[PATCH 3/5] Switch caches notification dynamically

[Pavel Emelyanov <xemul-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>]

The /sys/slab/<name>/cache_notify attribute controls
whether the cache <name> is to be accounted or not.

By default no caches are accountable. Simply make
  # echo -n 1 > /sys/slab/<name>cache_notify
to turn notification of this cache on.

If we turn accounting on on some cache and this cache 
is merged with some other, this "other" will be notified
as well. One can solve this by disabling of cache merging,
i.e. booting with slub_nomerge option to the kernel.

Turning the notifications "on" causes all te subsequent
allocations use the slow allocation path, so all the
per-cpu caches are flushed and all the partial pages
are marked as SlabDebug.

Turning the notification off is possible only when this 
cache is empty. The reason for this is that the pages,
that are full of objects are not linked in any list, so
we wouldn't be able to walk these pages and tell them
thay should not produce notifications any longer. 

To make "off" possible we need to rework the slub.c in 
respect to full slabs handling (this is currently 
available with SLUB_DEBUG only). This is in TODO.

Signed-off-by: Pavel Emelyanov <xemul-GEFAQzZX7r8dnm+yROfE0A@public.gmane.org>

---

diff --git a/mm/slub.c b/mm/slub.c
index 31d04a3..e066a0e 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3779,6 +3932,60 @@ static ssize_t defrag_ratio_store(struct
 SLAB_ATTR(defrag_ratio);
 #endif
 
+static ssize_t cache_notify_show(struct kmem_cache *s, char *buf)
+{
+	return sprintf(buf, "%d\n", !!(s->flags & SLAB_NOTIFY));
+}
+
+static ssize_t cache_notify_store(struct kmem_cache *s,
+		const char *buf, size_t length)
+{
+	int err;
+	int n;
+
+	if ((buf[0] == '1') && !(s->flags & SLAB_NOTIFY)) {
+		err = slub_on_notify(s);
+		if (err < 0)
+			return err;
+
+		s->flags |= SLAB_NOTIFY;
+
+		flush_all(s);
+		for_each_node_state(n, N_NORMAL_MEMORY) {
+			struct kmem_cache_node *node;
+			struct page *pg;
+			
+			node = get_node(s, n);
+			spin_lock_irq(&node->list_lock);
+			list_for_each_entry(pg, &node->partial, lru)
+				SetSlabDebug(pg);
+			spin_unlock_irq(&node->list_lock);
+		}
+		return length;
+	}
+
+	if ((buf[0] == '0') && (s->flags & SLAB_NOTIFY)) {
+		/*
+		 * TODO: make the notifications-off work
+		 */
+		if (any_slab_objects(s))
+			return -EBUSY;
+
+		s->flags &= ~SLAB_NOTIFY;
+		err = slub_off_notify(s);
+		if (err < 0) {
+			s->flags |= SLAB_NOTIFY;
+			return err;
+		}
+
+		return length;
+	}
+
+	return -EINVAL;
+}
+
+SLAB_ATTR(cache_notify);
+
 static struct attribute * slab_attrs[] = {
 	&slab_size_attr.attr,
 	&object_size_attr.attr,
@@ -3809,6 +4016,7 @@ static struct attribute * slab_attrs[] =
 #ifdef CONFIG_NUMA
 	&defrag_ratio_attr.attr,
 #endif
+	&cache_notify_attr.attr,
 	NULL
 };