[PATCH 5/5] lib/test_vmalloc: Switch to prandom_u32()

Uladzislau Rezki (Sony)
2022-06-07T09:36:10Z

A get_random_bytes() function can cause a high contention if it is
called across CPUs simultaneously. Because it shares one lock per
all CPUs:

<snip>
class name con-bounces contentions waittime-min waittime-max waittime-total waittime-avg acq-bounces acquisitions holdtime-min holdtime-max holdtime-total holdtime-avg
&crng->lock: 663145 665886 0.05 8.85 261966.66 0.39 7188152 13731279 0.04 11.89 2181582.30 0.16
-----------
&crng->lock 307835 [<00000000acba59cd>] _extract_crng+0x48/0x90
&crng->lock 358051 [<00000000f0075abc>] _crng_backtrack_protect+0x32/0x90
-----------
&crng->lock 234241 [<00000000f0075abc>] _crng_backtrack_protect+0x32/0x90
&crng->lock 431645 [<00000000acba59cd>] _extract_crng+0x48/0x90
<snip>

Switch from the get_random_bytes() to prandom_u32() that does not
have any internal contention when a random value is needed for the
tests.

The reason is to minimize CPU cycles introduced by the test-suite
itself from the vmalloc performance metrics.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
lib/test_vmalloc.c | 15 ++++++++-------
1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/lib/test_vmalloc.c b/lib/test_vmalloc.c
index cf41fd6df42a..4f2f2d1bac56 100644
--- a/lib/test_vmalloc.c
+++ b/lib/test_vmalloc.c
@@ -74,12 +74,13 @@ test_report_one_done(void)

static int random_size_align_alloc_test(void)
{
- unsigned long size, align, rnd;
+ unsigned long size, align;
+ unsigned int rnd;
void *ptr;
int i;

for (i = 0; i < test_loop_count; i++) {
- get_random_bytes(&rnd, sizeof(rnd));
+ rnd = prandom_u32();

/*
* Maximum 1024 pages, if PAGE_SIZE is 4096.
@@ -150,7 +151,7 @@ static int random_size_alloc_test(void)
int i;

for (i = 0; i < test_loop_count; i++) {
- get_random_bytes(&n, sizeof(i));
+ n = prandom_u32();
n = (n % 100) + 1;

p = vmalloc(n * PAGE_SIZE);
@@ -294,14 +295,14 @@ pcpu_alloc_test(void)
for (i = 0; i < 35000; i++) {
unsigned int r;

- get_random_bytes(&r, sizeof(i));
+ r = prandom_u32();
size = (r % (PAGE_SIZE / 4)) + 1;

/*
* Maximum PAGE_SIZE
*/
- get_random_bytes(&r, sizeof(i));
- align = 1 << ((i % 11) + 1);
+ r = prandom_u32();
+ align = 1 << ((r % 11) + 1);

pcpu[i] = __alloc_percpu(size, align);
if (!pcpu[i])
@@ -396,7 +397,7 @@ static void shuffle_array(int *arr, int n)
int i, j;

for (i = n - 1; i > 0; i--) {
- get_random_bytes(&rnd, sizeof(rnd));
+ rnd = prandom_u32();

/* Cut the range. */
j = rnd % i;
--
2.30.2