linux-kernel - [PATCH 1/4] mm/percpu: micro-optimize pcpu_is

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Date:   Sun, 18 Jul 2021 19:17:52 -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 1/4] mm/percpu: micro-optimize pcpu_is_populated()

bitmap_next_clear_region() calls find_next_zero_bit() and find_next_bit()
sequentially to find a range of clear bits. In case of pcpu_is_populated()
there's a chance to return earlier if bitmap has all bits set.

Signed-off-by: Yury Norov <yury.norov@...il.com>
---
 mm/percpu.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/mm/percpu.c b/mm/percpu.c
index 7f2e0151c4e2..25461571dcc5 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -1071,17 +1071,18 @@ static void pcpu_block_update_hint_free(struct pcpu_chunk *chunk, int bit_off,
 static bool pcpu_is_populated(struct pcpu_chunk *chunk, int bit_off, int bits,
 			      int *next_off)
 {
-	unsigned int page_start, page_end, rs, re;
+	unsigned int start, end;
 
-	page_start = PFN_DOWN(bit_off * PCPU_MIN_ALLOC_SIZE);
-	page_end = PFN_UP((bit_off + bits) * PCPU_MIN_ALLOC_SIZE);
+	start = PFN_DOWN(bit_off * PCPU_MIN_ALLOC_SIZE);
+	end = PFN_UP((bit_off + bits) * PCPU_MIN_ALLOC_SIZE);
 
-	rs = page_start;
-	bitmap_next_clear_region(chunk->populated, &rs, &re, page_end);
-	if (rs >= page_end)
+	start = find_next_zero_bit(chunk->populated, end, start);
+	if (start >= end)
 		return true;
 
-	*next_off = re * PAGE_SIZE / PCPU_MIN_ALLOC_SIZE;
+	end = find_next_bit(chunk->populated, end, start + 1);
+
+	*next_off = end * PAGE_SIZE / PCPU_MIN_ALLOC_SIZE;
 	return false;
 }
 
-- 
2.30.2