linux-kernel - [PATCH 29/33] kthread: Comment on the purpose and placement of kthread_affine

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Message-ID: <20251013203146.10162-30-frederic@kernel.org>
Date: Mon, 13 Oct 2025 22:31:42 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: LKML <linux-kernel@...r.kernel.org>
Cc: Frederic Weisbecker <frederic@...nel.org>,
	Michal Koutný <mkoutny@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bjorn Helgaas <bhelgaas@...gle.com>,
	Catalin Marinas <catalin.marinas@....com>,
	Danilo Krummrich <dakr@...nel.org>,
	"David S . Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Gabriele Monaco <gmonaco@...hat.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Ingo Molnar <mingo@...hat.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Jens Axboe <axboe@...nel.dk>,
	Johannes Weiner <hannes@...xchg.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Marco Crivellari <marco.crivellari@...e.com>,
	Michal Hocko <mhocko@...e.com>,
	Muchun Song <muchun.song@...ux.dev>,
	Paolo Abeni <pabeni@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Phil Auld <pauld@...hat.com>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Shakeel Butt <shakeel.butt@...ux.dev>,
	Simon Horman <horms@...nel.org>,
	Tejun Heo <tj@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vlastimil Babka <vbabka@...e.cz>,
	Waiman Long <longman@...hat.com>,
	Will Deacon <will@...nel.org>,
	cgroups@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-block@...r.kernel.org,
	linux-mm@...ck.org,
	linux-pci@...r.kernel.org,
	netdev@...r.kernel.org
Subject: [PATCH 29/33] kthread: Comment on the purpose and placement of kthread_affine_node() call

It may not appear obvious why kthread_affine_node() is not called before
the kthread creation completion instead of after the first wake-up.

The reason is that kthread_affine_node() applies a default affinity
behaviour that only takes place if no affinity preference have already
been passed by the kthread creation call site.

Add a comment to clarify that.

Reported-by: Peter Zijlstra <peterz@...radead.org>
Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
---
 kernel/kthread.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 4d3cc04e5e8b..d36bdfbd004e 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -453,6 +453,10 @@ static int kthread(void *_create)
 
 	self->started = 1;
 
+	/*
+	 * Apply default node affinity if no call to kthread_bind[_mask]() nor
+	 * kthread_affine_preferred() was issued before the first wake-up.
+	 */
 	if (!(current->flags & PF_NO_SETAFFINITY) && !self->preferred_affinity)
 		kthread_affine_node();
 
-- 
2.51.0