[PATCH v2 0/1] wifi: libertas: add WQ_UNBOUND to alloc_workqueue users

[PATCH v2 0/1] wifi: libertas: add WQ_UNBOUND to alloc_workqueue users

[Marco Crivellari]

Hi!

Below is a summary of a discussion about the Workqueue API and cpu isolation
considerations. Details and more information are available here:

        "workqueue: Always use wq_select_unbound_cpu() for WORK_CPU_UNBOUND."
        https://lore.kernel.org/all/20250221112003.1dSuoGyc@linutronix.de/

=== Current situation: problems ===

Let's consider a nohz_full system with isolated CPUs: wq_unbound_cpumask is
set to the housekeeping CPUs, for !WQ_UNBOUND the local CPU is selected.

This leads to different scenarios if a work item is scheduled on an isolated
CPU where "delay" value is 0 or greater then 0:
        schedule_delayed_work(, 0);

This will be handled by __queue_work() that will queue the work item on the
current local (isolated) CPU, while:

        schedule_delayed_work(, 1);

Will move the timer on an housekeeping CPU, and schedule the work there.

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.

=== Plan and future plans ===

This patchset is the first stone on a refactoring needed in order to
address the points aforementioned; it will have a positive impact also
on the cpu isolation, in the long term, moving away percpu workqueue in
favor to an unbound model.

These are the main steps:
1)  API refactoring (that this patch is introducing)
    -   Make more clear and uniform the system wq names, both per-cpu and
        unbound. This to avoid any possible confusion on what should be
        used.

    -   Introduction of WQ_PERCPU: this flag is the complement of WQ_UNBOUND,
        introduced in this patchset and used on all the callers that are not
        currently using WQ_UNBOUND.

        WQ_UNBOUND will be removed in a future release cycle.

        Most users don't need to be per-cpu, because they don't have
        locality requirements, because of that, a next future step will be
        make "unbound" the default behavior.

2)  Check who really needs to be per-cpu
    -   Remove the WQ_PERCPU flag when is not strictly required.

3)  Add a new API (prefer local cpu)
    -   There are users that don't require a local execution, like mentioned
        above; despite that, local execution yeld to performance gain.

        This new API will prefer the local execution, without requiring it.

=== Introduced Changes by this series ===

1) [P 1] add WQ_UNBOUND to alloc_workqueue() users

        Every alloc_workqueue() caller should use one among WQ_PERCPU or
        WQ_UNBOUND.

        WQ_UNBOUND will be removed in a next release cycle.

Thanks!

---
Changes in v2:
- libertas: WQ_PERCPU changed with WQ_UNBOUND

Marco Crivellari (1):
  wifi: libertas: WQ_UNBOUND added to alloc_workqueue users

 drivers/net/wireless/marvell/libertas/if_sdio.c | 3 ++-
 drivers/net/wireless/marvell/libertas/if_spi.c  | 3 ++-
 drivers/net/wireless/marvell/libertas_tf/main.c | 2 +-
 3 files changed, 5 insertions(+), 3 deletions(-)

-- 
2.51.0

[PATCH v2 1/1] wifi: libertas: WQ_UNBOUND added to alloc_workqueue users

[Marco Crivellari]

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 explicitly add the WQ_UNBOUND flag to alloc_workqueue() users,
to mark the workqueue unbound.

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.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
---
 drivers/net/wireless/marvell/libertas/if_sdio.c | 3 ++-
 drivers/net/wireless/marvell/libertas/if_spi.c  | 3 ++-
 drivers/net/wireless/marvell/libertas_tf/main.c | 2 +-
 3 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/net/wireless/marvell/libertas/if_sdio.c b/drivers/net/wireless/marvell/libertas/if_sdio.c
index 524034699972..fc5318035822 100644
--- a/drivers/net/wireless/marvell/libertas/if_sdio.c
+++ b/drivers/net/wireless/marvell/libertas/if_sdio.c
@@ -1181,7 +1181,8 @@ static int if_sdio_probe(struct sdio_func *func,
 	spin_lock_init(&card->lock);
 	INIT_LIST_HEAD(&card->packets);
 
-	card->workqueue = alloc_workqueue("libertas_sdio", WQ_MEM_RECLAIM, 0);
+	card->workqueue = alloc_workqueue("libertas_sdio",
+					  WQ_MEM_RECLAIM | WQ_UNBOUND, 0);
 	if (unlikely(!card->workqueue)) {
 		ret = -ENOMEM;
 		goto err_queue;
diff --git a/drivers/net/wireless/marvell/libertas/if_spi.c b/drivers/net/wireless/marvell/libertas/if_spi.c
index b722a6587fd3..8a2504a62840 100644
--- a/drivers/net/wireless/marvell/libertas/if_spi.c
+++ b/drivers/net/wireless/marvell/libertas/if_spi.c
@@ -1153,7 +1153,8 @@ static int if_spi_probe(struct spi_device *spi)
 	priv->fw_ready = 1;
 
 	/* Initialize interrupt handling stuff. */
-	card->workqueue = alloc_workqueue("libertas_spi", WQ_MEM_RECLAIM, 0);
+	card->workqueue = alloc_workqueue("libertas_spi",
+					  WQ_MEM_RECLAIM | WQ_UNBOUND, 0);
 	if (!card->workqueue) {
 		err = -ENOMEM;
 		goto remove_card;
diff --git a/drivers/net/wireless/marvell/libertas_tf/main.c b/drivers/net/wireless/marvell/libertas_tf/main.c
index d1067874428f..fb20fe31cd36 100644
--- a/drivers/net/wireless/marvell/libertas_tf/main.c
+++ b/drivers/net/wireless/marvell/libertas_tf/main.c
@@ -708,7 +708,7 @@ EXPORT_SYMBOL_GPL(lbtf_bcn_sent);
 static int __init lbtf_init_module(void)
 {
 	lbtf_deb_enter(LBTF_DEB_MAIN);
-	lbtf_wq = alloc_workqueue("libertastf", WQ_MEM_RECLAIM, 0);
+	lbtf_wq = alloc_workqueue("libertastf", WQ_MEM_RECLAIM | WQ_UNBOUND, 0);
 	if (lbtf_wq == NULL) {
 		printk(KERN_ERR "libertastf: couldn't create workqueue\n");
 		return -ENOMEM;
-- 
2.51.0