Re: [PATCH 5/6] Btrfs: wire up the free space tree to the extent tree

[Josef Bacik <jbacik@fb.com>]

On 09/01/2015 03:05 PM, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
>
> The free space tree is updated in tandem with the extent tree. There are
> only a handful of places where we need to hook in:
>
> 1. Block group creation
> 2. Block group deletion
> 3. Delayed refs (extent creation and deletion)
> 4. Block group caching
>
> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---
>   fs/btrfs/extent-tree.c | 73 +++++++++++++++++++++++++++++++++++++++++++++++---
>   1 file changed, 70 insertions(+), 3 deletions(-)
>
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 37179a569f40..3f10df3932f0 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -33,6 +33,7 @@
>   #include "raid56.h"
>   #include "locking.h"
>   #include "free-space-cache.h"
> +#include "free-space-tree.h"
>   #include "math.h"
>   #include "sysfs.h"
>   #include "qgroup.h"
> @@ -589,7 +590,41 @@ static int cache_block_group(struct btrfs_block_group_cache *cache,
>   	cache->cached = BTRFS_CACHE_FAST;
>   	spin_unlock(&cache->lock);
>
> -	if (fs_info->mount_opt & BTRFS_MOUNT_SPACE_CACHE) {
> +	if (btrfs_fs_compat_ro(fs_info, FREE_SPACE_TREE)) {
> +		if (load_cache_only) {
> +			spin_lock(&cache->lock);
> +			cache->caching_ctl = NULL;
> +			cache->cached = BTRFS_CACHE_NO;
> +			spin_unlock(&cache->lock);
> +			wake_up(&caching_ctl->wait);
> +		} else {
> +			mutex_lock(&caching_ctl->mutex);
> +			ret = load_free_space_tree(fs_info, cache);
> +			if (ret) {
> +				btrfs_warn(fs_info, "failed to load free space tree for %llu: %d",
> +					   cache->key.objectid, ret);
> +				spin_lock(&cache->lock);
> +				cache->caching_ctl = NULL;
> +				cache->cached = BTRFS_CACHE_ERROR;
> +				spin_unlock(&cache->lock);
> +				goto tree_out;
> +			}
> +
> +			spin_lock(&cache->lock);
> +			cache->caching_ctl = NULL;
> +			cache->cached = BTRFS_CACHE_FINISHED;
> +			cache->last_byte_to_unpin = (u64)-1;
> +			caching_ctl->progress = (u64)-1;
> +			spin_unlock(&cache->lock);
> +			mutex_unlock(&caching_ctl->mutex);
> +
> +tree_out:
> +			wake_up(&caching_ctl->wait);
> +			put_caching_control(caching_ctl);
> +			free_excluded_extents(fs_info->extent_root, cache);
> +			return 0;
> +		}
> +	} else if (fs_info->mount_opt & BTRFS_MOUNT_SPACE_CACHE) {
>   		mutex_lock(&caching_ctl->mutex);

So the reason we have this load_cache_only thing is because the free 
space cache could be loaded almost instantaneously since it was 
contiguously allocated.  This isn't the case with the free space tree, 
and although it is better than the no space cache way of doing things, 
we are still going to incur a bit of latency when seeking through a 
large free space tree.  So break this out and make the caching kthread 
either do the old style load or load the free space tree.  Then you can 
use the add free space helpers that will wake anybody up waiting on 
allocations and you incur less direct latency.  Thanks,

Josef