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

[Johannes Thumshirn <Johannes.Thumshirn@wdc.com>]

On 22.01.24 13:22, Naohiro Aota wrote:
> 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@wdc.com>
>> ---
>>   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 ?

Hmm yes, I've thought of adding a list_count() for the reclaim and 
unused lists, and only calling into should_reclaim if these lists have 
entries. Or even better !list_is_singular().

> 
> 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?

Oh sh*t! Right we should check bytes_used from all space_infos in 
btrfs_zoned_should_reclaim() and compare that to the disk total bytes.