[PATCH] 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
benchmarking the algorithms) instead use the higher precision local_time
for benchmarking. This patch performs 2,500 iterations of the benchmark
measurements which works out to be accurate enough for benchmarking the
raid algorithm data rates. Also add division by zero checking in case
timing measurements are bogus.

Measuring 100 re-boots on Intel(R) Core(TM) Ultra 9 285K with
improves raid64 benchmarking loop from ~68000 usecs to ~5300 usec.

This patch has been in use in Clear Linux for ~2 years w/o issues.

Signed-off-by: Colin Ian King <colin.king@intel.com>
---
 lib/raid6/algos.c | 53 ++++++++++++++++++++---------------------------
 1 file changed, 22 insertions(+), 31 deletions(-)

diff --git a/lib/raid6/algos.c b/lib/raid6/algos.c
index cd2e88ee1f14..b846635542bc 100644
--- a/lib/raid6/algos.c
+++ b/lib/raid6/algos.c
@@ -18,6 +18,8 @@
 #else
 #include <linux/module.h>
 #include <linux/gfp.h>
+#include <linux/sched/clock.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 +157,15 @@ 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 = 2500;
 	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 +175,20 @@ 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) ? n / ns : 0, ns);
 		}
 	}
 
@@ -206,31 +205,23 @@ 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) ? 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) ? n / ns : 0, ns);
 	}
-
 out:
 	return best;
 }
-- 
2.49.0