slab: don't batch kvfree_rcu() with SLUB_TINY

kvfree_rcu() is batched for better performance except on TINY_RCU, which
is a simple implementation for small UP systems. Similarly SLUB_TINY is
an option intended for small systems, whether or not used together with
TINY_RCU. In case SLUB_TINY is used with !TINY_RCU, it makes arguably
sense to not do the batching and limit the memory footprint. It's also
suboptimal to have RCU-specific #ifdefs in slab code.

With that, add CONFIG_KVFREE_RCU_BATCHED to determine whether batching
kvfree_rcu() implementation is used. It is not set by a user prompt, but
enabled by default and disabled in case TINY_RCU or SLUB_TINY are
enabled.

Use the new config for #ifdef's in slab code and extend their scope to
cover all code used by the batched kvfree_rcu(). For example there's no
need to perform kvfree_rcu_init() if the batching is disabled.

Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reviewed-by: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Tested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>

Author: tehcaster

include/linux/slab.h

@@ -1083,7 +1083,7 @@ extern void kvfree_sensitive(const void *addr, size_t len);
 
 unsigned int kmem_cache_size(struct kmem_cache *s);
 
-#ifdef CONFIG_TINY_RCU
+#ifndef CONFIG_KVFREE_RCU_BATCHED
 static inline void kvfree_rcu_barrier(void)
 {
 	rcu_barrier();

mm/Kconfig

@@ -242,6 +242,10 @@ menu "Slab allocator options"
 config SLUB
 	def_bool y
 
+config KVFREE_RCU_BATCHED
+	def_bool y
+	depends on !SLUB_TINY && !TINY_RCU
+
 config SLUB_TINY
 	bool "Configure for minimal memory footprint"
 	depends on EXPERT

mm/slab_common.c

@@ -1284,7 +1284,7 @@ EXPORT_TRACEPOINT_SYMBOL(kmem_cache_alloc);
 EXPORT_TRACEPOINT_SYMBOL(kfree);
 EXPORT_TRACEPOINT_SYMBOL(kmem_cache_free);
 
-#ifdef CONFIG_TINY_RCU
+#ifndef CONFIG_KVFREE_RCU_BATCHED
 
 void kvfree_call_rcu(struct rcu_head *head, void *ptr)
 {
@@ -1301,7 +1301,11 @@ void kvfree_call_rcu(struct rcu_head *head, void *ptr)
 }
 EXPORT_SYMBOL_GPL(kvfree_call_rcu);
 
-#endif
+void __init kvfree_rcu_init(void)
+{
+}
+
+#else /* CONFIG_KVFREE_RCU_BATCHED */
 
 /*
  * This rcu parameter is runtime-read-only. It reflects
@@ -1879,8 +1883,6 @@ add_ptr_to_bulk_krc_lock(struct kfree_rcu_cpu **krcp,
 	return true;
 }
 
-#if !defined(CONFIG_TINY_RCU)
-
 static enum hrtimer_restart
 schedule_page_work_fn(struct hrtimer *t)
 {
@@ -2089,8 +2091,6 @@ void kvfree_rcu_barrier(void)
 }
 EXPORT_SYMBOL_GPL(kvfree_rcu_barrier);
 
-#endif /* #if !defined(CONFIG_TINY_RCU) */
-
 static unsigned long
 kfree_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
 {
@@ -2180,3 +2180,6 @@ void __init kvfree_rcu_init(void)
 
 	shrinker_register(kfree_rcu_shrinker);
 }
+
+#endif /* CONFIG_KVFREE_RCU_BATCHED */
+