linux-kernel - Re: [PATCH 16/17] fs: Convert nr

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Message-Id: <20100929215322.ff635d3e.akpm@linux-foundation.org>
Date:	Wed, 29 Sep 2010 21:53:22 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 16/17] fs: Convert nr_inodes to a per-cpu counter

On Wed, 29 Sep 2010 22:18:48 +1000 Dave Chinner <david@...morbit.com> wrote:

> From: Eric Dumazet <dada1@...mosbay.com>
> 
> The number of inodes allocated does not need to be tied to the
> addition or removal of an inode to/from a list. If we are not tied
> to a list lock, we could update the counters when inodes are
> initialised or destroyed, but to do that we need to convert the
> counters to be per-cpu (i.e. independent of a lock). This means that
> we have the freedom to change the list/locking implementation
> without needing to care about the counters.
> 
>
> ...
>
> +int get_nr_inodes(void)
> +{
> +	int i;
> +	int sum = 0;
> +	for_each_possible_cpu(i)
> +		sum += per_cpu(nr_inodes, i);
> +	return sum < 0 ? 0 : sum;
> +}

This reimplements percpu_counter_sum_positive(), rather poorly

If one never intends to use the approximate percpu_counter_read() then
one could initialise the counter with a really large batch value, for a
very small performance gain.

> +int get_nr_inodes_unused(void)
> +{
> +	return inodes_stat.nr_unused;
> +}
>
> ...
>
> @@ -407,6 +407,8 @@ extern struct files_stat_struct files_stat;
>  extern int get_max_files(void);
>  extern int sysctl_nr_open;
>  extern struct inodes_stat_t inodes_stat;
> +extern int get_nr_inodes(void);
> +extern int get_nr_inodes_unused(void);

These are pretty cruddy names.  Unfotunately we don't really have a vfs
or "inode" subsystem name to prefix them with.

>
> ...
>

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