linux-kernel - Re: [PATCH] f2fs: reduce lock overhead of extent node releasing

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Date:	Sat, 4 Jul 2015 00:03:53 -0700
From:	Jaegeuk Kim <jaegeuk@...nel.org>
To:	Chao Yu <yuchaochina@...mail.com>
Cc:	linux-kernel@...r.kernel.org,
	linux-f2fs-devel@...ts.sourceforge.net
Subject: Re: [PATCH] f2fs: reduce lock overhead of extent node releasing

On Thu, Jul 02, 2015 at 08:40:12PM +0800, Chao Yu wrote:
> >From e5c6600d01c4462c4e1ee0c70ec1d9319862077d Mon Sep 17 00:00:00 2001
> From: Chao Yu <chao2.yu@...sung.com>
> Date: Thu, 2 Jul 2015 18:52:46 +0800
> Subject: [PATCH] f2fs: reduce lock overhead of extent node releasing
> 
> Open and close critical section for each extent node when traversing rb-tree
> results in high overhead of cpu, slows thing down.
> 
> This patch alternates to batch mode for removing extent nodes under spin lock.
> 
> Signed-off-by: Chao Yu <chao2.yu@...sung.com>
> ---
>  fs/f2fs/data.c | 28 ++++++++++++++++++++--------
>  1 file changed, 20 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> index 6a706dd..7fb56a0 100644
> --- a/fs/f2fs/data.c
> +++ b/fs/f2fs/data.c
> @@ -441,19 +441,31 @@ static unsigned int __free_extent_tree(struct f2fs_sb_info *sbi,
>  	struct extent_node *en;
>  	unsigned int count = et->count;
>  
> -	node = rb_first(&et->root);
> -	while (node) {
> -		next = rb_next(node);
> -		en = rb_entry(node, struct extent_node, rb_node);
> +	if (!et->count)
> +		return 0;
> +
> +	/* 1. remove all extent nodes in global lru list */
> +	if (free_all) {
> +		spin_lock(&sbi->extent_lock);
> +		node = rb_first(&et->root);
> +		while (node) {
> +			next = rb_next(node);
> +			en = rb_entry(node, struct extent_node, rb_node);
>  
> -		if (free_all) {
> -			spin_lock(&sbi->extent_lock);
>  			if (!list_empty(&en->list))
>  				list_del_init(&en->list);
> -			spin_unlock(&sbi->extent_lock);
> +			node = next;
>  		}
> +		spin_unlock(&sbi->extent_lock);
> +	}
> +
> +	/* 2. release all extent nodes which are not in global lru list */

Hmm,
Is there any overhead to traverse the rb_tree twice and any spin_lock delay
caused by contention?

Thanks,

> +	node = rb_first(&et->root);
> +	while (node) {
> +		next = rb_next(node);
> +		en = rb_entry(node, struct extent_node, rb_node);
>  
> -		if (free_all || list_empty(&en->list)) {
> +		if (list_empty(&en->list)) {
>  			__detach_extent_node(sbi, et, en);
>  			kmem_cache_free(extent_node_slab, en);
>  		}
> -- 
> 2.4.2
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