linux-kernel - Re: [PATCH 1/4] seq_file: don't call bitmap_scnprintf

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Date:	Mon, 13 Oct 2008 10:38:06 +0400
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Paul Menage <menage@...gle.com>, Paul Jackson <pj@....com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] seq_file: don't call bitmap_scnprintf_len()

On Sun, Oct 12, 2008 at 05:29:17PM +0800, Lai Jiangshan wrote:
> "m->count + len < m->size" is true commonly, so bitmap_scnprintf()
> is commonly called. this fix saves a call to bitmap_scnprintf_len().

This saves a call if seq buffer is already full which is not a common
situation, so you're optimising for rare situations and adding branch
for common ones.

> --- a/fs/seq_file.c
> +++ b/fs/seq_file.c
> @@ -452,17 +452,18 @@ int seq_dentry(struct seq_file *m, struct dentry *dentry, char *esc)
>  
>  int seq_bitmap(struct seq_file *m, unsigned long *bits, unsigned int nr_bits)
>  {
> -	size_t len = bitmap_scnprintf_len(nr_bits);
> -
> -	if (m->count + len < m->size) {
> -		bitmap_scnprintf(m->buf + m->count, m->size - m->count,
> -				 bits, nr_bits);
> -		m->count += len;
> -		return 0;
> +	if (m->count < m->size) {
> +		int len = bitmap_scnprintf(m->buf + m->count,
> +				m->size - m->count, bits, nr_bits);
> +		if (m->count + len < m->size) {
> +			m->count += len;
> +			return 0;
> +		}
>  	}
>  	m->count = m->size;
>  	return -1;
>  }
> +EXPORT_SYMBOL(seq_bitmap);

Modular users, where are they?
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