[PATCH v4 3/3] fs/file.c: add fast path in find_next_fd()

[Yu Ma]

Skip 2-levels searching via find_next_zero_bit() when there is free slot in the
word contains next_fd, as:
(1) next_fd indicates the lower bound for the first free fd.
(2) There is fast path inside of find_next_zero_bit() when size<=64 to speed up
searching.
(3) After fdt is expanded (the bitmap size doubled for each time of expansion),
it would never be shrunk. The search size increases but there are few open fds
available here.

This fast path is proposed by Mateusz Guzik <mjguzik@xxxxxxxxx>, and agreed by
Jan Kara <jack@xxxxxxx>, which is more generic and scalable than previous
versions. And on top of patch 1 and 2, it improves pts/blogbench-1.1.0 read by
8% and write by 4% on Intel ICX 160 cores configuration with v6.10-rc7.

Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
Signed-off-by: Yu Ma <yu.ma@xxxxxxxxx>
---
 fs/file.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/fs/file.c b/fs/file.c
index 1be2a5bcc7c4..a3ce6ba30c8c 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -488,9 +488,20 @@ struct files_struct init_files = {
 
 static unsigned int find_next_fd(struct fdtable *fdt, unsigned int start)
 {
+	unsigned int bitbit = start / BITS_PER_LONG;
+	unsigned int bit;
+
+	/*
+	 * Try to avoid looking at the second level bitmap
+	 */
+	bit = find_next_zero_bit(&fdt->open_fds[bitbit], BITS_PER_LONG,
+				 start & (BITS_PER_LONG -1));
+	if (bit < BITS_PER_LONG) {
+		return bit + bitbit * BITS_PER_LONG;
+	}
+
 	unsigned int maxfd = fdt->max_fds; /* always multiple of BITS_PER_LONG */
 	unsigned int maxbit = maxfd / BITS_PER_LONG;
-	unsigned int bitbit = start / BITS_PER_LONG;
 
 	bitbit = find_next_zero_bit(fdt->full_fds_bits, maxbit, bitbit) * BITS_PER_LONG;
 	if (bitbit >= maxfd)
-- 
2.43.0