[PATCH][next][V2] md/raid6 algorithms: scale test duration for speedier boots

[Colin Ian King <colin.king@intel.com>]

Instead of using jiffies and waiting for jiffies to wrap before
measuring use the higher precision local_time for benchmarking.
Measure 10,000 loops, which works out to be accurate enough for
benchmarking the raid algo data rates. Also add division by zero
checking in case timing measurements are bogus.

Speeds up raid benchmarkingm, reduces calibration time from
48,000 usecs to 21,000 usecs on a i9-19200.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>

---

V2: Increase max_perf loops to 10,000 for more stable benchmarking on
    slower devices and use div64_u64 for 64 bit unsigned int divisions
    to fix build problems on 32 bit systems.

---
 lib/raid6/algos.c | 57 +++++++++++++++++++++--------------------------
 1 file changed, 25 insertions(+), 32 deletions(-)

diff --git a/lib/raid6/algos.c b/lib/raid6/algos.c
index cd2e88ee1f14..141931563dd1 100644
--- a/lib/raid6/algos.c
+++ b/lib/raid6/algos.c
@@ -18,6 +18,9 @@
 #else
 #include <linux/module.h>
 #include <linux/gfp.h>
+#include <linux/sched/clock.h>
+#include <linux/math64.h>
+
 /* In .bss so it's zeroed */
 const char raid6_empty_zero_page[PAGE_SIZE] __attribute__((aligned(256)));
 EXPORT_SYMBOL(raid6_empty_zero_page);
@@ -155,12 +158,15 @@ static inline const struct raid6_recov_calls *raid6_choose_recov(void)
 static inline const struct raid6_calls *raid6_choose_gen(
 	void *(*const dptrs)[RAID6_TEST_DISKS], const int disks)
 {
-	unsigned long perf, bestgenperf, j0, j1;
+	unsigned long perf;
+	const unsigned long max_perf = 10000;
 	int start = (disks>>1)-1, stop = disks-3;	/* work on the second half of the disks */
 	const struct raid6_calls *const *algo;
 	const struct raid6_calls *best;
+	const u64 ns_per_mb = 1000000000 >> 20;
+	u64 n, ns, t, ns_best = ~0ULL;
 
-	for (bestgenperf = 0, best = NULL, algo = raid6_algos; *algo; algo++) {
+	for (best = NULL, algo = raid6_algos; *algo; algo++) {
 		if (!best || (*algo)->priority >= best->priority) {
 			if ((*algo)->valid && !(*algo)->valid())
 				continue;
@@ -170,26 +176,20 @@ static inline const struct raid6_calls *raid6_choose_gen(
 				break;
 			}
 
-			perf = 0;
-
 			preempt_disable();
-			j0 = jiffies;
-			while ((j1 = jiffies) == j0)
-				cpu_relax();
-			while (time_before(jiffies,
-					    j1 + (1<<RAID6_TIME_JIFFIES_LG2))) {
+			t = local_clock();
+			for (perf = 0; perf < max_perf; perf++)
 				(*algo)->gen_syndrome(disks, PAGE_SIZE, *dptrs);
-				perf++;
-			}
+			ns = local_clock() - t;
 			preempt_enable();
 
-			if (perf > bestgenperf) {
-				bestgenperf = perf;
+			if (ns < ns_best) {
+				ns_best = ns;
 				best = *algo;
 			}
-			pr_info("raid6: %-8s gen() %5ld MB/s\n", (*algo)->name,
-				(perf * HZ * (disks-2)) >>
-				(20 - PAGE_SHIFT + RAID6_TIME_JIFFIES_LG2));
+			n = max_perf * PAGE_SIZE * ns_per_mb * (disks - 2);
+			pr_info("raid6: %-8s gen() %5llu MB/s (%llu ns)\n", (*algo)->name,
+				(ns > 0) ? div64_u64(n, ns) : 0, ns);
 		}
 	}
 
@@ -206,31 +206,24 @@ static inline const struct raid6_calls *raid6_choose_gen(
 		goto out;
 	}
 
-	pr_info("raid6: using algorithm %s gen() %ld MB/s\n",
-		best->name,
-		(bestgenperf * HZ * (disks - 2)) >>
-		(20 - PAGE_SHIFT + RAID6_TIME_JIFFIES_LG2));
+	n = max_perf * PAGE_SIZE * ns_per_mb * (disks - 2);
+	pr_info("raid6: using algorithm %s gen() %llu MB/s (%llu ns)\n",
+		best->name, (ns_best > 0) ? div64_u64(n, ns_best) : 0, ns_best);
 
 	if (best->xor_syndrome) {
-		perf = 0;
-
 		preempt_disable();
-		j0 = jiffies;
-		while ((j1 = jiffies) == j0)
-			cpu_relax();
-		while (time_before(jiffies,
-				   j1 + (1 << RAID6_TIME_JIFFIES_LG2))) {
+		t = local_clock();
+		for (perf = 0; perf < max_perf; perf++) {
 			best->xor_syndrome(disks, start, stop,
 					   PAGE_SIZE, *dptrs);
-			perf++;
 		}
+		ns = local_clock() - t;
 		preempt_enable();
 
-		pr_info("raid6: .... xor() %ld MB/s, rmw enabled\n",
-			(perf * HZ * (disks - 2)) >>
-			(20 - PAGE_SHIFT + RAID6_TIME_JIFFIES_LG2 + 1));
+		n = max_perf * PAGE_SIZE * ns_per_mb * (disks - 2);
+		pr_info("raid6: .... xor() %llu MB/s, rmw enabled (%llu ns)\n",
+			(ns > 0) ? div64_u64(n, ns) : 0, ns);
 	}
-
 out:
 	return best;
 }
-- 
2.43.0