linux-kernel - [PATCH 3/3] drivers/block: WQ_PERCPU added to alloc

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Message-ID: <20250905085141.93357-4-marco.crivellari@suse.com>
Date: Fri,  5 Sep 2025 10:51:41 +0200
From: Marco Crivellari <marco.crivellari@...e.com>
To: linux-kernel@...r.kernel.org,
	linux-block@...r.kernel.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>,
	Jens Axboe <axboe@...nel.dk>
Subject: [PATCH 3/3] drivers/block: WQ_PERCPU added 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 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 patch adds a new WQ_PERCPU flag to explicitly request the use of
the per-CPU behavior. Both flags coexist for one release cycle to allow
callers to transition their calls.

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

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.

All existing users have been updated accordingly.

Suggested-by: Tejun Heo <tj@...nel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@...e.com>
---
 drivers/block/aoe/aoemain.c   | 2 +-
 drivers/block/rbd.c           | 2 +-
 drivers/block/rnbd/rnbd-clt.c | 2 +-
 drivers/block/sunvdc.c        | 2 +-
 drivers/block/virtio_blk.c    | 2 +-
 5 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/block/aoe/aoemain.c b/drivers/block/aoe/aoemain.c
index cdf6e4041bb9..3b21750038ee 100644
--- a/drivers/block/aoe/aoemain.c
+++ b/drivers/block/aoe/aoemain.c
@@ -44,7 +44,7 @@ aoe_init(void)
 {
 	int ret;

-	aoe_wq = alloc_workqueue("aoe_wq", 0, 0);
+	aoe_wq = alloc_workqueue("aoe_wq", WQ_PERCPU, 0);
 	if (!aoe_wq)
 		return -ENOMEM;

diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
index faafd7ff43d6..af0e21149dbc 100644
--- a/drivers/block/rbd.c
+++ b/drivers/block/rbd.c
@@ -7389,7 +7389,7 @@ static int __init rbd_init(void)
 	 * The number of active work items is limited by the number of
 	 * rbd devices * queue depth, so leave @max_active at default.
 	 */
-	rbd_wq = alloc_workqueue(RBD_DRV_NAME, WQ_MEM_RECLAIM, 0);
+	rbd_wq = alloc_workqueue(RBD_DRV_NAME, WQ_MEM_RECLAIM | WQ_PERCPU, 0);
 	if (!rbd_wq) {
 		rc = -ENOMEM;
 		goto err_out_slab;
diff --git a/drivers/block/rnbd/rnbd-clt.c b/drivers/block/rnbd/rnbd-clt.c
index 15627417f12e..b3a0470f9e80 100644
--- a/drivers/block/rnbd/rnbd-clt.c
+++ b/drivers/block/rnbd/rnbd-clt.c
@@ -1809,7 +1809,7 @@ static int __init rnbd_client_init(void)
 		unregister_blkdev(rnbd_client_major, "rnbd");
 		return err;
 	}
-	rnbd_clt_wq = alloc_workqueue("rnbd_clt_wq", 0, 0);
+	rnbd_clt_wq = alloc_workqueue("rnbd_clt_wq", WQ_PERCPU, 0);
 	if (!rnbd_clt_wq) {
 		pr_err("Failed to load module, alloc_workqueue failed.\n");
 		rnbd_clt_destroy_sysfs_files();
diff --git a/drivers/block/sunvdc.c b/drivers/block/sunvdc.c
index 442546b05df8..851763e5dd18 100644
--- a/drivers/block/sunvdc.c
+++ b/drivers/block/sunvdc.c
@@ -1215,7 +1215,7 @@ static int __init vdc_init(void)
 {
 	int err;

-	sunvdc_wq = alloc_workqueue("sunvdc", 0, 0);
+	sunvdc_wq = alloc_workqueue("sunvdc", WQ_PERCPU, 0);
 	if (!sunvdc_wq)
 		return -ENOMEM;

diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
index 7cffea01d868..a5a48f976a20 100644
--- a/drivers/block/virtio_blk.c
+++ b/drivers/block/virtio_blk.c
@@ -1683,7 +1683,7 @@ static int __init virtio_blk_init(void)
 {
 	int error;

-	virtblk_wq = alloc_workqueue("virtio-blk", 0, 0);
+	virtblk_wq = alloc_workqueue("virtio-blk", WQ_PERCPU, 0);
 	if (!virtblk_wq)
 		return -ENOMEM;

-- 
2.51.0