[PATCH] [RFC PATCH v2]mm/slub: Optimize slub memory usage

[Jay Patel <jaypatel@linux.ibm.com>]

In the previous version [1], we were able to reduce slub memory
wastage, but the total memory was also increasing so to solve
this problem have modified the patch as follow:

1) If min_objects * object_size > PAGE_ALLOC_COSTLY_ORDER, then it
will return with PAGE_ALLOC_COSTLY_ORDER.
2) Similarly, if min_objects * object_size < PAGE_SIZE, then it will
return with slub_min_order.
3) Additionally, I changed slub_max_order to 2. There is no specific
reason for using the value 2, but it provided the best results in
terms of performance without any noticeable impact.

[1]
https://lore.kernel.org/linux-mm/20230612085535.275206-1-jaypatel@linux.ibm.com/

I have conducted tests on systems with 160 CPUs and 16 CPUs using 4K
and 64K page sizes. The tests showed that the patch successfully
reduces the total and wastage of slab memory without any noticeable
performance degradation in the hackbench test.

Test Results are as follows:
1) On 160 CPUs with 4K Page size

+----------------+----------------+----------------+
|          Total wastage in slub memory            |
+----------------+----------------+----------------+
|                | After Boot     | After Hackbench|
| Normal         | 2090 Kb        | 3204 Kb        |
| With Patch     | 1825 Kb        | 3088 Kb        |
| Wastage reduce | ~12%           | ~4%            |
+----------------+----------------+----------------+

+-----------------+----------------+----------------+
|            Total slub memory                      |
+-----------------+----------------+----------------+
|                 | After Boot     | After Hackbench|
| Normal          | 500572         | 713568         |
| With Patch      | 482036         | 688312         |
| Memory reduce   | ~4%            | ~3%            |
+-----------------+----------------+----------------+

hackbench-process-sockets
+-------+-----+----------+----------+-----------+
|             |  Normal  |With Patch|           |
+-------+-----+----------+----------+-----------+
| Amean |  1  |  1.3237  |  1.2737  | ( 3.78%)  |
| Amean |   4 |   1.5923 |   1.6023 | ( -0.63%) |
| Amean |   7 |   2.3727 |   2.4260 | ( -2.25%) |
| Amean |  12 |   3.9813 |   4.1290 | ( -3.71%) |
| Amean |  21 |   6.9680 |   7.0630 | ( -1.36%) |
| Amean |  30 |  10.1480 |  10.2170 | ( -0.68%) |
| Amean |  48 |  16.7793 |  16.8780 | ( -0.59%) |
| Amean |  79 |  28.9537 |  28.8187 | ( 0.47%)  |
| Amean | 110 |  39.5507 |  40.0157 | ( -1.18%) |
| Amean | 141 |  51.5670 |  51.8200 | ( -0.49%) |
| Amean | 172 |  62.8710 |  63.2540 | ( -0.61%) |
| Amean | 203 |  74.6417 |  75.2520 | ( -0.82%) |
| Amean | 234 |  86.0853 |  86.5653 | ( -0.56%) |
| Amean | 265 |  97.9203 |  98.4617 | ( -0.55%) |
| Amean | 296 | 108.6243 | 109.8770 | ( -1.15%) |
+-------+-----+----------+----------+-----------+

2) On 160 CPUs with 64K Page size
+-----------------+----------------+----------------+
|          Total wastage in slub memory             |
+-----------------+----------------+----------------+
|                 | After Boot     |After Hackbench |
| Normal          | 919 Kb         | 1880 Kb        |
| With Patch      | 807 Kb         | 1684 Kb        |
| Wastage reduce  | ~12%           | ~10%           |
+-----------------+----------------+----------------+

+-----------------+----------------+----------------+
|            Total slub memory                      |
+-----------------+----------------+----------------+
|                 | After Boot     | After Hackbench|
| Normal          | 1862592        | 3023744        |
| With Patch      | 1644416        | 2675776        |
| Memory reduce   | ~12%           | ~11%           |
+-----------------+----------------+----------------+

hackbench-process-sockets
+-------+-----+----------+----------+-----------+
|             |  Normal  |With Patch|           |
+-------+-----+----------+----------+-----------+
| Amean |  1  |  1.2547  |  1.2677  | ( -1.04%) |
| Amean |   4 |   1.5523 |   1.5783 | ( -1.67%) |
| Amean |   7 |   2.4157 |   2.3883 | ( 1.13%)  |
| Amean |  12 |   3.9807 |   3.9793 | ( 0.03%)  |
| Amean |  21 |   6.9687 |   6.9703 | ( -0.02%) |
| Amean |  30 |  10.1403 |  10.1297 | ( 0.11%)  |
| Amean |  48 |  16.7477 |  16.6893 | ( 0.35%)  |
| Amean |  79 |  27.9510 |  28.0463 | ( -0.34%) |
| Amean | 110 |  39.6833 |  39.5687 | ( 0.29%)  |
| Amean | 141 |  51.5673 |  51.4477 | ( 0.23%)  |
| Amean | 172 |  62.9643 |  63.1647 | ( -0.32%) |
| Amean | 203 |  74.6220 |  73.7900 | ( 1.11%)  |
| Amean | 234 |  85.1783 |  85.3420 | ( -0.19%) |
| Amean | 265 |  96.6627 |  96.7903 | ( -0.13%) |
| Amean | 296 | 108.2543 | 108.2253 | ( 0.03%)  |
+-------+-----+----------+----------+-----------+

3) On 16 CPUs with 4K Page size
+-----------------+----------------+------------------+
|          Total wastage in slub memory               |
+-----------------+----------------+------------------+
|                 | After Boot     | After Hackbench  |
| Normal          | 491 Kb         | 727 Kb           |
| With Patch      | 483 Kb         | 670 Kb           |
| Wastage reduce  | ~1%            | ~8%              |
+-----------------+----------------+------------------+

+-----------------+----------------+----------------+
|            Total slub memory                      |
+-----------------+----------------+----------------+
|                 | After Boot      | After Hackbench|
| Normal          | 105340          |  153116        |
| With Patch      | 103620          | 147412         |
| Memory reduce   | ~1.6%           | ~4%            |
+-----------------+----------------+----------------+

hackbench-process-sockets
+-------+-----+----------+----------+---------+
|             |  Normal  |With Patch|         |
+-------+-----+----------+----------+---------+
| Amean | 1  | 1.0963   | 1.1070  | ( -0.97%) |
| Amean |  4 |  3.7963) |  3.7957 | ( 0.02%)  |
| Amean |  7 |  6.5947) |  6.6017 | ( -0.11%) |
| Amean | 12 | 11.1993) | 11.1730 | ( 0.24%)  |
| Amean | 21 | 19.4097) | 19.3647 | ( 0.23%)  |
| Amean | 30 | 27.7023) | 27.6040 | ( 0.35%)  |
| Amean | 48 | 44.1287) | 43.9630 | ( 0.38%)  |
| Amean | 64 | 58.8147) | 58.5753 | ( 0.41%)  |
+-------+----+---------+----------+-----------+

4) On 16 CPUs with 64K Page size
+----------------+----------------+----------------+
|          Total wastage in slub memory            |
+----------------+----------------+----------------+
|                | After Boot     | After Hackbench|
| Normal         | 194 Kb         | 349 Kb         |
| With Patch     | 191 Kb         | 344 Kb         |
| Wastage reduce | ~1%            | ~1%            |
+----------------+----------------+----------------+

+-----------------+----------------+----------------+
|            Total slub memory                      |
+-----------------+----------------+----------------+
|                 | After Boot      | After Hackbench|
| Normal          | 330304          | 472960        |
| With Patch      | 319808          | 458944        |
| Memory reduce   | ~3%             | ~3%           |
+-----------------+----------------+----------------+

hackbench-process-sockets
+-------+-----+----------+----------+---------+
|             |  Normal  |With Patch|         |
+-------+----+----------+----------+----------+
| Amean | 1  |  1.9030  |  1.8967  | ( 0.33%) |
| Amean |  4 |   7.2117 |   7.1283 | ( 1.16%) |
| Amean |  7 |  12.5247 |  12.3460 | ( 1.43%) |
| Amean | 12 |  21.7157 |  21.4753 | ( 1.11%) |
| Amean | 21 |  38.2693 |  37.6670 | ( 1.57%) |
| Amean | 30 |  54.5930 |  53.8657 | ( 1.33%) |
| Amean | 48 |  87.6700 |  86.3690 | ( 1.48%) |
| Amean | 64 | 117.1227 | 115.4893 | ( 1.39%) |
+-------+----+----------+----------+----------+

Signed-off-by: Jay Patel <jaypatel@linux.ibm.com>
---
 mm/slub.c | 52 +++++++++++++++++++++++++---------------------------
 1 file changed, 25 insertions(+), 27 deletions(-)

diff --git a/mm/slub.c b/mm/slub.c
index c87628cd8a9a..0a1090c528da 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -4058,7 +4058,7 @@ EXPORT_SYMBOL(kmem_cache_alloc_bulk);
  */
 static unsigned int slub_min_order;
 static unsigned int slub_max_order =
-	IS_ENABLED(CONFIG_SLUB_TINY) ? 1 : PAGE_ALLOC_COSTLY_ORDER;
+	IS_ENABLED(CONFIG_SLUB_TINY) ? 1 : 2;
 static unsigned int slub_min_objects;
 
 /*
@@ -4087,11 +4087,10 @@ static unsigned int slub_min_objects;
  * the smallest order which will fit the object.
  */
 static inline unsigned int calc_slab_order(unsigned int size,
-		unsigned int min_objects, unsigned int max_order,
-		unsigned int fract_leftover)
+		unsigned int min_objects, unsigned int max_order)
 {
 	unsigned int min_order = slub_min_order;
-	unsigned int order;
+	unsigned int order, min_wastage = size, min_wastage_order = MAX_ORDER+1;
 
 	if (order_objects(min_order, size) > MAX_OBJS_PER_PAGE)
 		return get_order(size * MAX_OBJS_PER_PAGE) - 1;
@@ -4104,11 +4103,17 @@ static inline unsigned int calc_slab_order(unsigned int size,
 
 		rem = slab_size % size;
 
-		if (rem <= slab_size / fract_leftover)
-			break;
+		if (rem < min_wastage) {
+			min_wastage = rem;
+			min_wastage_order = order;
+		}
 	}
 
-	return order;
+	if (min_wastage_order <= slub_max_order)
+		return min_wastage_order;
+	else
+		return order;
+
 }
 
 static inline int calculate_order(unsigned int size)
@@ -4142,35 +4147,28 @@ static inline int calculate_order(unsigned int size)
 			nr_cpus = nr_cpu_ids;
 		min_objects = 4 * (fls(nr_cpus) + 1);
 	}
+
+	if ((min_objects * size) > (PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER))
+		return PAGE_ALLOC_COSTLY_ORDER;
+
+	if ((min_objects * size) <= PAGE_SIZE)
+		return slub_min_order;
+
 	max_objects = order_objects(slub_max_order, size);
 	min_objects = min(min_objects, max_objects);
 
-	while (min_objects > 1) {
-		unsigned int fraction;
-
-		fraction = 16;
-		while (fraction >= 4) {
-			order = calc_slab_order(size, min_objects,
-					slub_max_order, fraction);
-			if (order <= slub_max_order)
-				return order;
-			fraction /= 2;
-		}
+	while (min_objects >= 1) {
+		order = calc_slab_order(size, min_objects,
+		slub_max_order);
+		if (order <= slub_max_order)
+			return order;
 		min_objects--;
 	}
 
-	/*
-	 * We were unable to place multiple objects in a slab. Now
-	 * lets see if we can place a single object there.
-	 */
-	order = calc_slab_order(size, 1, slub_max_order, 1);
-	if (order <= slub_max_order)
-		return order;
-
 	/*
 	 * Doh this slab cannot be placed using slub_max_order.
 	 */
-	order = calc_slab_order(size, 1, MAX_ORDER, 1);
+	order = calc_slab_order(size, 1, MAX_ORDER);
 	if (order <= MAX_ORDER)
 		return order;
 	return -ENOSYS;
-- 
2.39.1