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

[Tim Chen <tim.c.chen@linux.intel.com>]

On Sat, 2024-06-15 at 08:31 +0200, Mateusz Guzik wrote:
> On Fri, Jun 14, 2024 at 12:34:14PM -0400, 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) There is fast path inside of find_next_zero_bit() when size<=64 to speed up
> > searching.
> > 
> > With the fast path added in alloc_fd() through one-time bitmap searching,
> > pts/blogbench-1.1.0 read is improved by 20% and write by 10% on Intel ICX 160
> > cores configuration with v6.8-rc6.
> > 
> > Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
> > Signed-off-by: Yu Ma <yu.ma@intel.com>
> > ---
> >  fs/file.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/file.c b/fs/file.c
> > index 3b683b9101d8..e8d2f9ef7fd1 100644
> > --- a/fs/file.c
> > +++ b/fs/file.c
> > @@ -510,8 +510,13 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
> >  	if (fd < files->next_fd)
> >  		fd = files->next_fd;
> >  
> > -	if (fd < fdt->max_fds)
> > +	if (fd < fdt->max_fds) {
> > +		if (~fdt->open_fds[0]) {
> > +			fd = find_next_zero_bit(fdt->open_fds, BITS_PER_LONG, fd);

Will adding a check here work to ensure fd < end?

			if (unlikely(fd >= end)) {
				error = -EMFILE;
				goto out;
			}
					
> > +			goto success;
> > +		}
> >  		fd = find_next_fd(fdt, fd);
> > +	}
> >  
> >  	/*
> >  	 * N.B. For clone tasks sharing a files structure, this test
> > @@ -531,7 +536,7 @@ static int alloc_fd(unsigned start, unsigned end, unsigned flags)
> >  	 */
> >  	if (error)
> >  		goto repeat;
> > -
> > +success:
> >  	if (start <= files->next_fd)
> >  		files->next_fd = fd + 1;
> >  
> 
> As indicated in my other e-mail it may be a process can reach a certain
> fd number and then lower its rlimit(NOFILE). In that case the max_fds
> field can happen to be higher and the above patch will fail to check for
> the (fd < end) case.
> 
Thanks.

Tim