linux-kernel - Re: [PATCH v4 3/3] fs/file.c: add fast path in find_next

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Message-ID: <20240716111908.tocqtq435d6bc3q3@quack3>
Date: Tue, 16 Jul 2024 13:19:08 +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 v4 3/3] fs/file.c: add fast path in find_next_fd()

On Fri 12-07-24 22:39:17, Yu Ma wrote:
> Skip 2-levels searching via find_next_zero_bit() when there is free slot in the
> word contains next_fd, as:
> (1) next_fd indicates the lower bound for the first free fd.
> (2) There is fast path inside of find_next_zero_bit() when size<=64 to speed up
> searching.
> (3) 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.
> 
> This fast path is proposed by Mateusz Guzik <mjguzik@...il.com>, and agreed by
> Jan Kara <jack@...e.cz>, which is more generic and scalable than previous
> versions. And on top of patch 1 and 2, it improves pts/blogbench-1.1.0 read by
> 8% and write by 4% on Intel ICX 160 cores configuration with v6.10-rc7.
> 
> Reviewed-by: Tim Chen <tim.c.chen@...ux.intel.com>
> Signed-off-by: Yu Ma <yu.ma@...el.com>

Looks good. Just some code style nits below.

> diff --git a/fs/file.c b/fs/file.c
> index 1be2a5bcc7c4..a3ce6ba30c8c 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -488,9 +488,20 @@ struct files_struct init_files = {
>  
>  static unsigned int find_next_fd(struct fdtable *fdt, unsigned int start)
>  {
> +	unsigned int bitbit = start / BITS_PER_LONG;
> +	unsigned int bit;
> +
> +	/*
> +	 * Try to avoid looking at the second level bitmap
> +	 */
> +	bit = find_next_zero_bit(&fdt->open_fds[bitbit], BITS_PER_LONG,
> +				 start & (BITS_PER_LONG -1));
							^^ Either
(BITS_PER_LONG-1) or (BITS_PER_LONG - 1) please. Your combination looks
particularly weird :)

> +	if (bit < BITS_PER_LONG) {
> +		return bit + bitbit * BITS_PER_LONG;
> +	}

No need for braces around the above block.

>  	unsigned int maxfd = fdt->max_fds; /* always multiple of BITS_PER_LONG */
>  	unsigned int maxbit = maxfd / BITS_PER_LONG;

We keep declarations at the beginning of the block. Usually it keeps the
code more readable and the compiler should be clever enough to perform the
loads & arithmetics only when needed.

After fixing these style nits feel free to add:

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

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