linux-kernel - Re: [PATCH v3 3/3] fs/file.c: add fast path in find_next

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Message-ID: <20240704100308.6hczzyqhmpty4avx@quack3>
Date: Thu, 4 Jul 2024 12:03:08 +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 v3 3/3] fs/file.c: add fast path in find_next_fd()

On Wed 03-07-24 10:33:11, 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.
> 
> As suggested by Mateusz Guzik <mjguzik gmail.com> and Jan Kara <jack@...e.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@...ux.intel.com>
> Signed-off-by: Yu Ma <yu.ma@...el.com>

Nice! The patch looks good to me. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

One style nit below:

> 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;

Empty line here please to separate variable declaration and code...

> +	bit = find_next_zero_bit(fdt->open_fds, BITS_PER_LONG, start);
> +	if (bit < BITS_PER_LONG)
> +		return bit;
> +
>  	unsigned int maxfd = fdt->max_fds; /* always multiple of BITS_PER_LONG */
>  	unsigned int maxbit = maxfd / BITS_PER_LONG;
>  	unsigned int bitbit = start / BITS_PER_LONG;

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