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

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Message-ID: <20240516151953.GM22557@noisy.programming.kicks-ass.net>
Date: Thu, 16 May 2024 17:19:53 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Frederic Weisbecker <frederic@...nel.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 05:02:51PM +0200, Frederic Weisbecker wrote:

> > I'm confused, none of that makes sense. If you're part of a
> > load-balancer, you're part of a load-balancer, no ifs buts or other
> > nonsense.
> > 
> > idle load balancer is no different from regular load balancing.
> > 
> > Fundamentally, you can't disable the tick if you're part of a
> > load-balance group, the load-balancer needs the tick.
> > 
> > The only possible way to use nohz_full is to not be part of a
> > load-balancer, and the only way that is so is by having (lots of) single
> > CPU partitions.
> 
> So you're suggesting that nohz_full should just be part of the whole
> ilb machinery by default (that is, not fiddle with ilb internals) and
> then it's up to CPU partitioning (through cpuset or isolcpus) to disable
> ilb naturally. Right?

Yes, but stronger, as long as the CPU is part of a load-balance domain,
it must not disable the tick while running anything.

that is, NOHZ_FULL must not become active unless it's running on a
single CPU partition.