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

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Message-ID: <20240516175321.GN22557@noisy.programming.kicks-ass.net>
Date: Thu, 16 May 2024 19:53:21 +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 05:32:56PM +0200, Frederic Weisbecker wrote:
> On Thu, May 16, 2024 at 05:19:53PM +0200, Peter Zijlstra wrote:

> > 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.
> 
> I like the idea but I'm afraid to introduce regressions while doing so,
> with people currently using nohz_full without proper partionning...

There is no regression, if this is possible today it is utterly broken.

This should never have been possible.