[PATCH v3 3/3] fs/file.c: add fast path in find_next_fd()

[Yu Ma <yu.ma@intel.com>]

There is available fd in the lower 64 bits of open_fds bitmap for most cases
when we look for an available fd slot. Skip 2-levels searching via
find_next_zero_bit() for this common fast path.

Look directly for an open bit in the lower 64 bits of open_fds bitmap when a
free slot is available there, as:
(1) The fd allocation algorithm would always allocate fd from small to large.
Lower bits in open_fds bitmap would be used much more frequently than higher
bits.
(2) 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.
(3) There is fast path inside of find_next_zero_bit() when size<=64 to speed up
searching.

As suggested by Mateusz Guzik <mjguzik gmail.com> and Jan Kara <jack@suse.cz>,
update the fast path from alloc_fd() to find_next_fd(). With which, on top of
patch 1 and 2, pts/blogbench-1.1.0 read is improved by 13% and write by 7% on
Intel ICX 160 cores configuration with v6.10-rc6.

Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Yu Ma <yu.ma@intel.com>
---
 fs/file.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/file.c b/fs/file.c
index a15317db3119..f25eca311f51 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -488,6 +488,11 @@ struct files_struct init_files = {
 
 static unsigned int find_next_fd(struct fdtable *fdt, unsigned int start)
 {
+	unsigned int bit;
+	bit = find_next_zero_bit(fdt->open_fds, BITS_PER_LONG, start);
+	if (bit < BITS_PER_LONG)
+		return bit;
+
 	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;
-- 
2.43.0