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

Jan Kara
2024-06-25T12:53:11Z

On Tue 25-06-24 13:52:57, Jan Kara wrote:
> 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?

OK, I've misunderstood the next_fd logic. next_fd is the lowest 0-bit in
the open_fds bitmap so if next_fd is big, the ~fdt->open_fds[0] should
be false. As such the above condition could be rewritten as:

if (!start && files->next_fd < BITS_PER_LONG)

to avoid loading the first bitmap long if we know it is full? Or we could
maybe go as far as:

if (!start && fd < BITS_PER_LONG && !test_bit(fd, fdt->open_fds))
goto fastreturn;

because AFAIU this should work in exactly the same cases as your code?

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