linux-kernel - Re: [PATCH 11/20] sched: Handle CPU isolation on last resort fallback rq selection

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Message-ID: <ZwUlTiDpvmb9ysfn@localhost.localdomain>
Date: Tue, 8 Oct 2024 14:27:58 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Will Deacon <will@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Michal Hocko <mhocko@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>,
	Uladzislau Rezki <urezki@...il.com>, rcu@...r.kernel.org,
	Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH 11/20] sched: Handle CPU isolation on last resort
 fallback rq selection

Le Tue, Oct 08, 2024 at 11:54:35AM +0100, Will Deacon a écrit :
> On Fri, Sep 27, 2024 at 12:48:59AM +0200, Frederic Weisbecker wrote:
> > When a kthread or any other task has an affinity mask that is fully
> > offline or unallowed, the scheduler reaffines the task to all possible
> > CPUs as a last resort.
> > 
> > This default decision doesn't mix up very well with nohz_full CPUs that
> > are part of the possible cpumask but don't want to be disturbed by
> > unbound kthreads or even detached pinned user tasks.
> > 
> > Make the fallback affinity setting aware of nohz_full. This applies to
> > all architectures supporting nohz_full except arm32. However this
> > architecture that overrides the task possible mask is unlikely to be
> > willing to integrate new development.
> 
> I'm not sure I understand this last sentence. The possible mask is
> overridden for 32-bit tasks on an *arm64* kernel when running on an SoC
> featuring some CPUs that can execute only 64-bit tasks. Who is unwilling
> to integrate what?

Right I've been lazy on that, assuming that nohz_full is a niche, and
nohz_full on arm 32 bits tasks must be even more a niche. But I can make
it a macro just like task_cpu_possible_mask() so that architectures
can override it?

Thanks.