Re: [PATCH 03/12] btrfs: limit max discard size for async discard

[David Sterba <dsterba@suse.cz>]

On Thu, Jan 02, 2020 at 04:26:37PM -0500, Dennis Zhou wrote:
> Throttle the maximum size of a discard so that we can provide an upper
> bound for the rate of async discard. While the block layer is able to
> split discards into the appropriate sized discards, we want to be able
> to account more accurately the rate at which we are consuming ncq slots
> as well as limit the upper bound of work for a discard.
> 
> Signed-off-by: Dennis Zhou <dennis@kernel.org>
> Reviewed-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/discard.h          |  5 +++++
>  fs/btrfs/free-space-cache.c | 41 +++++++++++++++++++++++++++++--------
>  2 files changed, 37 insertions(+), 9 deletions(-)
> 
> diff --git a/fs/btrfs/discard.h b/fs/btrfs/discard.h
> index 5250fe178e49..562c60fab77a 100644
> --- a/fs/btrfs/discard.h
> +++ b/fs/btrfs/discard.h
> @@ -3,10 +3,15 @@
>  #ifndef BTRFS_DISCARD_H
>  #define BTRFS_DISCARD_H
>  
> +#include <linux/sizes.h>
> +
>  struct btrfs_fs_info;
>  struct btrfs_discard_ctl;
>  struct btrfs_block_group;
>  
> +/* Discard size limits */
> +#define BTRFS_ASYNC_DISCARD_MAX_SIZE	(SZ_64M)
> +
>  /* Work operations */
>  void btrfs_discard_cancel_work(struct btrfs_discard_ctl *discard_ctl,
>  			       struct btrfs_block_group *block_group);
> diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
> index 40fb918a82f4..34291c777998 100644
> --- a/fs/btrfs/free-space-cache.c
> +++ b/fs/btrfs/free-space-cache.c
> @@ -3466,16 +3466,36 @@ static int trim_no_bitmap(struct btrfs_block_group *block_group,
>  		extent_start = entry->offset;
>  		extent_bytes = entry->bytes;
>  		extent_trim_state = entry->trim_state;
> -		start = max(start, extent_start);
> -		bytes = min(extent_start + extent_bytes, end) - start;
> -		if (bytes < minlen) {
> -			spin_unlock(&ctl->tree_lock);
> -			mutex_unlock(&ctl->cache_writeout_mutex);
> -			goto next;
> -		}
> +		if (async) {
> +			start = entry->offset;
> +			bytes = entry->bytes;
> +			if (bytes < minlen) {
> +				spin_unlock(&ctl->tree_lock);
> +				mutex_unlock(&ctl->cache_writeout_mutex);
> +				goto next;
> +			}
> +			unlink_free_space(ctl, entry);
> +			if (bytes > BTRFS_ASYNC_DISCARD_MAX_SIZE) {
> +				bytes = extent_bytes =
> +					BTRFS_ASYNC_DISCARD_MAX_SIZE;

You still left one chain assignment here, fixed and also in the followup
patch that switches to the variable limit.