Re: [PATCH 2/2] btrfs: zoned: wake up cleaner sooner if needed

[Naohiro Aota]

On Mon, Jan 22, 2024 at 02:51:04AM -0800, Johannes Thumshirn wrote:
> On very fast but small devices, waiting for a transaction commit can be
> too long of a wait in order to wake up the cleaner kthread to remove unused
> and reclaimable block-groups.
> 
> Check every time we're adding back free space to a block group, if we need
> to activate the cleaner kthread.
> 
> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@xxxxxxx>
> ---
>  fs/btrfs/free-space-cache.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
> index d372c7ce0e6b..2d98b9ca0e83 100644
> --- a/fs/btrfs/free-space-cache.c
> +++ b/fs/btrfs/free-space-cache.c
> @@ -30,6 +30,7 @@
>  #include "file-item.h"
>  #include "file.h"
>  #include "super.h"
> +#include "zoned.h"
>  
>  #define BITS_PER_BITMAP		(PAGE_SIZE * 8UL)
>  #define MAX_CACHE_BYTES_PER_GIG	SZ_64K
> @@ -2694,6 +2695,7 @@ int __btrfs_add_free_space(struct btrfs_block_group *block_group,
>  static int __btrfs_add_free_space_zoned(struct btrfs_block_group *block_group,
>  					u64 bytenr, u64 size, bool used)
>  {
> +	struct btrfs_fs_info *fs_info = block_group->fs_info;
>  	struct btrfs_space_info *sinfo = block_group->space_info;
>  	struct btrfs_free_space_ctl *ctl = block_group->free_space_ctl;
>  	u64 offset = bytenr - block_group->start;
> @@ -2745,6 +2747,10 @@ static int __btrfs_add_free_space_zoned(struct btrfs_block_group *block_group,
>  		btrfs_mark_bg_to_reclaim(block_group);
>  	}
>  
> +	if (btrfs_zoned_should_reclaim(fs_info) &&
> +	    !test_bit(BTRFS_FS_CLEANER_RUNNING, &fs_info->flags))
> +		wake_up_process(fs_info->cleaner_kthread);
> +

Isn't it too costly to call btrfs_zoned_should_reclaim() every time
something updated? Can we wake up it in btrfs_mark_bg_to_reclaim and
btrfs_mark_bg_unused ?

Also, looking into btrfs_zoned_should_reclaim(), it sums device->bytes_used
for each fs_devices->devices. And, device->bytes_used is set at
create_chunk() or at btrfs_remove_chunk(). Isn't it feasible to do the
calculation only there?

>  	return 0;
>  }
>  
> 
> -- 
> 2.43.0
>