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

Jan Kara
2024-07-04T10:07:17Z

On Wed 03-07-24 16:17:01, Mateusz Guzik wrote:
> On Wed, Jul 3, 2024 at 4:07 PM Yu Ma <yu.ma@intel.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@suse.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@linux.intel.com>
> > Signed-off-by: Yu Ma <yu.ma@intel.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;
> > +
>
> The rest of the patchset looks good on cursory read.
>
> As for this one, the suggestion was to make it work across the entire range.

I'm not sure what do you exactly mean by "make it work across the entire
range" because what Ma has implemented is exactly what I originally had in
mind - i.e., search the first word of open_fds starting from next_fd (note
that 'start' in this function is already set to max(start, next_fd)), if
that fails, go through the two level bitmap.

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