mm: slub: move flush_cpu_slab() invocations __free_slab() invocations out of IRQ context

flush_all() flushes a specific SLAB cache on each CPU (where the cache
is present). The deactivate_slab()/__free_slab() invocation happens
within IPI handler and is problematic for PREEMPT_RT.

The flush operation is not a frequent operation or a hot path. The
per-CPU flush operation can be moved to within a workqueue.

Because a workqueue handler, unlike IPI handler, does not disable irqs,
flush_slab() now has to disable them for working with the kmem_cache_cpu
fields. deactivate_slab() is safe to call with irqs enabled.

[[email protected]: adapt to new SLUB changes]
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Vlastimil Babka <[email protected]>

Author: Sebastian Andrzej Siewior

mm/slab_common.c

@@ -502,6 +502,7 @@ void kmem_cache_destroy(struct kmem_cache *s)
 	if (unlikely(!s))
 		return;
 
+	cpus_read_lock();
 	mutex_lock(&slab_mutex);
 
 	s->refcount--;
@@ -516,6 +517,7 @@ void kmem_cache_destroy(struct kmem_cache *s)
 	}
 out_unlock:
 	mutex_unlock(&slab_mutex);
+	cpus_read_unlock();
 }
 EXPORT_SYMBOL(kmem_cache_destroy);
 

mm/slub.c

@@ -2496,16 +2496,25 @@ static void put_cpu_partial(struct kmem_cache *s, struct page *page, int drain)
 
 static inline void flush_slab(struct kmem_cache *s, struct kmem_cache_cpu *c)
 {
-	void *freelist = c->freelist;
-	struct page *page = c->page;
+	unsigned long flags;
+	struct page *page;
+	void *freelist;
+
+	local_irq_save(flags);
+
+	page = c->page;
+	freelist = c->freelist;
 
 	c->page = NULL;
 	c->freelist = NULL;
 	c->tid = next_tid(c->tid);
 
-	deactivate_slab(s, page, freelist);
+	local_irq_restore(flags);
 
-	stat(s, CPUSLAB_FLUSH);
+	if (page) {
+		deactivate_slab(s, page, freelist);
+		stat(s, CPUSLAB_FLUSH);
+	}
 }
 
 static inline void __flush_cpu_slab(struct kmem_cache *s, int cpu)
@@ -2526,33 +2535,79 @@ static inline void __flush_cpu_slab(struct kmem_cache *s, int cpu)
 	unfreeze_partials_cpu(s, c);
 }
 
+struct slub_flush_work {
+	struct work_struct work;
+	struct kmem_cache *s;
+	bool skip;
+};
+
 /*
  * Flush cpu slab.
  *
- * Called from IPI handler with interrupts disabled.
+ * Called from CPU work handler with migration disabled.
  */
-static void flush_cpu_slab(void *d)
+static void flush_cpu_slab(struct work_struct *w)
 {
-	struct kmem_cache *s = d;
-	struct kmem_cache_cpu *c = this_cpu_ptr(s->cpu_slab);
+	struct kmem_cache *s;
+	struct kmem_cache_cpu *c;
+	struct slub_flush_work *sfw;
+
+	sfw = container_of(w, struct slub_flush_work, work);
+
+	s = sfw->s;
+	c = this_cpu_ptr(s->cpu_slab);
 
 	if (c->page)
 		flush_slab(s, c);
 
 	unfreeze_partials(s);
 }
 
-static bool has_cpu_slab(int cpu, void *info)
+static bool has_cpu_slab(int cpu, struct kmem_cache *s)
 {
-	struct kmem_cache *s = info;
 	struct kmem_cache_cpu *c = per_cpu_ptr(s->cpu_slab, cpu);
 
 	return c->page || slub_percpu_partial(c);
 }
 
+static DEFINE_MUTEX(flush_lock);
+static DEFINE_PER_CPU(struct slub_flush_work, slub_flush);
+
+static void flush_all_cpus_locked(struct kmem_cache *s)
+{
+	struct slub_flush_work *sfw;
+	unsigned int cpu;
+
+	lockdep_assert_cpus_held();
+	mutex_lock(&flush_lock);
+
+	for_each_online_cpu(cpu) {
+		sfw = &per_cpu(slub_flush, cpu);
+		if (!has_cpu_slab(cpu, s)) {
+			sfw->skip = true;
+			continue;
+		}
+		INIT_WORK(&sfw->work, flush_cpu_slab);
+		sfw->skip = false;
+		sfw->s = s;
+		schedule_work_on(cpu, &sfw->work);
+	}
+
+	for_each_online_cpu(cpu) {
+		sfw = &per_cpu(slub_flush, cpu);
+		if (sfw->skip)
+			continue;
+		flush_work(&sfw->work);
+	}
+
+	mutex_unlock(&flush_lock);
+}
+
 static void flush_all(struct kmem_cache *s)
 {
-	on_each_cpu_cond(has_cpu_slab, flush_cpu_slab, s, 1);
+	cpus_read_lock();
+	flush_all_cpus_locked(s);
+	cpus_read_unlock();
 }
 
 /*
@@ -4097,7 +4152,7 @@ int __kmem_cache_shutdown(struct kmem_cache *s)
 	int node;
 	struct kmem_cache_node *n;
 
-	flush_all(s);
+	flush_all_cpus_locked(s);
 	/* Attempt to free all objects */
 	for_each_kmem_cache_node(s, node, n) {
 		free_partial(s, n);
@@ -4373,7 +4428,7 @@ EXPORT_SYMBOL(kfree);
  * being allocated from last increasing the chance that the last objects
  * are freed in them.
  */
-int __kmem_cache_shrink(struct kmem_cache *s)
+static int __kmem_cache_do_shrink(struct kmem_cache *s)
 {
 	int node;
 	int i;
@@ -4385,7 +4440,6 @@ int __kmem_cache_shrink(struct kmem_cache *s)
 	unsigned long flags;
 	int ret = 0;
 
-	flush_all(s);
 	for_each_kmem_cache_node(s, node, n) {
 		INIT_LIST_HEAD(&discard);
 		for (i = 0; i < SHRINK_PROMOTE_MAX; i++)
@@ -4435,13 +4489,21 @@ int __kmem_cache_shrink(struct kmem_cache *s)
 	return ret;
 }
 
+int __kmem_cache_shrink(struct kmem_cache *s)
+{
+	flush_all(s);
+	return __kmem_cache_do_shrink(s);
+}
+
 static int slab_mem_going_offline_callback(void *arg)
 {
 	struct kmem_cache *s;
 
 	mutex_lock(&slab_mutex);
-	list_for_each_entry(s, &slab_caches, list)
-		__kmem_cache_shrink(s);
+	list_for_each_entry(s, &slab_caches, list) {
+		flush_all_cpus_locked(s);
+		__kmem_cache_do_shrink(s);
+	}
 	mutex_unlock(&slab_mutex);
 
 	return 0;