mm/slub: Optimize slub memory usage

In the current implementation of the slub memory allocator, the slab
order selection process follows these criteria:

1) Determine the minimum order required to serve the minimum number of
objects (min_objects). This calculation is based on the formula (order
= min_objects * object_size / PAGE_SIZE).
2) If the minimum order is greater than the maximum allowed order
(slub_max_order), set slub_max_order as the order for this slab.
3) If the minimum order is less than the slub_max_order, iterate
through a loop from minimum order to slub_max_order and check if the
condition (rem <= slab_size / fract_leftover) holds true. Here,
slab_size is calculated as (PAGE_SIZE << order), rem is (slab_size %
object_size), and fract_leftover can have values of 16, 8, or 4. If
the condition is true, select that order for the slab.

However, in point 3, when calculating the fraction left over, it can
result in a large range of values (like 1 Kb to 256 bytes on 4K page
size & 4 Kb to 16 Kb on 64K page size with order 0 and goes on
increasing with higher order) when compared to the remainder (rem). This
can lead to the selection of an order that results in more memory
wastage. To mitigate such wastage, we have modified point 3 as follows:
To adjust the value of fract_leftover based on the page size, while
retaining the current value as the default for a 4K page size.

Test results are as follows:

1) On 160 CPUs with 64K Page size

+-----------------+----------------+----------------+
|          Total wastage in slub memory             |
+-----------------+----------------+----------------+
|                 | After Boot     |After Hackbench |
| Normal          | 932 Kb         | 1812 Kb        |
| With Patch      | 729 Kb         | 1636 Kb        |
| Wastage reduce  | ~22%           | ~10%           |
+-----------------+----------------+----------------+

+-----------------+----------------+----------------+
|            Total slub memory                      |
+-----------------+----------------+----------------+
|                 | After Boot     | After Hackbench|
| Normal          | 1855296        | 2944576        |
| With Patch      | 1544576        | 2692032        |
| Memory reduce   | ~17%           | ~9%            |
+-----------------+----------------+----------------+

hackbench-process-sockets
+-------+-----+----------+----------+-----------+
| Amean | 1   | 1.2727   | 1.2450   | ( 2.22%)  |
| Amean | 4   | 1.6063   | 1.5810   | ( 1.60%)  |
| Amean | 7   | 2.4190   | 2.3983   | ( 0.86%)  |
| Amean | 12  | 3.9730   | 3.9347   | ( 0.97%)  |
| Amean | 21  | 6.9823   | 6.8957   | ( 1.26%)  |
| Amean | 30  | 10.1867  | 10.0600  | ( 1.26%)  |
| Amean | 48  | 16.7490  | 16.4853  | ( 1.60%)  |
| Amean | 79  | 28.1870  | 27.8673  | ( 1.15%)  |
| Amean | 110 | 39.8363  | 39.3793  | ( 1.16%) |
| Amean | 141 | 51.5277  | 51.4907  | ( 0.07%)  |
| Amean | 172 | 62.9700  | 62.7300  | ( 0.38%)  |
| Amean | 203 | 74.5037  | 74.0630  | ( 0.59%)  |
| Amean | 234 | 85.6560  | 85.3587  | ( 0.35%)  |
| Amean | 265 | 96.9883  | 96.3770  | ( 0.63%)  |
| Amean | 296 | 108.6893 | 108.0870 | ( 0.56%)  |
+-------+-----+----------+----------+-----------+

2) On 16 CPUs with 64K Page size

+----------------+----------------+----------------+
|          Total wastage in slub memory            |
+----------------+----------------+----------------+
|                | After Boot     | After Hackbench|
| Normal         | 273 Kb         | 544 Kb         |
| With Patch     | 260 Kb         | 500 Kb         |
| Wastage reduce | ~5%            | ~9%            |
+----------------+----------------+----------------+

+-----------------+----------------+----------------+
|            Total slub memory                      |
+-----------------+----------------+----------------+
|                 | After Boot     | After Hackbench|
| Normal          | 275840          | 412480        |
| With Patch      | 272768          | 406208        |
| Memory reduce   | ~1%             | ~2%           |
+-----------------+----------------+----------------+

hackbench-process-sockets
+-------+----+---------+---------+-----------+
| Amean | 1  | 0.9513  | 0.9250  | ( 2.77%)  |
| Amean | 4  | 2.9630  | 2.9570  | ( 0.20%)  |
| Amean | 7  | 5.1780  | 5.1763  | ( 0.03%)  |
| Amean | 12 | 8.8833  | 8.8817  | ( 0.02%)  |
| Amean | 21 | 15.7577 | 15.6883 | ( 0.44%)  |
| Amean | 30 | 22.2063 | 22.2843 | ( -0.35%) |
| Amean | 48 | 36.0587 | 36.1390 | ( -0.22%) |
| Amean | 64 | 49.7803 | 49.3457 | ( 0.87%)  |
+-------+----+---------+---------+-----------+

Signed-off-by: Jay Patel <jaypatel@linux.ibm.com>