linux-kernel - [PATCH 4/4] vsprintf: rework bitmap_list

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Date:   Sun, 18 Jul 2021 19:17:55 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-mmc@...r.kernel.org
Cc:     Yury Norov <yury.norov@...il.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Tejun Heo <tj@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Dennis Zhou <dennis@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH 4/4] vsprintf: rework bitmap_list_string

bitmap_list_string() is very ineffective when printing bitmaps with long
ranges of set bits because it calls find_next_bit() for each bit in the
bitmap.  We can do better by detecting ranges of set bits.

In my environment, before/after is 943008/31008 ns.

Signed-off-by: Yury Norov <yury.norov@...il.com>
---
 lib/vsprintf.c | 24 +++++++-----------------
 1 file changed, 7 insertions(+), 17 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 3b8b3f20051a..361799075706 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -1241,20 +1241,13 @@ char *bitmap_list_string(char *buf, char *end, unsigned long *bitmap,
 			 struct printf_spec spec, const char *fmt)
 {
 	int nr_bits = max_t(int, spec.field_width, 0);
-	/* current bit is 'cur', most recently seen range is [rbot, rtop] */
-	int cur, rbot, rtop;
 	bool first = true;
+	int rbot, rtop;
 
 	if (check_pointer(&buf, end, bitmap, spec))
 		return buf;
 
-	rbot = cur = find_first_bit(bitmap, nr_bits);
-	while (cur < nr_bits) {
-		rtop = cur;
-		cur = find_next_bit(bitmap, nr_bits, cur + 1);
-		if (cur < nr_bits && cur <= rtop + 1)
-			continue;
-
+	for_each_set_bitrange(rbot, rtop, bitmap, nr_bits) {
 		if (!first) {
 			if (buf < end)
 				*buf = ',';
@@ -1263,15 +1256,12 @@ char *bitmap_list_string(char *buf, char *end, unsigned long *bitmap,
 		first = false;
 
 		buf = number(buf, end, rbot, default_dec_spec);
-		if (rbot < rtop) {
-			if (buf < end)
-				*buf = '-';
-			buf++;
-
-			buf = number(buf, end, rtop, default_dec_spec);
-		}
+		if (rtop == rbot + 1)
+			continue;
 
-		rbot = cur;
+		if (buf < end)
+			*buf = '-';
+		buf = number(++buf, end, rtop - 1, default_dec_spec);
 	}
 	return buf;
 }
-- 
2.30.2