[PATCH 2/8] xor: improve the runtime selection benchmark

[Christoph Hellwig]

Use plain ktime_get_ns for the timing, use 4 + 1 disks for a realistic
load, and report the throughput on the data disks instead of the that
on the parity disk, which isn't all that useful.

Signed-off-by: Christoph Hellwig <hch@xxxxxx>
---
 lib/raid/xor/xor-core.c | 55 +++++++++++++++++++----------------------
 1 file changed, 26 insertions(+), 29 deletions(-)

diff --git a/lib/raid/xor/xor-core.c b/lib/raid/xor/xor-core.c
index 50931fbf0324..a890cec21bbc 100644
--- a/lib/raid/xor/xor-core.c
+++ b/lib/raid/xor/xor-core.c
@@ -10,7 +10,6 @@
 #include <linux/gfp.h>
 #include <linux/slab.h>
 #include <linux/raid/xor.h>
-#include <linux/jiffies.h>
 #include <linux/preempt.h>
 #include <linux/static_call.h>
 #include "xor_impl.h"
@@ -73,59 +72,56 @@ void __init xor_force(struct xor_block_template *tmpl)
 	forced_template = tmpl;
 }
 
-#define BENCH_SIZE	4096
+#define BENCH_SIZE	SZ_4K
+#define NR_SRCS		4
 #define REPS		800U
 
-static void __init
-do_xor_speed(struct xor_block_template *tmpl, void *b1, void *b2)
+static void __init do_xor_speed(struct xor_block_template *tmpl, void *dest,
+		void *srcs[NR_SRCS])
 {
-	int speed;
-	unsigned long reps;
-	ktime_t min, start, t0;
-	void *srcs[1] = { b2 };
+	u64 t;
+	int i;
 
 	preempt_disable();
-
-	reps = 0;
-	t0 = ktime_get();
-	/* delay start until time has advanced */
-	while ((start = ktime_get()) == t0)
-		cpu_relax();
-	do {
+	t = ktime_get_ns();
+	for (i = 0; i < REPS; i++) {
 		mb(); /* prevent loop optimization */
-		tmpl->xor_gen(b1, srcs, 1, BENCH_SIZE);
+		tmpl->xor_gen(dest, srcs, NR_SRCS, BENCH_SIZE);
 		mb();
-	} while (reps++ < REPS || (t0 = ktime_get()) == start);
-	min = ktime_sub(t0, start);
-
+	}
+	t = ktime_get_ns() - t;
 	preempt_enable();
 
-	// bytes/ns == GB/s, multiply by 1000 to get MB/s [not MiB/s]
-	speed = (1000 * reps * BENCH_SIZE) / (unsigned int)ktime_to_ns(min);
-	tmpl->speed = speed;
+	/* bytes/ns == GB/s, multiply by 1000 to get MB/s [not MiB/s] */
+	tmpl->speed = div64_u64((u64)BENCH_SIZE * REPS * NR_SRCS * 1000, t);
 
-	pr_info("   %-16s: %5d MB/sec\n", tmpl->name, speed);
+	pr_info("   %-16s: %5d MB/sec\n", tmpl->name, tmpl->speed);
 }
 
 static int __init calibrate_xor_blocks(void)
 {
-	void *b1, *b2;
 	struct xor_block_template *f, *fastest;
+	void *srcs[NR_SRCS];
+	void *buf, *dest;
+	int i;
 
 	if (forced_template)
 		return 0;
 
-	b1 = kmalloc(PAGE_SIZE * 4, GFP_KERNEL);
-	if (!b1) {
+	buf = kmalloc(BENCH_SIZE * (NR_SRCS + 1), GFP_KERNEL);
+	if (!buf) {
 		pr_warn("xor: Yikes!  No memory available.\n");
 		return -ENOMEM;
 	}
-	b2 = b1 + 2*PAGE_SIZE + BENCH_SIZE;
+	get_random_bytes(buf, BENCH_SIZE * (NR_SRCS + 1));
+	dest = buf;
+	for (i = 0; i < NR_SRCS; i++)
+		srcs[i] = buf + (i + 1) * BENCH_SIZE;
 
 	pr_info("xor: measuring software checksum speed\n");
 	fastest = template_list;
 	for (f = template_list; f; f = f->next) {
-		do_xor_speed(f, b1, b2);
+		do_xor_speed(f, dest, srcs);
 		if (f->speed > fastest->speed)
 			fastest = f;
 	}
@@ -133,9 +129,10 @@ static int __init calibrate_xor_blocks(void)
 	pr_info("xor: using function: %s (%d MB/sec)\n",
 	       fastest->name, fastest->speed);
 
-	kfree(b1);
+	kfree(buf);
 	return 0;
 }
+#undef NR_SRCS
 
 #ifdef CONFIG_XOR_BLOCKS_ARCH
 #include "xor_arch.h" /* $SRCARCH/xor_arch.h */
-- 
2.53.0