Re: [PATCH 1/3] fs/file.c: add fast path in alloc_fd()

[Mateusz Guzik]

On Fri, Jun 14, 2024 at 12:34:14PM -0400, Yu Ma wrote:
> 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.
> 
> With the fast path added in alloc_fd() through one-time bitmap searching,
> pts/blogbench-1.1.0 read is improved by 20% and write by 10% on Intel ICX 160
> cores configuration with v6.8-rc6.
> 
> Reviewed-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> Signed-off-by: Yu Ma <yu.ma@xxxxxxxxx>
> ---
>  fs/file.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/file.c b/fs/file.c
> index 3b683b9101d8..e8d2f9ef7fd1 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -510,8 +510,13 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
>  	if (fd < files->next_fd)
>  		fd = files->next_fd;
>  
> -	if (fd < fdt->max_fds)
> +	if (fd < fdt->max_fds) {
> +		if (~fdt->open_fds[0]) {
> +			fd = find_next_zero_bit(fdt->open_fds, BITS_PER_LONG, fd);
> +			goto success;
> +		}
>  		fd = find_next_fd(fdt, fd);
> +	}
>  
>  	/*
>  	 * N.B. For clone tasks sharing a files structure, this test
> @@ -531,7 +536,7 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
>  	 */
>  	if (error)
>  		goto repeat;
> -
> +success:
>  	if (start <= files->next_fd)
>  		files->next_fd = fd + 1;
>  

As indicated in my other e-mail it may be a process can reach a certain
fd number and then lower its rlimit(NOFILE). In that case the max_fds
field can happen to be higher and the above patch will fail to check for
the (fd < end) case.


> -- 
> 2.43.0
>