linux-kernel - Re: [PATCH v5 3/3] fs/file.c: add fast path in find_next

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Message-ID: <CAGudoHHQSjbeuSevyL=W=fhjOOo=bCjq4ixHfEMN_XdRLLdPbQ@mail.gmail.com>
Date: Fri, 19 Jul 2024 19:53:01 +0200
From: Mateusz Guzik <mjguzik@...il.com>
To: Yu Ma <yu.ma@...el.com>
Cc: brauner@...nel.org, jack@...e.cz, edumazet@...gle.com, 
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org, 
	pan.deng@...el.com, tianyou.li@...el.com, tim.c.chen@...el.com, 
	tim.c.chen@...ux.intel.com, viro@...iv.linux.org.uk
Subject: Re: [PATCH v5 3/3] fs/file.c: add fast path in find_next_fd()

On Wed, Jul 17, 2024 at 4:24 PM Yu Ma <yu.ma@...el.com> wrote:
>
> 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.

this is stale -- now the fast path searches up to 64 fds in the lower bitmap

> (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 <mjguzik@...il.com>, and agreed by
> Jan Kara <jack@...e.cz>, which is more generic and scalable than previous
> versions.

I think this paragraph is droppable. You already got an ack from Jan
below, so stating he agrees with the patch is redundant. As for me I
don't think this warrants mentioning. Just remove it, perhaps
Christian will be willing to massage it by himself to avoid another
series posting.

> 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 <jack@...e.cz>
> Reviewed-by: Tim Chen <tim.c.chen@...ux.intel.com>
> Signed-off-by: Yu Ma <yu.ma@...el.com>
> ---
>  fs/file.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/fs/file.c b/fs/file.c
> index 1be2a5bcc7c4..729c07a4fc28 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -491,6 +491,15 @@ 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)
> --
> 2.43.0
>


-- 
Mateusz Guzik <mjguzik gmail.com>