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

[Mateusz Guzik]

On Wed, Jul 3, 2024 at 4:07 PM Yu Ma <yu.ma@xxxxxxxxx> 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.
>
> As suggested by Mateusz Guzik <mjguzik gmail.com> and Jan Kara <jack@xxxxxxx>,
> 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@xxxxxxxxxxxxxxx>
> Signed-off-by: Yu Ma <yu.ma@xxxxxxxxx>
> ---
>  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
>

I had something like this in mind:
diff --git a/fs/file.c b/fs/file.c
index a3b72aa64f11..4d3307e39db7 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -489,6 +489,16 @@ 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 map.
+        */
+       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)

can you please test it out. I expect it to provide a tiny improvement
over your patch.

-- 
Mateusz Guzik <mjguzik gmail.com>