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Date:	Mon, 10 Mar 2014 13:50:24 +0900
From:	Jaegeuk Kim <jaegeuk.kim@...sung.com>
To:	Gu Zheng <guz.fnst@...fujitsu.com>
Cc:	f2fs <linux-f2fs-devel@...ts.sourceforge.net>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/5] f2fs: add a wait queue to avoid unnecessary,
 build_free_nid

Hi Gu,

2014-03-07 (금), 18:43 +0800, Gu Zheng:
> Previously, when we try to alloc free nid while the build free nid
> is going, the allocer will be run into the flow that waiting for
> "nm_i->build_lock", see following:
> 	/* We should not use stale free nids created by build_free_nids */
> ---->	if (nm_i->fcnt && !on_build_free_nids(nm_i)) {
> 		f2fs_bug_on(list_empty(&nm_i->free_nid_list));
> 		list_for_each(this, &nm_i->free_nid_list) {
> 			i = list_entry(this, struct free_nid, list);
> 			if (i->state == NID_NEW)
> 				break;
> 		}
> 
> 		f2fs_bug_on(i->state != NID_NEW);
> 		*nid = i->nid;
> 		i->state = NID_ALLOC;
> 		nm_i->fcnt--;
> 		spin_unlock(&nm_i->free_nid_list_lock);
> 		return true;
> 	}
> 	spin_unlock(&nm_i->free_nid_list_lock);
> 
> 	/* Let's scan nat pages and its caches to get free nids */
> ---->	mutex_lock(&nm_i->build_lock);
> 	build_free_nids(sbi);
> 	mutex_unlock(&nm_i->build_lock);
> and this will cause another unnecessary building free nid if the current
> building free nid job is done.

Could you support any performance number for this?
Since, IMO, the contended building processes will be released right away
because of the following condition check inside build_free_nids().

if (nm_i->fcnt > NAT_ENTRY_PER_BLOCK)
	return;

So, I don't think this gives us any high latency.
Can the wakeup_all() become another overhead all the time?
Thanks,

> So here we introduce a wait_queue to avoid this issue.
> 
> Signed-off-by: Gu Zheng <guz.fnst@...fujitsu.com>
> ---
>  fs/f2fs/f2fs.h |    1 +
>  fs/f2fs/node.c |   10 +++++++++-
>  2 files changed, 10 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index f845e92..7ae193e 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -256,6 +256,7 @@ struct f2fs_nm_info {
>  	spinlock_t free_nid_list_lock;	/* protect free nid list */
>  	unsigned int fcnt;		/* the number of free node id */
>  	struct mutex build_lock;	/* lock for build free nids */
> +	wait_queue_head_t build_wq;	/* wait queue for build free nids */
>  
>  	/* for checkpoint */
>  	char *nat_bitmap;		/* NAT bitmap pointer */
> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
> index 4b7861d..ab44711 100644
> --- a/fs/f2fs/node.c
> +++ b/fs/f2fs/node.c
> @@ -1422,7 +1422,13 @@ retry:
>  	spin_lock(&nm_i->free_nid_list_lock);
>  
>  	/* We should not use stale free nids created by build_free_nids */
> -	if (nm_i->fcnt && !on_build_free_nids(nm_i)) {
> +	if (on_build_free_nids(nm_i)) {
> +		spin_unlock(&nm_i->free_nid_list_lock);
> +		wait_event(nm_i->build_wq, !on_build_free_nids(nm_i));
> +		goto retry;
> +	}
> +
> +	if (nm_i->fcnt) {
>  		f2fs_bug_on(list_empty(&nm_i->free_nid_list));
>  		list_for_each(this, &nm_i->free_nid_list) {
>  			i = list_entry(this, struct free_nid, list);
> @@ -1443,6 +1449,7 @@ retry:
>  	mutex_lock(&nm_i->build_lock);
>  	build_free_nids(sbi);
>  	mutex_unlock(&nm_i->build_lock);
> +	wake_up_all(&nm_i->build_wq);
>  	goto retry;
>  }
>  
> @@ -1813,6 +1820,7 @@ static int init_node_manager(struct f2fs_sb_info *sbi)
>  	INIT_LIST_HEAD(&nm_i->dirty_nat_entries);
>  
>  	mutex_init(&nm_i->build_lock);
> +	init_waitqueue_head(&nm_i->build_wq);
>  	spin_lock_init(&nm_i->free_nid_list_lock);
>  	rwlock_init(&nm_i->nat_tree_lock);
>  

-- 
Jaegeuk Kim
Samsung

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