[09/11] fs/file.c: add fast path in find_next_fd()

Commit Message

Al Viro Aug. 12, 2024, 6:44 a.m. UTC

From: Yu Ma <[e-mail]>

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 <[e-mail]>, and agreed by
Jan Kara <[e-mail]>, 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: Jan Kara <[e-mail]>
Reviewed-by: Tim Chen <[e-mail]>
Signed-off-by: Yu Ma <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Christian Brauner <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>
---
 fs/file.c | 9 +++++++++
 1 file changed, 9 insertions(+)

Patch

diff --git a/fs/file.c b/fs/file.c
index 0340c811b22a..1ee85e061ade 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -490,6 +490,15 @@  static unsigned int find_next_fd(struct fdtable *fdt, unsigned int start)
 	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;
+	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;
 
 	bitbit = find_next_zero_bit(fdt->full_fds_bits, maxbit, bitbit) * BITS_PER_LONG;
 	if (bitbit >= maxfd)