Subject: Re: [PATCH v3 3/7] fs: Introduce a per_cpu last_ino allocator

From: "Paul E. McKenney" <[email-masked]>
Date: Tue, 16 Dec 2008 13:26:38 -0800

On Thu, Dec 11, 2008 at 11:39:18PM +0100, Eric Dumazet wrote:
> new_inode() dirties a contended cache line to get increasing
> inode numbers.
> 
> Solve this problem by providing to each cpu a per_cpu variable,
> feeded by the shared last_ino, but once every 1024 allocations.
> 
> This reduce contention on the shared last_ino, and give same
> spreading ino numbers than before.
> (same wraparound after 2^32 allocations)

One question below, but just a clarification.  Works correctly as is,
though a bit strangely.

Reviewed-by: Paul E. McKenney <[email-masked]>

> Signed-off-by: Eric Dumazet <[email-masked]>
> ---
>  fs/inode.c |   35 ++++++++++++++++++++++++++++++++---
>  1 files changed, 32 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/inode.c b/fs/inode.c
> index f94f889..dc8e72a 100644
> --- a/fs/inode.c
> +++ b/fs/inode.c
> @@ -556,6 +556,36 @@ repeat:
>  	return node ? inode : NULL;
>  }
> 
> +#ifdef CONFIG_SMP
> +/*
> + * Each cpu owns a range of 1024 numbers.
> + * 'shared_last_ino' is dirtied only once out of 1024 allocations,
> + * to renew the exhausted range.
> + */
> +static DEFINE_PER_CPU(int, last_ino);
> +
> +static int last_ino_get(void)
> +{
> +	static atomic_t shared_last_ino;
> +	int *p = &get_cpu_var(last_ino);
> +	int res = *p;
> +
> +	if (unlikely((res & 1023) == 0))
> +		res = atomic_add_return(1024, &shared_last_ino) - 1024;
> +
> +	*p = ++res;

So the first CPU gets the range [1:1024], the second [1025:2048], and
so on, eventually wrapping to [4294966273:0].  Is that the intent?

(I don't see a problem with this, just seems a bit strange.)

> +	put_cpu_var(last_ino);
> +	return res;
> +}
> +#else
> +static int last_ino_get(void)
> +{
> +	static int last_ino;
> +
> +	return ++last_ino;
> +}
> +#endif
> +
>  /**
>   *	new_inode 	- obtain an inode
>   *	@sb: superblock
> @@ -575,7 +605,6 @@ struct inode *new_inode(struct super_block *sb)
>  	 * error if st_ino won't fit in target struct field. Use 32bit counter
>  	 * here to attempt to avoid that.
>  	 */
> -	static unsigned int last_ino;
>  	struct inode * inode;
> 
>  	spin_lock_prefetch(&inode_lock);
> @@ -583,11 +612,11 @@ struct inode *new_inode(struct super_block *sb)
>  	inode = alloc_inode(sb);
>  	if (inode) {
>  		percpu_counter_inc(&nr_inodes);
> +		inode->i_state = 0;
> +		inode->i_ino = last_ino_get();
>  		spin_lock(&inode_lock);
>  		list_add(&inode->i_list, &inode_in_use);
>  		list_add(&inode->i_sb_list, &sb->s_inodes);
> -		inode->i_ino = ++last_ino;
> -		inode->i_state = 0;
>  		spin_unlock(&inode_lock);
>  	}
>  	return inode;