Message-ID: <20251202101626.783736-1-harry.yoo@oracle.com>
Date: Tue,  2 Dec 2025 19:16:26 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: vbabka@...e.cz
Cc: surenb@...gle.com, Liam.Howlett@...cle.com, cl@...two.org,
        rientjes@...gle.com, roman.gushchin@...ux.dev, harry.yoo@...cle.com,
        urezki@...il.com, sidhartha.kumar@...cle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, rcu@...r.kernel.org,
        maple-tree@...ts.infradead.org, linux-modules@...r.kernel.org,
        mcgrof@...nel.org, petr.pavlu@...e.com, samitolvanen@...gle.com,
        atomlin@...mlin.com, lucas.demarchi@...el.com,
        akpm@...ux-foundation.org, jonathanh@...dia.com,
        stable@...r.kernel.org, Daniel Gomez <da.gomez@...sung.com>
Subject: [PATCH V2] mm/slab: introduce kvfree_rcu_barrier_on_cache() for cache destruction

Currently, kvfree_rcu_barrier() flushes RCU sheaves across all slab
caches when a cache is destroyed. This is unnecessary; only the RCU
sheaves belonging to the cache being destroyed need to be flushed.

As suggested by Vlastimil Babka, introduce a weaker form of
kvfree_rcu_barrier() that operates on a specific slab cache.

Factor out flush_rcu_sheaves_on_cache() from flush_all_rcu_sheaves() and
call it from flush_all_rcu_sheaves() and kvfree_rcu_barrier_on_cache().

Call kvfree_rcu_barrier_on_cache() instead of kvfree_rcu_barrier() on
cache destruction.

The performance benefit is evaluated on a 12 core 24 threads AMD Ryzen
5900X machine (1 socket), by loading slub_kunit module.

Before:
  Total calls: 19
  Average latency (us): 18127
  Total time (us): 344414

After:
  Total calls: 19
  Average latency (us): 10066
  Total time (us): 191264

Two performance regression have been reported:
  - stress module loader test's runtime increases by 50-60% (Daniel)
  - internal graphics test's runtime on Tegra23 increases by 35% (Jon)

They are fixed by this change.

Suggested-by: Vlastimil Babka <vbabka@...e.cz>
Fixes: ec66e0d59952 ("slab: add sheaf support for batching kfree_rcu() operations")
Cc: <stable@...r.kernel.org>
Link: https://lore.kernel.org/linux-mm/1bda09da-93be-4737-aef0-d47f8c5c9301@suse.cz
Reported-and-tested-by: Daniel Gomez <da.gomez@...sung.com>
Closes: https://lore.kernel.org/linux-mm/0406562e-2066-4cf8-9902-b2b0616dd742@kernel.org
Reported-and-tested-by: Jon Hunter <jonathanh@...dia.com>
Closes: https://lore.kernel.org/linux-mm/e988eff6-1287-425e-a06c-805af5bbf262@nvidia.com
Signed-off-by: Harry Yoo <harry.yoo@...cle.com>
---

No code change, added proper tags and updated changelog.

 include/linux/slab.h |  5 ++++
 mm/slab.h            |  1 +
 mm/slab_common.c     | 52 +++++++++++++++++++++++++++++------------
 mm/slub.c            | 55 ++++++++++++++++++++++++--------------------
 4 files changed, 73 insertions(+), 40 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index cf443f064a66..937c93d44e8c 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -1149,6 +1149,10 @@ static inline void kvfree_rcu_barrier(void)
 {
 	rcu_barrier();
 }
+static inline void kvfree_rcu_barrier_on_cache(struct kmem_cache *s)
+{
+	rcu_barrier();
+}
 
 static inline void kfree_rcu_scheduler_running(void) { }
 #else
@@ -1156,6 +1160,7 @@ void kvfree_rcu_barrier(void);
 
 void kfree_rcu_scheduler_running(void);
 #endif
+void kvfree_rcu_barrier_on_cache(struct kmem_cache *s);
 
 /**
  * kmalloc_size_roundup - Report allocation bucket size for the given size
diff --git a/mm/slab.h b/mm/slab.h
index f730e012553c..e767aa7e91b0 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -422,6 +422,7 @@ static inline bool is_kmalloc_normal(struct kmem_cache *s)
 
 bool __kfree_rcu_sheaf(struct kmem_cache *s, void *obj);
 void flush_all_rcu_sheaves(void);
+void flush_rcu_sheaves_on_cache(struct kmem_cache *s);
 
 #define SLAB_CORE_FLAGS (SLAB_HWCACHE_ALIGN | SLAB_CACHE_DMA | \
 			 SLAB_CACHE_DMA32 | SLAB_PANIC | \
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 84dfff4f7b1f..dd8a49d6f9cc 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -492,7 +492,7 @@ void kmem_cache_destroy(struct kmem_cache *s)
 		return;
 
 	/* in-flight kfree_rcu()'s may include objects from our cache */
-	kvfree_rcu_barrier();
+	kvfree_rcu_barrier_on_cache(s);
 
 	if (IS_ENABLED(CONFIG_SLUB_RCU_DEBUG) &&
 	    (s->flags & SLAB_TYPESAFE_BY_RCU)) {
@@ -2038,25 +2038,13 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
 }
 EXPORT_SYMBOL_GPL(kvfree_call_rcu);
 
-/**
- * kvfree_rcu_barrier - Wait until all in-flight kvfree_rcu() complete.
- *
- * Note that a single argument of kvfree_rcu() call has a slow path that
- * triggers synchronize_rcu() following by freeing a pointer. It is done
- * before the return from the function. Therefore for any single-argument
- * call that will result in a kfree() to a cache that is to be destroyed
- * during module exit, it is developer's responsibility to ensure that all
- * such calls have returned before the call to kmem_cache_destroy().
- */
-void kvfree_rcu_barrier(void)
+static inline void __kvfree_rcu_barrier(void)
 {
 	struct kfree_rcu_cpu_work *krwp;
 	struct kfree_rcu_cpu *krcp;
 	bool queued;
 	int i, cpu;
 
-	flush_all_rcu_sheaves();
-
 	/*
 	 * Firstly we detach objects and queue them over an RCU-batch
 	 * for all CPUs. Finally queued works are flushed for each CPU.
@@ -2118,8 +2106,43 @@ void kvfree_rcu_barrier(void)
 		}
 	}
 }
+
+/**
+ * kvfree_rcu_barrier - Wait until all in-flight kvfree_rcu() complete.
+ *
+ * Note that a single argument of kvfree_rcu() call has a slow path that
+ * triggers synchronize_rcu() following by freeing a pointer. It is done
+ * before the return from the function. Therefore for any single-argument
+ * call that will result in a kfree() to a cache that is to be destroyed
+ * during module exit, it is developer's responsibility to ensure that all
+ * such calls have returned before the call to kmem_cache_destroy().
+ */
+void kvfree_rcu_barrier(void)
+{
+	flush_all_rcu_sheaves();
+	__kvfree_rcu_barrier();
+}
 EXPORT_SYMBOL_GPL(kvfree_rcu_barrier);
 
+/**
+ * kvfree_rcu_barrier_on_cache - Wait for in-flight kvfree_rcu() calls on a
+ *                               specific slab cache.
+ * @s: slab cache to wait for
+ *
+ * See the description of kvfree_rcu_barrier() for details.
+ */
+void kvfree_rcu_barrier_on_cache(struct kmem_cache *s)
+{
+	if (s->cpu_sheaves)
+		flush_rcu_sheaves_on_cache(s);
+	/*
+	 * TODO: Introduce a version of __kvfree_rcu_barrier() that works
+	 * on a specific slab cache.
+	 */
+	__kvfree_rcu_barrier();
+}
+EXPORT_SYMBOL_GPL(kvfree_rcu_barrier_on_cache);
+
 static unsigned long
 kfree_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
 {
@@ -2215,4 +2238,3 @@ void __init kvfree_rcu_init(void)
 }
 
 #endif /* CONFIG_KVFREE_RCU_BATCHED */
-
diff --git a/mm/slub.c b/mm/slub.c
index 785e25a14999..7cec2220712b 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4118,42 +4118,47 @@ static void flush_rcu_sheaf(struct work_struct *w)
 
 
 /* needed for kvfree_rcu_barrier() */
-void flush_all_rcu_sheaves(void)
+void flush_rcu_sheaves_on_cache(struct kmem_cache *s)
 {
 	struct slub_flush_work *sfw;
-	struct kmem_cache *s;
 	unsigned int cpu;
 
-	cpus_read_lock();
-	mutex_lock(&slab_mutex);
+	mutex_lock(&flush_lock);
 
-	list_for_each_entry(s, &slab_caches, list) {
-		if (!s->cpu_sheaves)
-			continue;
+	for_each_online_cpu(cpu) {
+		sfw = &per_cpu(slub_flush, cpu);
 
-		mutex_lock(&flush_lock);
+		/*
+		 * we don't check if rcu_free sheaf exists - racing
+		 * __kfree_rcu_sheaf() might have just removed it.
+		 * by executing flush_rcu_sheaf() on the cpu we make
+		 * sure the __kfree_rcu_sheaf() finished its call_rcu()
+		 */
 
-		for_each_online_cpu(cpu) {
-			sfw = &per_cpu(slub_flush, cpu);
+		INIT_WORK(&sfw->work, flush_rcu_sheaf);
+		sfw->s = s;
+		queue_work_on(cpu, flushwq, &sfw->work);
+	}
 
-			/*
-			 * we don't check if rcu_free sheaf exists - racing
-			 * __kfree_rcu_sheaf() might have just removed it.
-			 * by executing flush_rcu_sheaf() on the cpu we make
-			 * sure the __kfree_rcu_sheaf() finished its call_rcu()
-			 */
+	for_each_online_cpu(cpu) {
+		sfw = &per_cpu(slub_flush, cpu);
+		flush_work(&sfw->work);
+	}
 
-			INIT_WORK(&sfw->work, flush_rcu_sheaf);
-			sfw->s = s;
-			queue_work_on(cpu, flushwq, &sfw->work);
-		}
+	mutex_unlock(&flush_lock);
+}
 
-		for_each_online_cpu(cpu) {
-			sfw = &per_cpu(slub_flush, cpu);
-			flush_work(&sfw->work);
-		}
+void flush_all_rcu_sheaves(void)
+{
+	struct kmem_cache *s;
+
+	cpus_read_lock();
+	mutex_lock(&slab_mutex);
 
-		mutex_unlock(&flush_lock);
+	list_for_each_entry(s, &slab_caches, list) {
+		if (!s->cpu_sheaves)
+			continue;
+		flush_rcu_sheaves_on_cache(s);
 	}
 
 	mutex_unlock(&slab_mutex);
-- 
2.43.0

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