linux-kernel - [PATCH 1/3] mm: replace use of system_unbound_wq with system_dfl

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Message-ID: <20250905090323.103401-2-marco.crivellari@suse.com>
Date: Fri,  5 Sep 2025 11:03:21 +0200
From: Marco Crivellari <marco.crivellari@...e.com>
To: linux-kernel@...r.kernel.org,
	linux-mm@...ck.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>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH 1/3] mm: replace use of system_unbound_wq with system_dfl_wq

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.

system_unbound_wq should be the default workqueue so as not to enforce
locality constraints for random work whenever it's not required.

Adding system_dfl_wq to encourage its use when unbound work should be used.

queue_work() / queue_delayed_work() / mod_delayed_work() will now use the
new unbound wq: whether the user still use the old wq a warn will be
printed along with a wq redirect to the new one.

The old system_unbound_wq will be kept for a few release cycles.

Suggested-by: Tejun Heo <tj@...nel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@...e.com>
---
 mm/backing-dev.c | 2 +-
 mm/kfence/core.c | 6 +++---
 mm/memcontrol.c  | 4 ++--
 3 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 783904d8c5ef..e9f9fdcfe052 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -934,7 +934,7 @@ void wb_memcg_offline(struct mem_cgroup *memcg)
 	memcg_cgwb_list->next = NULL;	/* prevent new wb's */
 	spin_unlock_irq(&cgwb_lock);
 
-	queue_work(system_unbound_wq, &cleanup_offline_cgwbs_work);
+	queue_work(system_dfl_wq, &cleanup_offline_cgwbs_work);
 }
 
 /**
diff --git a/mm/kfence/core.c b/mm/kfence/core.c
index 102048821c22..f26d87d59296 100644
--- a/mm/kfence/core.c
+++ b/mm/kfence/core.c
@@ -854,7 +854,7 @@ static void toggle_allocation_gate(struct work_struct *work)
 	/* Disable static key and reset timer. */
 	static_branch_disable(&kfence_allocation_key);
 #endif
-	queue_delayed_work(system_unbound_wq, &kfence_timer,
+	queue_delayed_work(system_dfl_wq, &kfence_timer,
 			   msecs_to_jiffies(kfence_sample_interval));
 }
 
@@ -900,7 +900,7 @@ static void kfence_init_enable(void)
 		atomic_notifier_chain_register(&panic_notifier_list, &kfence_check_canary_notifier);
 
 	WRITE_ONCE(kfence_enabled, true);
-	queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
+	queue_delayed_work(system_dfl_wq, &kfence_timer, 0);
 
 	pr_info("initialized - using %lu bytes for %d objects at 0x%p-0x%p\n", KFENCE_POOL_SIZE,
 		CONFIG_KFENCE_NUM_OBJECTS, (void *)__kfence_pool,
@@ -996,7 +996,7 @@ static int kfence_enable_late(void)
 		return kfence_init_late();
 
 	WRITE_ONCE(kfence_enabled, true);
-	queue_delayed_work(system_unbound_wq, &kfence_timer, 0);
+	queue_delayed_work(system_dfl_wq, &kfence_timer, 0);
 	pr_info("re-enabled\n");
 	return 0;
 }
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 421740f1bcdc..c2944bc83378 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -651,7 +651,7 @@ static void flush_memcg_stats_dwork(struct work_struct *w)
 	 * in latency-sensitive paths is as cheap as possible.
 	 */
 	__mem_cgroup_flush_stats(root_mem_cgroup, true);
-	queue_delayed_work(system_unbound_wq, &stats_flush_dwork, FLUSH_TIME);
+	queue_delayed_work(system_dfl_wq, &stats_flush_dwork, FLUSH_TIME);
 }
 
 unsigned long memcg_page_state(struct mem_cgroup *memcg, int idx)
@@ -3732,7 +3732,7 @@ static int mem_cgroup_css_online(struct cgroup_subsys_state *css)
 		goto offline_kmem;
 
 	if (unlikely(mem_cgroup_is_root(memcg)) && !mem_cgroup_disabled())
-		queue_delayed_work(system_unbound_wq, &stats_flush_dwork,
+		queue_delayed_work(system_dfl_wq, &stats_flush_dwork,
 				   FLUSH_TIME);
 	lru_gen_online_memcg(memcg);
 
-- 
2.51.0