linux-kernel - Re: [PATCH 00/27] cpuset/isolation: Honour kthreads preferred affinity

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Message-ID: <20250620160847.GA1295810@bhelgaas>
Date: Fri, 20 Jun 2025 11:08:47 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Marco Crivellari <marco.crivellari@...e.com>,
	Michal Hocko <mhocko@...e.com>,
	Peter Zijlstra <peterz@...radead.org>, Tejun Heo <tj@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vlastimil Babka <vbabka@...e.cz>, Waiman Long <longman@...hat.com>
Subject: Re: [PATCH 00/27] cpuset/isolation: Honour kthreads preferred
 affinity

On Fri, Jun 20, 2025 at 05:22:41PM +0200, Frederic Weisbecker wrote:
> The kthread code was enhanced lately to provide an infrastructure which
> manages the preferred affinity of unbound kthreads (node or custom
> cpumask) against housekeeping constraints and CPU hotplug events.
> 
> One crucial missing piece is cpuset: when an isolated partition is
> created, deleted, or its CPUs updated, all the unbound kthreads in the
> top cpuset are affine to _all_ the non-isolated CPUs, possibly breaking
> their preferred affinity along the way
> 
> Solve this with performing the kthreads affinity update from cpuset to
> the kthreads consolidated relevant code instead so that preferred
> affinities are honoured.
> 
> The dispatch of the new cpumasks to workqueues and kthreads is performed
> by housekeeping, as per the nice Tejun's suggestion.
> 
> As a welcome side effect, HK_TYPE_DOMAIN then integrates both the set
> from isolcpus= and cpuset isolated partitions. Housekeeping cpumasks are
> now modifyable with specific synchronization. A big step toward making
> nohz_full= also mutable through cpuset in the future.

Is there anything in Documentation/ that covers the "housekeeping"
feature (and isolation in general) and how to use it?  I see a few
mentions in kernel-parameters.txt and kernel-per-CPU-kthreads.rst, but
they are only incidental.

Bjorn