Re: [PATCH v2 4/4] btrfs: zoned: make auto-reclaim less aggressive

[David Sterba <dsterba@suse.cz>]

On Tue, Mar 29, 2022 at 01:56:09AM -0700, Johannes Thumshirn wrote:
> The current auto-reclaim algorithm starts reclaiming all block-group's

Please write it as 'block group' in the text, also the use of 's is not
plural.

> with a zone_unusable value above a configured threshold. This is causing a
> lot of reclaim IO even if there would be enough free zones on the device.
> 
> Instead of only accounting a block-group's zone_unusable value, also take
> the ratio of free and not usable (written as well as zone_unusable) bytes
> a device has into account.
> 
> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> ---
>  fs/btrfs/block-group.c | 10 ++++++++++
>  fs/btrfs/zoned.c       | 28 ++++++++++++++++++++++++++++
>  fs/btrfs/zoned.h       |  6 ++++++
>  3 files changed, 44 insertions(+)
> 
> diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
> index 628741ecb97b..12454304bb85 100644
> --- a/fs/btrfs/block-group.c
> +++ b/fs/btrfs/block-group.c
> @@ -1512,6 +1512,13 @@ static int reclaim_bgs_cmp(void *unused, const struct list_head *a,
>  	return bg1->used > bg2->used;
>  }
>  
> +static inline bool btrfs_should_reclaim(struct btrfs_fs_info *fs_info)
> +{
> +	if (btrfs_is_zoned(fs_info))
> +		return btrfs_zoned_should_reclaim(fs_info);
> +	return true;
> +}
> +
>  void btrfs_reclaim_bgs_work(struct work_struct *work)
>  {
>  	struct btrfs_fs_info *fs_info =
> @@ -1522,6 +1529,9 @@ void btrfs_reclaim_bgs_work(struct work_struct *work)
>  	if (!test_bit(BTRFS_FS_OPEN, &fs_info->flags))
>  		return;
>  
> +	if (!btrfs_should_reclaim(fs_info))
> +		return;
> +
>  	sb_start_write(fs_info->sb);
>  
>  	if (!btrfs_exclop_start(fs_info, BTRFS_EXCLOP_BALANCE)) {
> diff --git a/fs/btrfs/zoned.c b/fs/btrfs/zoned.c
> index 1b1b310c3c51..c0c460749b74 100644
> --- a/fs/btrfs/zoned.c
> +++ b/fs/btrfs/zoned.c
> @@ -2079,3 +2079,31 @@ void btrfs_free_zone_cache(struct btrfs_fs_info *fs_info)
>  	}
>  	mutex_unlock(&fs_devices->device_list_mutex);
>  }
> +
> +bool btrfs_zoned_should_reclaim(struct btrfs_fs_info *fs_info)
> +{
> +	struct btrfs_fs_devices *fs_devices = fs_info->fs_devices;
> +	struct btrfs_device *device;
> +	u64 used = 0;
> +	u64 total = 0;
> +	u64 factor;
> +
> +	ASSERT(btrfs_is_zoned(fs_info));
> +
> +	if (!fs_info->bg_reclaim_threshold)

For integer values it's IMHO better to use == 0 as ! is for bool
variables.

> +		return false;
> +
> +	mutex_lock(&fs_devices->device_list_mutex);
> +	list_for_each_entry(device, &fs_devices->devices, dev_list) {
> +		if (!device->bdev)
> +			continue;
> +
> +		total += device->disk_total_bytes;
> +		used += device->bytes_used;
> +
> +	}
> +	mutex_unlock(&fs_devices->device_list_mutex);
> +
> +	factor = div64_u64(used * 100, total);

Seems we can't avoid 64bit division here, at least it's not perf
critical.

> +	return factor >= fs_info->bg_reclaim_threshold;
> +}