[PATCH 3/4] Workqueue: add WQ_PERCPU

[Marco Crivellari <marco.crivellari@suse.com>]

Currently 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 patch adds a new WQ_PERCPU flag to explicitly request the legacy
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.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
---
 include/linux/workqueue.h | 1 +
 kernel/workqueue.c        | 7 +++++++
 2 files changed, 8 insertions(+)

diff --git a/include/linux/workqueue.h b/include/linux/workqueue.h
index 9dea10a09cc5..697aabbd6dcb 100644
--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -374,6 +374,7 @@ enum wq_flags {
 	WQ_HIGHPRI		= 1 << 4, /* high priority */
 	WQ_CPU_INTENSIVE	= 1 << 5, /* cpu intensive workqueue */
 	WQ_SYSFS		= 1 << 6, /* visible in sysfs, see workqueue_sysfs_register() */
+	WQ_PERCPU		= 1 << 7, /* bound to a specific cpu */
 
 	/*
 	 * Per-cpu workqueues are generally preferred because they tend to
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 98cbfc685f39..0a088a2bd6a3 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -5670,6 +5670,13 @@ static struct workqueue_struct *__alloc_workqueue(const char *fmt,
 			return NULL;
 	}
 
+	/* one among WQ_UNBOUND and WQ_PERCPU should always be present */
+	if (WARN_ON_ONCE(flags & WQ_UNBOUND && flags & WQ_PERCPU))
+		return NULL;
+
+	if (WARN_ON_ONCE(!(flags & WQ_UNBOUND) && !(flags & WQ_PERCPU)))
+		return NULL;
+
 	/* see the comment above the definition of WQ_POWER_EFFICIENT */
 	if ((flags & WQ_POWER_EFFICIENT) && wq_power_efficient)
 		flags |= WQ_UNBOUND;
-- 
2.49.0