes <joel@...lfernandes.org>,
	Vineeth Pillai <vineeth@...byteword.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	anna-maria@...utronix.de, mingo@...nel.org, tglx@...utronix.de,
	Markus.Elfring@....de, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] time/tick-sched: idle load balancing when nohz_full
 cpu becomes idle.

On Thu, May 16, 2024 at 04:00:03PM +0200, Peter Zijlstra wrote:
> > If I make you annoyed I'm sorry in advance but let me clarify please.
> > 
> > 1. In case of none-HK-TICK-housekeeping cpu (a.k.a nohz_full cpu),
> >     It should be on the null_domain. right?
> > 
> > 2. If (1) is true, when none-HK-TICK is set, should it set none-HK-DOMAIN
> >     to prevent on any sched_domain (cpusets filter out none-HK-DOMAIN cpu)?
> > 
> > 3. If (1) is true, Is HK_SCHED still necessary? There seems to be no use case
> >     and the check for this can be replaced by on_null_domain().
> 
> I've no idea about all those HK knobs, it's all insane if you ask me.
> 
> Frederic, afaict all the HK_ goo in kernel/sched/fair.c is total
> nonsense, can you please explain?

Yes. Lemme unearth this patch:
https://lore.kernel.org/all/20230203232409.163847-2-frederic@kernel.org/

Because all we need now is:

_ HK_TYPE_KERNEL_NOISE: nohz_full= or isolcpus=nohz
_ HK_TYPE_DOMAIN: isolcpus=domain (or classic isolcpus= alone)
_ HK_TYPE_MANAGED_IRQ: isolcpus=managed_irq

And that's it. Then let's remove HK_TYPE_SCHED that is unused. And then
lemme comment the HK_TYPE_* uses within sched/* within the same
patchset.

Just a question, correct me if I'm wrong, we don't want nohz_full= to ever
take the idle load balancer duty (this is what HK_TYPE_MISC prevents in
find_new_ilb) because the nohz_full CPU going back to userspace concurrently
doesn't want to be disturbed by a loose IPI telling it to do idle balancing. But
we still want nohz_full CPUs to be part of nohz.idle_cpus_mask so that the
idle balancing can be performed on them by a non isolated CPU. Right?

Thanks.

> 
> If the CPU participates in load-balancing, it gets to fully participate.
> If you want to get out of load-balancing, you get single CPU partitions.