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

[Naohiro Aota]

On Mon, Jan 22, 2024 at 12:30:52PM +0000, Johannes Thumshirn wrote:
> 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@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 ?
> 
> 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().

That sounds reasonable.

> > 
> > 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.

You mean device->bytes_used? space_info->bytes_used does not count free
space and zone_unusable in BGs, so using that changes the behavior. Even,
it won't kick the thread if there are many zone_unusable but small used
space.

>