linux-kernel - [PATCH 2/3] nvme: add WQ_PERCPU to alloc

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Message-ID: <20251104112535.139450-3-marco.crivellari@suse.com>
Date: Tue,  4 Nov 2025 12:25:27 +0100
From: Marco Crivellari <marco.crivellari@...e.com>
To: linux-kernel@...r.kernel.org,
	linux-nvme@...ts.infradead.org
Cc: Tejun Heo <tj@...nel.org>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Frederic Weisbecker <frederic@...nel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Marco Crivellari <marco.crivellari@...e.com>,
	Michal Hocko <mhocko@...e.com>,
	Christoph Hellwig <hch@....de>,
	Sagi Grimberg <sagi@...mberg.me>,
	Chaitanya Kulkarni <kch@...dia.com>
Subject: [PATCH 2/3] nvme: add WQ_PERCPU to alloc_workqueue users

Currently if a user enqueue a work item using schedule_delayed_work() the
used wq is "system_wq" (per-cpu wq) while queue_delayed_work() use
WORK_CPU_UNBOUND (used when a cpu is not specified). The same applies to
schedule_work() that is using system_wq and queue_work(), that makes use
again of WORK_CPU_UNBOUND.
This lack of consistentcy cannot be addressed without refactoring the API.

alloc_workqueue() treats all queues as per-CPU by default, while unbound
workqueues must opt-in via WQ_UNBOUND.

This default is suboptimal: most workloads benefit from unbound queues,
allowing the scheduler to place worker threads where they’re needed and
reducing noise when CPUs are isolated.

This change adds a new WQ_PERCPU flag to explicitly request alloc_workqueue()
to be per-cpu when WQ_UNBOUND has not been specified.

This patch continues the effort to refactor worqueue APIs, which has begun
with the change introducing new workqueues and a new alloc_workqueue flag:

commit 128ea9f6ccfb ("workqueue: Add system_percpu_wq and system_dfl_wq")
commit 930c2ea566af ("workqueue: Add new WQ_PERCPU flag")

With the introduction of the WQ_PERCPU flag (equivalent to !WQ_UNBOUND),
any alloc_workqueue() caller that doesn’t explicitly specify WQ_UNBOUND
must now use WQ_PERCPU.

Once migration is complete, WQ_UNBOUND can be removed and unbound will
become the implicit default.

Suggested-by: Tejun Heo <tj@...nel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@...e.com>
---
 drivers/nvme/target/core.c | 5 +++--
 drivers/nvme/target/tcp.c  | 2 +-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/nvme/target/core.c b/drivers/nvme/target/core.c
index 5d7d483bfbe3..ae8e903b55ad 100644
--- a/drivers/nvme/target/core.c
+++ b/drivers/nvme/target/core.c
@@ -1943,12 +1943,13 @@ static int __init nvmet_init(void)
 	if (!nvmet_bvec_cache)
 		return -ENOMEM;

-	zbd_wq = alloc_workqueue("nvmet-zbd-wq", WQ_MEM_RECLAIM, 0);
+	zbd_wq = alloc_workqueue("nvmet-zbd-wq", WQ_MEM_RECLAIM | WQ_PERCPU,
+				 0);
 	if (!zbd_wq)
 		goto out_destroy_bvec_cache;

 	buffered_io_wq = alloc_workqueue("nvmet-buffered-io-wq",
-			WQ_MEM_RECLAIM, 0);
+			WQ_MEM_RECLAIM | WQ_PERCPU, 0);
 	if (!buffered_io_wq)
 		goto out_free_zbd_work_queue;

diff --git a/drivers/nvme/target/tcp.c b/drivers/nvme/target/tcp.c
index 470bf37e5a63..fa7a56626bcc 100644
--- a/drivers/nvme/target/tcp.c
+++ b/drivers/nvme/target/tcp.c
@@ -2198,7 +2198,7 @@ static int __init nvmet_tcp_init(void)
 	int ret;

 	nvmet_tcp_wq = alloc_workqueue("nvmet_tcp_wq",
-				WQ_MEM_RECLAIM | WQ_HIGHPRI, 0);
+				WQ_MEM_RECLAIM | WQ_HIGHPRI | WQ_PERCPU, 0);
 	if (!nvmet_tcp_wq)
 		return -ENOMEM;

-- 
2.51.1