lib/test_vmalloc: switch to prandom_u32()

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.

Link: https://lkml.kernel.org/r/20220607093449.3100-6-urezki@gmail.com
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oleksiy Avramchenko <oleksiy.avramchenko@sony.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/lib/test_vmalloc.c b/lib/test_vmalloc.c
index cf41fd6df42a08563175d558df645fff377e2e10..4f2f2d1bac562dee432d9ec23b8e303d4fbb96c8 100644 (file)
--- 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;