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Subject[RFC 2/7] slub: Add defrag_ratio field and sysfs support
"defrag_ratio" is used to set the threshold at which defragmentation
should be attempted on a slab page.

"defrag_ratio" is percentage in the range of 1 - 100. If more than
that percentage of slots in a slab page are unused the the slab page
will become subject to defragmentation.

Add a defrag ratio field and set it to 30% by default. A limit of 30% specifies
that less than 3 out of 10 available slots for objects need to be leftover
before slab defragmentation will be attempted on the remaining objects.

Signed-off-by: Christoph Lameter <cl@linux.com>

---
Documentation/ABI/testing/sysfs-kernel-slab | 13 +++++++++++++
include/linux/slub_def.h | 6 ++++++
mm/slub.c | 23 +++++++++++++++++++++++
3 files changed, 42 insertions(+)

Index: linux/mm/slub.c
===================================================================
--- linux.orig/mm/slub.c
+++ linux/mm/slub.c
@@ -3628,6 +3628,7 @@ static int kmem_cache_open(struct kmem_c

set_cpu_partial(s);

+ s->defrag_ratio = 30;
#ifdef CONFIG_NUMA
s->remote_node_defrag_ratio = 1000;
#endif
@@ -5113,6 +5114,27 @@ static ssize_t destroy_by_rcu_show(struc
}
SLAB_ATTR_RO(destroy_by_rcu);

+static ssize_t defrag_ratio_show(struct kmem_cache *s, char *buf)
+{
+ return sprintf(buf, "%d\n", s->defrag_ratio);
+}
+
+static ssize_t defrag_ratio_store(struct kmem_cache *s,
+ const char *buf, size_t length)
+{
+ unsigned long ratio;
+ int err;
+
+ err = kstrtoul(buf, 10, &ratio);
+ if (err)
+ return err;
+
+ if (ratio < 100)
+ s->defrag_ratio = ratio;
+ return length;
+}
+SLAB_ATTR(defrag_ratio);
+
#ifdef CONFIG_SLUB_DEBUG
static ssize_t slabs_show(struct kmem_cache *s, char *buf)
{
@@ -5437,6 +5459,7 @@ static struct attribute *slab_attrs[] =
&validate_attr.attr,
&alloc_calls_attr.attr,
&free_calls_attr.attr,
+ &defrag_ratio_attr.attr,
#endif
#ifdef CONFIG_ZONE_DMA
&cache_dma_attr.attr,
Index: linux/Documentation/ABI/testing/sysfs-kernel-slab
===================================================================
--- linux.orig/Documentation/ABI/testing/sysfs-kernel-slab
+++ linux/Documentation/ABI/testing/sysfs-kernel-slab
@@ -180,6 +180,19 @@ Description:
list. It can be written to clear the current count.
Available when CONFIG_SLUB_STATS is enabled.

+What: /sys/kernel/slab/cache/defrag_ratio
+Date: December 2018
+KernelVersion: 4.18
+Contact: Christoph Lameter <cl@linux-foundation.org>
+ Pekka Enberg <penberg@cs.helsinki.fi>,
+Description:
+ The defrag_ratio files allows the control of how agressive
+ slab fragmentation reduction works at reclaiming objects from
+ sparsely populated slabs. This is a percentage. If a slab
+ has more than this percentage of available object then reclaim
+ will attempt to reclaim objects so that the whole slab
+ page can be freed. The default is 30%.
+
What: /sys/kernel/slab/cache/deactivate_to_tail
Date: February 2008
KernelVersion: 2.6.25
Index: linux/include/linux/slub_def.h
===================================================================
--- linux.orig/include/linux/slub_def.h
+++ linux/include/linux/slub_def.h
@@ -104,6 +104,12 @@ struct kmem_cache {
unsigned int red_left_pad; /* Left redzone padding size */
const char *name; /* Name (only for display!) */
struct list_head list; /* List of slab caches */
+ int defrag_ratio; /*
+ * Ratio used to check the percentage of
+ * objects allocate in a slab page.
+ * If less than this ratio is allocated
+ * then reclaim attempts are made.
+ */
#ifdef CONFIG_SYSFS
struct kobject kobj; /* For sysfs */
struct work_struct kobj_remove_work;
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