linux-kernel - Re: [PATCH v3 3/3] fs/file.c: add fast path in find_next

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Message-ID: <20240704215507.mr6st2d423lvkepu@quack3>
Date: Thu, 4 Jul 2024 23:55:07 +0200
From: Jan Kara <jack@...e.cz>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: Yu Ma <yu.ma@...el.com>, viro@...iv.linux.org.uk, 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
Subject: Re: [PATCH v3 3/3] fs/file.c: add fast path in find_next_fd()

On Thu 04-07-24 19:44:10, Mateusz Guzik wrote:
> On Wed, Jul 3, 2024 at 4:07 PM Yu Ma <yu.ma@...el.com> 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@...e.cz>,
> > 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@...ux.intel.com>
> > Signed-off-by: Yu Ma <yu.ma@...el.com>
> > ---
> >  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;
> +       }

Drat, you're right. I missed that Ma did not add the proper offset to
open_fds. *This* is what I meant :)

								Honza

-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR