fs/file.c: add fast path in find_next_fd()

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 <[email protected]>, and agreed by Jan Kara <[email protected]>, 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 <[email protected]> Reviewed-by: Tim Chen <[email protected]> Signed-off-by: Yu Ma <[email protected]> Link: https://lore.kernel.org/r/[email protected] Signed-off-by: Christian Brauner <[email protected]> Signed-off-by: Al Viro <[email protected]> (cherry picked from commit 0c40bf47cf2d9e1413b1e62826c89c2341e66e40)
deepin-community · Jan 17, 2025 · a8fb2d5 · a8fb2d5
1 parent a643cb8
commit a8fb2d5
Showing 1 changed file with 9 additions and 0 deletions.
diff --git a/fs/file.c b/fs/file.c
@@ -467,6 +467,15 @@ 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)