linux-kernel - Re: [PATCH v2 1/3] fs/file.c: add fast path in alloc

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Date: Tue, 25 Jun 2024 14:53:09 +0200
From: Jan Kara <jack@...e.cz>
To: Yu Ma <yu.ma@...el.com>
Cc: viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz,
	mjguzik@...il.com, 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 v2 1/3] fs/file.c: add fast path in alloc_fd()

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@...ux.intel.com>
> > Signed-off-by: Yu Ma <yu.ma@...el.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@...e.com>
> SUSE Labs, CR
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR