linux-kernel - [PATCH 5/5] lib/test_vmalloc: Switch to prandom

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Message-Id: <20220607093449.3100-6-urezki@gmail.com>
Date:   Tue,  7 Jun 2022 11:34:49 +0200
From:   "Uladzislau Rezki (Sony)" <urezki@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Uladzislau Rezki <urezki@...il.com>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: [PATCH 5/5] 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.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.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