[PATCH] random: give up adding randomness to the entropy pool when block devices are busy

Hi,

This gives up adding randomness to the entropy pool when block devices
are busy, to avoid lock contention in I/O intensive situation.

Currently randomness is added to the entropy pool for each 4096 I/O
completion, and holds a global spinlock during its process.
Contention for the spinlock occurs In I/O intensive situation that
many cores process I/O completions, even each core processes
completion for different block device.

This just replaces spin_lock_irqsave() in mix_pool_bytes()
with spin_trylock_irqsave(), and gives up adding randomness
when trylock is failed.
And, it moves trace_mix_pool_bytes() after lock, to trace
events only where addition of randomness is executed.

Thank you,

Signed-off-by: Kazuya Hisaki <kazuya.hisaki.bx@hitachi.com>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-kernel@vger.kernel.org
[PATCH v2] zsmalloc: merge size_class to reduce fragmentation

zsmalloc has many size_classes to reduce fragmentation and they are
in 16 bytes unit, for example, 16, 32, 48, etc., if PAGE_SIZE is 4096.
And, zsmalloc has constraint that each zspage has 4 pages at maximum.

In this situation, we can see interesting aspect.
Let's think about size_class for 1488, 1472, ..., 1376.
To prevent external fragmentation, they uses 4 pages per zspage and
so all they can contain 11 objects at maximum.

16384 (4096 * 4) = 1488 * 11 + remains
16384 (4096 * 4) = 1472 * 11 + remains
16384 (4096 * 4) = ...
16384 (4096 * 4) = 1376 * 11 + remains

It means that they have same characteristics and classification between
them isn't needed. If we use one size_class for them, we can reduce
fragementation and save some memory. For this purpose, this patch
implement size_class merging. If there is size_class that have
same pages_per_zspage and same number of objects per zspage with previous
size_class, we don't create and use new size_class. Instead, we use
previous, same characteristic size_class. With this way, above example
sizes (1488, 1472, ..., 1376) use just one size_class so we can get much
more memory utilization.

Below is result of my simple test.

TEST ENV: EXT4 on zram, mount with discard option
WORKLOAD: untar kernel source code, remove directory in descending order
in size. (drivers arch fs sound include net Documentation firmware
kernel tools)

Each line represents orig_data_size, compr_data_size, mem_used_total,
fragmentation overhead (mem_used - compr_data_size) and overhead ratio
(overhead to compr_data_size), respectively, after untar and remove
operation is executed.

* untar-nomerge.out

orig_size compr_size used_size overhead overhead_ratio
525.88MB 199.16MB 210.23MB  11.08MB 5.56%
288.32MB  97.43MB 105.63MB   8.20MB 8.41%
177.32MB  61.12MB  69.40MB   8.28MB 13.55%
146.47MB  47.32MB  56.10MB   8.78MB 18.55%
124.16MB  38.85MB  48.41MB   9.55MB 24.58%
103.93MB  31.68MB  40.93MB   9.25MB 29.21%
 84.34MB  22.86MB  32.72MB   9.86MB 43.13%
 66.87MB  14.83MB  23.83MB   9.00MB 60.70%
 60.67MB  11.11MB  18.60MB   7.49MB 67.48%
 55.86MB   8.83MB  16.61MB   7.77MB 88.03%
 53.32MB   8.01MB  15.32MB   7.31MB 91.24%

* untar-merge.out

orig_size compr_size used_size overhead overhead_ratio
526.23MB 199.18MB 209.81MB  10.64MB 5.34%
288.68MB  97.45MB 104.08MB   6.63MB 6.80%
177.68MB  61.14MB  66.93MB   5.79MB 9.47%
146.83MB  47.34MB  52.79MB   5.45MB 11.51%
124.52MB  38.87MB  44.30MB   5.43MB 13.96%
104.29MB  31.70MB  36.83MB   5.13MB 16.19%
 84.70MB  22.88MB  27.92MB   5.04MB 22.04%
 67.11MB  14.83MB  19.26MB   4.43MB 29.86%
 60.82MB  11.10MB  14.90MB   3.79MB 34.17%
 55.90MB   8.82MB  12.61MB   3.79MB 42.97%
 53.32MB   8.01MB  11.73MB   3.73MB 46.53%

As you can see above result, merged one has better utilization (overhead
ratio, 5th column) and uses less memory (mem_used_total, 3rd column).

Changed from v1:
- More commit description about what to do in this patch.
- Remove nr_obj in size_class, because it isn't need after initialization.
- Rename __size_class to size_class, size_class to merged_size_class.
- Add code comment for merged_size_class of struct zs_pool.
- Add code comment how merging works in zs_create_pool().

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>