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

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Date: Tue, 25 Jun 2024 13:52:57 +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 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?

Finally, AFAIU this speeds up the lookup for cases where fd < 64 is
available at the cost of cases where the first long is full (there we
unnecessarily load open_fds[0] into cache). Did you check if the cost is
visible (e.g. by making blogbench occupy first 64 fds before starting its
load)?

								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