Re: [PATCH v2 1/3] fs/file.c: add fast path in alloc_fd()

Jan Kara
2024-06-25T11:52:59Z

On Sat 22-06-24 11:49:02, 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) find_next_zero_bit() itself has a fast path inside to speed up searching
> when size<=64.
>
> Besides, "!start" is added to fast path condition to ensure the allocated fd is
> greater than start (i.e. >=0), given alloc_fd() is only called in two scenarios:
> (1) Allocating a new fd (the most common usage scenario) via
> get_unused_fd_flags() to find fd start from bit 0 in fdt (i.e. start==0).
> (2) Duplicating a fd (less common usage) via dup_fd() to find a fd start from
> old_fd's index in fdt, which is only called by syscall fcntl.
>
> With the fast path added in alloc_fd(), pts/blogbench-1.1.0 read is improved
> by 17% and write by 9% on Intel ICX 160 cores configuration with v6.10-rc4.
>
> Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
> Signed-off-by: Yu Ma <yu.ma@intel.com>
> ---
> fs/file.c | 35 +++++++++++++++++++++--------------
> 1 file changed, 21 insertions(+), 14 deletions(-)
>
> diff --git a/fs/file.c b/fs/file.c
> index a3b72aa64f11..50e900a47107 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -515,28 +515,35 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
> if (fd < files->next_fd)
> fd = files->next_fd;
>
> - if (fd < fdt->max_fds)
> + error = -EMFILE;
> + if (likely(fd < fdt->max_fds)) {
> + if (~fdt->open_fds[0] && !start) {
> + fd = find_next_zero_bit(fdt->open_fds, BITS_PER_LONG, fd);

So I don't think this is quite correct. If files->next_fd is set, we could
end up calling find_next_zero_bit() starting from quite high offset causing
a regression? Also because we don't expand in this case, we could cause access
beyond end of fdtable?

Finally, AFAIU this speeds up the lookup for cases where fd < 64 is
available at the cost of cases where the first long is full (there we
unnecessarily load open_fds[0] into cache). Did you check if the cost is
visible (e.g. by making blogbench occupy first 64 fds before starting its
load)?

Honza

> + goto fastreturn;
> + }
> fd = find_next_fd(fdt, fd);
> + }
> +
> + if (unlikely(fd >= fdt->max_fds)) {
> + error = expand_files(files, fd);
> + if (error < 0)
> + goto out;
> + /*
> + * If we needed to expand the fs array we
> + * might have blocked - try again.
> + */
> + if (error)
> + goto repeat;
> + }
>
> +fastreturn:
> /*
> * N.B. For clone tasks sharing a files structure, this test
> * will limit the total number of files that can be opened.
> */
> - error = -EMFILE;
> - if (fd >= end)
> + if (unlikely(fd >= end))
> goto out;
>
> - error = expand_files(files, fd);
> - if (error < 0)
> - goto out;
> -
> - /*
> - * If we needed to expand the fs array we
> - * might have blocked - try again.
> - */
> - if (error)
> - goto repeat;
> -
> if (start <= files->next_fd)
> files->next_fd = fd + 1;
>
> --
> 2.43.0
>
--
Jan Kara <jack@suse.com>
SUSE Labs, CR