[PATCH 05/17] lib: add find_first_and_bit()

Yury Norov
2021-08-14T21:18:21Z

Currently find_first_and_bit() is an alias to find_next_and_bit(). However,
it is widely used in cpumask, so it worth to optimize it. This patch adds
its own implementation for find_first_and_bit().

On x86_64 find_bit_benchmark says:

Before (#define find_first_and_bit(...) find_next_and_bit(..., 0):
Start testing find_bit() with random-filled bitmap
[ 140.291468] find_first_and_bit: 46890919 ns, 32671 iterations
Start testing find_bit() with sparse bitmap
[ 140.295028] find_first_and_bit: 7103 ns, 1 iterations

After:
Start testing find_bit() with random-filled bitmap
[ 162.574907] find_first_and_bit: 25045813 ns, 32846 iterations
Start testing find_bit() with sparse bitmap
[ 162.578458] find_first_and_bit: 4900 ns, 1 iterations

(Thanks to Alexey Klimov for thorough testing.)

Signed-off-by: Yury Norov <yury.norov@gmail.com>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Alexey Klimov <aklimov@redhat.com>
---
include/linux/find.h | 27 +++++++++++++++++++++++++++
lib/find_bit.c | 21 +++++++++++++++++++++
lib/find_bit_benchmark.c | 21 +++++++++++++++++++++
3 files changed, 69 insertions(+)

diff --git a/include/linux/find.h b/include/linux/find.h
index ea57f7f38c49..6048f8c97418 100644
--- a/include/linux/find.h
+++ b/include/linux/find.h
@@ -12,6 +12,8 @@ extern unsigned long _find_next_bit(const unsigned long *addr1,
const unsigned long *addr2, unsigned long nbits,
unsigned long start, unsigned long invert, unsigned long le);
extern unsigned long _find_first_bit(const unsigned long *addr, unsigned long size);
+extern unsigned long _find_first_and_bit(const unsigned long *addr1,
+ const unsigned long *addr2, unsigned long size);
extern unsigned long _find_first_zero_bit(const unsigned long *addr, unsigned long size);
extern unsigned long _find_last_bit(const unsigned long *addr, unsigned long size);

@@ -123,6 +125,31 @@ unsigned long find_first_bit(const unsigned long *addr, unsigned long size)
}
#endif

+#ifndef find_first_and_bit
+/**
+ * find_first_and_bit - find the first set bit in both memory regions
+ * @addr1: The first address to base the search on
+ * @addr2: The second address to base the search on
+ * @size: The bitmap size in bits
+ *
+ * Returns the bit number for the next set bit
+ * If no bits are set, returns @size.
+ */
+static inline
+unsigned long find_first_and_bit(const unsigned long *addr1,
+ const unsigned long *addr2,
+ unsigned long size)
+{
+ if (small_const_nbits(size)) {
+ unsigned long val = *addr1 & *addr2 & GENMASK(size - 1, 0);
+
+ return val ? __ffs(val) : size;
+ }
+
+ return _find_first_and_bit(addr1, addr2, size);
+}
+#endif
+
#ifndef find_first_zero_bit
/**
* find_first_zero_bit - find the first cleared bit in a memory region
diff --git a/lib/find_bit.c b/lib/find_bit.c
index 0f8e2e369b1d..1b8e4b2a9cba 100644
--- a/lib/find_bit.c
+++ b/lib/find_bit.c
@@ -89,6 +89,27 @@ unsigned long _find_first_bit(const unsigned long *addr, unsigned long size)
EXPORT_SYMBOL(_find_first_bit);
#endif

+#ifndef find_first_and_bit
+/*
+ * Find the first set bit in two memory regions.
+ */
+unsigned long _find_first_and_bit(const unsigned long *addr1,
+ const unsigned long *addr2,
+ unsigned long size)
+{
+ unsigned long idx, val;
+
+ for (idx = 0; idx * BITS_PER_LONG < size; idx++) {
+ val = addr1[idx] & addr2[idx];
+ if (val)
+ return min(idx * BITS_PER_LONG + __ffs(val), size);
+ }
+
+ return size;
+}
+EXPORT_SYMBOL(_find_first_and_bit);
+#endif
+
#ifndef find_first_zero_bit
/*
* Find the first cleared bit in a memory region.
diff --git a/lib/find_bit_benchmark.c b/lib/find_bit_benchmark.c
index 5637c5711db9..db904b57d4b8 100644
--- a/lib/find_bit_benchmark.c
+++ b/lib/find_bit_benchmark.c
@@ -49,6 +49,25 @@ static int __init test_find_first_bit(void *bitmap, unsigned long len)
return 0;
}

+static int __init test_find_first_and_bit(void *bitmap, const void *bitmap2, unsigned long len)
+{
+ static DECLARE_BITMAP(cp, BITMAP_LEN) __initdata;
+ unsigned long i, cnt;
+ ktime_t time;
+
+ bitmap_copy(cp, bitmap, BITMAP_LEN);
+
+ time = ktime_get();
+ for (cnt = i = 0; i < len; cnt++) {
+ i = find_first_and_bit(cp, bitmap2, len);
+ __clear_bit(i, cp);
+ }
+ time = ktime_get() - time;
+ pr_err("find_first_and_bit: %18llu ns, %6ld iterations\n", time, cnt);
+
+ return 0;
+}
+
static int __init test_find_next_bit(const void *bitmap, unsigned long len)
{
unsigned long i, cnt;
@@ -129,6 +148,7 @@ static int __init find_bit_test(void)
* traverse only part of bitmap to avoid soft lockup.
*/
test_find_first_bit(bitmap, BITMAP_LEN / 10);
+ test_find_first_and_bit(bitmap, bitmap2, BITMAP_LEN / 2);
test_find_next_and_bit(bitmap, bitmap2, BITMAP_LEN);

pr_err("\nStart testing find_bit() with sparse bitmap\n");
@@ -145,6 +165,7 @@ static int __init find_bit_test(void)
test_find_next_zero_bit(bitmap, BITMAP_LEN);
test_find_last_bit(bitmap, BITMAP_LEN);
test_find_first_bit(bitmap, BITMAP_LEN);
+ test_find_first_and_bit(bitmap, bitmap2, BITMAP_LEN);
test_find_next_and_bit(bitmap, bitmap2, BITMAP_LEN);

/*
--
2.30.2