zsmalloc: turn zspage order into runtime variable

zsmalloc has 255 size classes.  Size classes contain a number of zspages,
which store objects of the same size.  zspage can consist of up to four
physical pages.  The exact (most optimal) zspage size is calculated for
each size class during zsmalloc pool creation.

As a reasonable optimization, zsmalloc merges size classes that have
similar characteristics: number of pages per zspage and number of objects
zspage can store.

For example, let's look at the following size classes:

class  size almost_full almost_empty obj_allocated   obj_used pages_used pages_per_zspage freeable
..
   94  1536           0            0             0          0          0                3        0
  100  1632           0            0             0          0          0                2        0
..

Size classes torvalds#95-99 are merged with size class torvalds#100. That is, each time
we store an object of size, say, 1568 bytes instead of using class torvalds#96
we end up storing it in size class torvalds#100. Class torvalds#100 is for objects of
1632 bytes in size, hence every 1568 bytes object wastes 1632-1568 bytes.
Class torvalds#100 zspages consist of 2 physical pages and can hold 5 objects.
When we need to store, say, 13 objects of size 1568 we end up allocating
three zspages; in other words, 6 physical pages.

However, if we'll look closer at size class torvalds#96 (which should hold objects
of size 1568 bytes) and trace get_pages_per_zspage():

    pages per zspage      wasted bytes     used%
           1                  960           76
           2                  352           95
           3                 1312           89
           4                  704           95
           5                   96           99

We'd notice that the most optimal zspage configuration for this class is
when it consists of 5 physical pages, but currently we never let zspages
to consists of more than 4 pages. A 5 page class torvalds#96 configuration would
store 13 objects of size 1568 in a single zspage, allocating 5 physical
pages, as opposed to 6 physical pages that class torvalds#100 will allocate.

A higher order zspage for class torvalds#96 also changes its key characteristics:
pages per-zspage and objects per-zspage. As a result classes torvalds#96 and torvalds#100
are not merged anymore, which gives us more compact zsmalloc.

Of course the described effect does not apply only to size classes torvalds#96 and
We still merge classes, but less often so. In other words classes are grouped
in a more compact way, which decreases memory wastage:

zspage order               # unique size classes
     2                                69
     3                               123
     4                               191

Let's take a closer look at the bottom of /sys/kernel/debug/zsmalloc/zram0/classes:

class  size almost_full almost_empty obj_allocated   obj_used pages_used pages_per_zspage freeable
...
  202  3264           0            0             0          0          0                4        0
  254  4096           0            0             0          0          0                1        0
...

For exactly same reason - maximum 4 pages per zspage - the last non-huge
size class is torvalds#202, which stores objects of size 3264 bytes. Any object
larger than 3264 bytes, hence, is considered to be huge and lands in size
class torvalds#254, which uses a whole physical page to store every object. To put
it slightly differently - objects in huge classes don't share physical pages.

3264 bytes is too low of a watermark and we have too many huge classes:
classes from torvalds#203 to torvalds#254. Similarly to class size torvalds#96 above, higher order
zspages change key characteristics for some of those huge size classes and
thus those classes become normal classes, where stored objects share physical
pages.

Hence yet another consequence of higher order zspages: we move the huge
size class watermark with higher order zspages, have less huge classes and
store large objects in a more compact way.

For order 3, huge class watermark becomes 3632 bytes:

class  size almost_full almost_empty obj_allocated   obj_used pages_used pages_per_zspage freeable
...
  202  3264           0            0             0          0          0                4        0
  211  3408           0            0             0          0          0                5        0
  217  3504           0            0             0          0          0                6        0
  222  3584           0            0             0          0          0                7        0
  225  3632           0            0             0          0          0                8        0
  254  4096           0            0             0          0          0                1        0
...

For order 4, huge class watermark becomes 3840 bytes:

class  size almost_full almost_empty obj_allocated   obj_used pages_used pages_per_zspage freeable
...
  202  3264           0            0             0          0          0                4        0
  206  3328           0            0             0          0          0               13        0
  207  3344           0            0             0          0          0                9        0
  208  3360           0            0             0          0          0               14        0
  211  3408           0            0             0          0          0                5        0
  212  3424           0            0             0          0          0               16        0
  214  3456           0            0             0          0          0               11        0
  217  3504           0            0             0          0          0                6        0
  219  3536           0            0             0          0          0               13        0
  222  3584           0            0             0          0          0                7        0
  223  3600           0            0             0          0          0               15        0
  225  3632           0            0             0          0          0                8        0
  228  3680           0            0             0          0          0                9        0
  230  3712           0            0             0          0          0               10        0
  232  3744           0            0             0          0          0               11        0
  234  3776           0            0             0          0          0               12        0
  235  3792           0            0             0          0          0               13        0
  236  3808           0            0             0          0          0               14        0
  238  3840           0            0             0          0          0               15        0
  254  4096           0            0             0          0          0                1        0
...

TESTS
=====

Test untars linux-6.0.tar.xz and compiles the kernel.

zram is configured as a block device with ext4 file system, lzo-rle
compression algorithm. We captured /sys/block/zram0/mm_stat after
every test and rebooted the VM.

orig_data_size       mem_used_total     mem_used_max       pages_compacted
          compr_data_size         mem_limit         same_pages       huge_pages

ORDER 2 (BASE) zspage

1691791360 628086729 655171584        0 655171584       60        0    34043
1691787264 628089196 655175680        0 655175680       60        0    34046
1691803648 628098840 655187968        0 655187968       59        0    34047
1691795456 628091503 655183872        0 655183872       60        0    34044
1691799552 628086877 655183872        0 655183872       60        0    34047

ORDER 3 zspage

1691803648 627792993 641794048        0 641794048       60        0    33591
1691787264 627779342 641708032        0 641708032       59        0    33591
1691811840 627786616 641769472        0 641769472       60        0    33591
1691803648 627794468 641818624        0 641818624       59        0    33592
1691783168 627780882 641794048        0 641794048       61        0    33591

ORDER 4 zspage

1691803648 627726635 639655936        0 639655936       60        0    33435
1691811840 627733348 639643648        0 639643648       61        0    33434
1691795456 627726290 639614976        0 639614976       60        0    33435
1691803648 627730458 639688704        0 639688704       60        0    33434
1691811840 627727771 639688704        0 639688704       60        0    33434

Order 3 and order 4 show statistically significant improvement in
`mem_used_max` metrics.

T-test for order 3:

x order-2-maxmem
+ order-3-maxmem
    N           Min           Max        Median           Avg        Stddev
x   5 6.5517158e+08 6.5518797e+08 6.5518387e+08  6.551806e+08     6730.4157
+   5 6.4170803e+08 6.4181862e+08 6.4179405e+08 6.4177684e+08     42210.666
Difference at 95.0% confidence
	-1.34038e+07 +/- 44080.7
	-2.04581% +/- 0.00672802%
	(Student's t, pooled s = 30224.5)

T-test for order 4:

x order-2-maxmem
+ order-4-maxmem
    N           Min           Max        Median           Avg        Stddev
x   5 6.5517158e+08 6.5518797e+08 6.5518387e+08  6.551806e+08     6730.4157
+   5 6.3961498e+08  6.396887e+08 6.3965594e+08 6.3965839e+08     31408.602
Difference at 95.0% confidence
	-1.55222e+07 +/- 33126.2
	-2.36915% +/- 0.00505604%
	(Student's t, pooled s = 22713.4)

This test tends to benefit more from order 4 zspages, due to test's data
patterns.

zsmalloc object distribution analysis
=============================================================================

Order 2 (4 pages per zspage) tends to put many objects in size class 2048,
which is merged with size classes torvalds#112-torvalds#125:

class  size almost_full almost_empty obj_allocated   obj_used pages_used pages_per_zspage freeable
...
    71  1168           0            0          6146       6146       1756                2        0
    74  1216           0            1          4560       4552       1368                3        0
    76  1248           0            1          2938       2934        904                4        0
    83  1360           0            0         10971      10971       3657                1        0
    91  1488           0            0         16126      16126       5864                4        0
    94  1536           0            1          5912       5908       2217                3        0
   100  1632           0            0         11990      11990       4796                2        0
   107  1744           0            1         15771      15768       6759                3        0
   111  1808           0            1         10386      10380       4616                4        0
   126  2048           0            0         45444      45444      22722                1        0
   144  2336           0            0         47446      47446      27112                4        0
   151  2448           1            0         10760      10759       6456                3        0
   168  2720           0            0         10173      10173       6782                2        0
   190  3072           0            1          1700       1697       1275                3        0
   202  3264           0            1           290        286        232                4        0
   254  4096           0            0         34051      34051      34051                1        0

Order 3 (8 pages per zspage) changed pool characteristics and unmerged
some of the size classes, which resulted in less objects being put into
size class 2048, because there are lower size classes are now available
for more compact object storage:

class  size almost_full almost_empty obj_allocated   obj_used pages_used pages_per_zspage freeable
...
    71  1168           0            1          2996       2994        856                2        0
    72  1184           0            1          1632       1609        476                7        0
    73  1200           1            0          1445       1442        425                5        0
    74  1216           0            0          1510       1510        453                3        0
    75  1232           0            1          1495       1479        455                7        0
    76  1248           0            1          1456       1451        448                4        0
    78  1280           0            1          3040       3033        950                5        0
    79  1296           0            1          1584       1571        504                7        0
    83  1360           0            0          6375       6375       2125                1        0
    84  1376           0            1          1817       1796        632                8        0
    87  1424           0            1          6020       6006       2107                7        0
    88  1440           0            1          2108       2101        744                6        0
    89  1456           0            1          2072       2064        740                5        0
    91  1488           0            1          4169       4159       1516                4        0
    92  1504           0            1          2014       2007        742                7        0
    94  1536           0            1          3904       3900       1464                3        0
    95  1552           0            1          1890       1873        720                8        0
    96  1568           0            1          1963       1958        755                5        0
    97  1584           0            1          1980       1974        770                7        0
   100  1632           0            1          6190       6187       2476                2        0
   103  1680           0            0          6477       6477       2667                7        0
   104  1696           0            1          2256       2253        940                5        0
   105  1712           0            1          2356       2340        992                8        0
   107  1744           1            0          4697       4696       2013                3        0
   110  1792           0            1          7744       7734       3388                7        0
   111  1808           0            1          2655       2649       1180                4        0
   114  1856           0            1          8371       8365       3805                5        0
   116  1888           1            0          5863       5862       2706                6        0
   117  1904           0            1          2955       2942       1379                7        0
   118  1920           0            1          3009       2997       1416                8        0
   126  2048           0            0         25276      25276      12638                1        0
   128  2080           0            1          6060       6052       3232                8        0
   129  2096           1            0          3081       3080       1659                7        0
   134  2176           0            1         14835      14830       7912                8        0
   135  2192           0            1          2769       2758       1491                7        0
   137  2224           0            1          5082       5077       2772                6        0
   140  2272           0            1          7236       7232       4020                5        0
   144  2336           0            1          8428       8423       4816                4        0
   147  2384           0            1          5316       5313       3101                7        0
   151  2448           0            1          5445       5443       3267                3        0
   155  2512           0            0          4121       4121       2536                8        0
   158  2560           0            1          2208       2205       1380                5        0
   160  2592           0            0          1133       1133        721                7        0
   168  2720           0            0          2712       2712       1808                2        0
   177  2864           1            0          1100       1098        770                7        0
   180  2912           0            1           189        183        135                5        0
   184  2976           0            1           176        166        128                8        0
   190  3072           0            0           252        252        189                3        0
   197  3184           0            1           198        192        154                7        0
   202  3264           0            1           100         96         80                4        0
   211  3408           0            1           210        208        175                5        0
   217  3504           0            1            98         94         84                6        0
   222  3584           0            0           104        104         91                7        0
   225  3632           0            1            54         50         48                8        0
   254  4096           0            0         33591      33591      33591                1        0

Note, the huge size watermark is above 3632 and there are a number of new
normal classes available that previously were merged with the huge class. 
For instance, size class torvalds#211 holds 210 objects of size 3408 and uses 175
physical pages, while previously for those objects we would have used 210
physical pages.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Sergey Senozhatsky <[email protected]>
Cc: Alexey Romanov <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Nitin Gupta <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: sergey-senozhatsky

include/linux/zsmalloc.h

@@ -33,6 +33,18 @@ enum zs_mapmode {
 	 */
 };
 
+#define ZS_PAGE_ORDER_2		2
+#define ZS_PAGE_ORDER_4		4
+
+/*
+ * A single 'zspage' is composed of up to 2^N discontiguous 0-order (single)
+ * pages. ZS_MAX_PAGE_ORDER defines upper limit on N, ZS_MIN_PAGE_ORDER
+ * defines lower limit on N. ZS_DEFAULT_PAGE_ORDER is recommended value.
+ */
+#define ZS_MIN_PAGE_ORDER	ZS_PAGE_ORDER_2
+#define ZS_MAX_PAGE_ORDER	ZS_PAGE_ORDER_4
+#define ZS_DEFAULT_PAGE_ORDER	ZS_PAGE_ORDER_2
+
 struct zs_pool_stats {
 	/* How many pages were migrated (freed) */
 	atomic_long_t pages_compacted;

mm/zsmalloc.c

@@ -73,12 +73,7 @@
  */
 #define ZS_ALIGN		8
 
-/*
- * A single 'zspage' is composed of up to 2^N discontiguous 0-order (single)
- * pages. ZS_MAX_ZSPAGE_ORDER defines upper limit on N.
- */
-#define ZS_MAX_ZSPAGE_ORDER 2
-#define ZS_MAX_PAGES_PER_ZSPAGE (_AC(1, UL) << ZS_MAX_ZSPAGE_ORDER)
+#define ZS_MAX_PAGES_PER_ZSPAGE	(_AC(1, UL) << ZS_MAX_PAGE_ORDER)
 
 #define ZS_HANDLE_SIZE (sizeof(unsigned long))
 
@@ -123,10 +118,8 @@
 #define ISOLATED_BITS	3
 #define MAGIC_VAL_BITS	8
 
-#define MAX(a, b) ((a) >= (b) ? (a) : (b))
-/* ZS_MIN_ALLOC_SIZE must be multiple of ZS_ALIGN */
-#define ZS_MIN_ALLOC_SIZE \
-	MAX(32, (ZS_MAX_PAGES_PER_ZSPAGE << PAGE_SHIFT >> OBJ_INDEX_BITS))
+#define ZS_MIN_ALLOC_SIZE	32U
+
 /* each chunk includes extra space to keep handle */
 #define ZS_MAX_ALLOC_SIZE	PAGE_SIZE
 
@@ -140,12 +133,10 @@
  *    determined). NOTE: all those class sizes must be set as multiple of
  *    ZS_ALIGN to make sure link_free itself never has to span 2 pages.
  *
- *  ZS_MIN_ALLOC_SIZE and ZS_SIZE_CLASS_DELTA must be multiple of ZS_ALIGN
- *  (reason above)
+ *  pool->min_alloc_size (ZS_MIN_ALLOC_SIZE) and ZS_SIZE_CLASS_DELTA must
+ *  be multiple of ZS_ALIGN (reason above)
  */
 #define ZS_SIZE_CLASS_DELTA	(PAGE_SIZE >> CLASS_BITS)
-#define ZS_SIZE_CLASSES	(DIV_ROUND_UP(ZS_MAX_ALLOC_SIZE - ZS_MIN_ALLOC_SIZE, \
-				      ZS_SIZE_CLASS_DELTA) + 1)
 
 enum fullness_group {
 	ZS_EMPTY,
@@ -234,12 +225,15 @@ struct zs_ops {
 struct zs_pool {
 	const char *name;
 
-	struct size_class *size_class[ZS_SIZE_CLASSES];
+	struct size_class **size_class;
 	struct kmem_cache *handle_cachep;
 	struct kmem_cache *zspage_cachep;
 
 	atomic_long_t pages_allocated;
 
+	u32 num_size_classes;
+	u32 min_alloc_size;
+
 	struct zs_pool_stats stats;
 
 	/* Compact classes */
@@ -595,15 +589,15 @@ static void set_zspage_mapping(struct zspage *zspage,
  * classes depending on its size. This function returns index of the
  * size class which has chunk size big enough to hold the given size.
  */
-static int get_size_class_index(int size)
+static int get_size_class_index(struct zs_pool *pool, int size)
 {
 	int idx = 0;
 
-	if (likely(size > ZS_MIN_ALLOC_SIZE))
-		idx = DIV_ROUND_UP(size - ZS_MIN_ALLOC_SIZE,
+	if (likely(size > pool->min_alloc_size))
+		idx = DIV_ROUND_UP(size - pool->min_alloc_size,
 				ZS_SIZE_CLASS_DELTA);
 
-	return min_t(int, ZS_SIZE_CLASSES - 1, idx);
+	return min_t(int, pool->num_size_classes - 1, idx);
 }
 
 /* type can be of enum type class_stat_type or fullness_group */
@@ -663,7 +657,7 @@ static int zs_stats_size_show(struct seq_file *s, void *v)
 			"obj_allocated", "obj_used", "pages_used",
 			"pages_per_zspage", "freeable");
 
-	for (i = 0; i < ZS_SIZE_CLASSES; i++) {
+	for (i = 0; i < pool->num_size_classes; i++) {
 		class = pool->size_class[i];
 
 		if (class->index != i)
@@ -849,13 +843,13 @@ static enum fullness_group fix_fullness_group(struct size_class *class,
  * link together 3 PAGE_SIZE sized pages to form a zspage
  * since then we can perfectly fit in 8 such objects.
  */
-static int get_pages_per_zspage(int class_size)
+static int get_pages_per_zspage(u32 class_size, u32 num_pages)
 {
 	int i, max_usedpc = 0;
 	/* zspage order which gives maximum used size per KB */
 	int max_usedpc_order = 1;
 
-	for (i = 1; i <= ZS_MAX_PAGES_PER_ZSPAGE; i++) {
+	for (i = 1; i <= num_pages; i++) {
 		int zspage_size;
 		int waste, usedpc;
 
@@ -1315,7 +1309,7 @@ unsigned int zs_lookup_class_index(struct zs_pool *pool, unsigned int size)
 {
 	struct size_class *class;
 
-	class = pool->size_class[get_size_class_index(size)];
+	class = pool->size_class[get_size_class_index(pool, size)];
 
 	return class->index;
 }
@@ -1526,7 +1520,7 @@ unsigned long zs_malloc(struct zs_pool *pool, size_t size, gfp_t gfp)
 
 	/* extra space in chunk to keep the handle */
 	size += ZS_HANDLE_SIZE;
-	class = pool->size_class[get_size_class_index(size)];
+	class = pool->size_class[get_size_class_index(pool, size)];
 
 	/* pool->lock effectively protects the zpage migration */
 	spin_lock(&pool->lock);
@@ -2102,7 +2096,7 @@ static void async_free_zspage(struct work_struct *work)
 	struct zs_pool *pool = container_of(work, struct zs_pool,
 					free_work);
 
-	for (i = 0; i < ZS_SIZE_CLASSES; i++) {
+	for (i = 0; i < pool->num_size_classes; i++) {
 		class = pool->size_class[i];
 		if (class->index != i)
 			continue;
@@ -2250,7 +2244,7 @@ unsigned long zs_compact(struct zs_pool *pool)
 	struct size_class *class;
 	unsigned long pages_freed = 0;
 
-	for (i = ZS_SIZE_CLASSES - 1; i >= 0; i--) {
+	for (i = pool->num_size_classes - 1; i >= 0; i--) {
 		class = pool->size_class[i];
 		if (class->index != i)
 			continue;
@@ -2294,7 +2288,7 @@ static unsigned long zs_shrinker_count(struct shrinker *shrinker,
 	struct zs_pool *pool = container_of(shrinker, struct zs_pool,
 			shrinker);
 
-	for (i = ZS_SIZE_CLASSES - 1; i >= 0; i--) {
+	for (i = pool->num_size_classes - 1; i >= 0; i--) {
 		class = pool->size_class[i];
 		if (class->index != i)
 			continue;
@@ -2336,11 +2330,27 @@ struct zs_pool *zs_create_pool(const char *name)
 	int i;
 	struct zs_pool *pool;
 	struct size_class *prev_class = NULL;
+	unsigned long num_pages;
 
 	pool = kzalloc(sizeof(*pool), GFP_KERNEL);
 	if (!pool)
 		return NULL;
 
+	num_pages = 1UL << ZS_DEFAULT_PAGE_ORDER;
+	/* min_alloc_size must be multiple of ZS_ALIGN */
+	pool->min_alloc_size = num_pages << PAGE_SHIFT >> OBJ_INDEX_BITS;
+	pool->min_alloc_size = max(pool->min_alloc_size, ZS_MIN_ALLOC_SIZE);
+
+	pool->num_size_classes =
+		DIV_ROUND_UP(ZS_MAX_ALLOC_SIZE - pool->min_alloc_size,
+			     ZS_SIZE_CLASS_DELTA) + 1;
+
+	pool->size_class = kmalloc_array(pool->num_size_classes,
+					 sizeof(struct size_class *),
+					 GFP_KERNEL | __GFP_ZERO);
+	if (!pool->size_class)
+		goto err;
+
 	init_deferred_free(pool);
 	spin_lock_init(&pool->lock);
 
@@ -2355,17 +2365,17 @@ struct zs_pool *zs_create_pool(const char *name)
 	 * Iterate reversely, because, size of size_class that we want to use
 	 * for merging should be larger or equal to current size.
 	 */
-	for (i = ZS_SIZE_CLASSES - 1; i >= 0; i--) {
+	for (i = pool->num_size_classes - 1; i >= 0; i--) {
 		int size;
 		int pages_per_zspage;
 		int objs_per_zspage;
 		struct size_class *class;
 		int fullness = 0;
 
-		size = ZS_MIN_ALLOC_SIZE + i * ZS_SIZE_CLASS_DELTA;
+		size = pool->min_alloc_size + i * ZS_SIZE_CLASS_DELTA;
 		if (size > ZS_MAX_ALLOC_SIZE)
 			size = ZS_MAX_ALLOC_SIZE;
-		pages_per_zspage = get_pages_per_zspage(size);
+		pages_per_zspage = get_pages_per_zspage(size, num_pages);
 		objs_per_zspage = pages_per_zspage * PAGE_SIZE / size;
 
 		/*
@@ -2450,7 +2460,7 @@ void zs_destroy_pool(struct zs_pool *pool)
 	zs_flush_migration(pool);
 	zs_pool_stat_destroy(pool);
 
-	for (i = 0; i < ZS_SIZE_CLASSES; i++) {
+	for (i = 0; i < pool->num_size_classes; i++) {
 		int fg;
 		struct size_class *class = pool->size_class[i];
 
@@ -2470,6 +2480,7 @@ void zs_destroy_pool(struct zs_pool *pool)
 	}
 
 	destroy_cache(pool);
+	kfree(pool->size_class);
 	kfree(pool->name);
 	kfree(pool);
 }