Re: [PATCH 2/5] btrfs: delayed refs pre-flushing should only run the heads we have

[Nikolay Borisov <nborisov@suse.com>]

On 13.03.20 г. 23:12 ч., Josef Bacik wrote:
> Previously our delayed ref running used the total number of items as the
> items to run.  However we changed that to number of heads to run with
> the delayed_refs_rsv, as generally we want to run all of the operations
> for one bytenr.
> 
> But with btrfs_run_delayed_refs(trans, 0) we set our count to 2x the
> number of items that we have.  This is generally fine, but if we have
> some operation generation loads of delayed refs while we're doing this
> pre-flushing in the transaction commit, we'll just spin forever doing
> delayed refs.
> 
> Fix this to simply pick the number of delayed refs we currently have,
> that way we do not end up doing a lot of extra work that's being
> generated in other threads.

Indeed there is a mismatch between delayed_refs->num_entries and what we
account in __btrfs_run_delayed_refs. In the function we count on a
per-head (which can include multiple delayed refs ops) granularity not
on per-refs-per-head. So this fix makes sense.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/extent-tree.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 8e5b49baad98..2925b3ad77a1 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -2196,7 +2196,7 @@ int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
>  
>  	delayed_refs = &trans->transaction->delayed_refs;
>  	if (count == 0)
> -		count = atomic_read(&delayed_refs->num_entries) * 2;
> +		count = delayed_refs->num_heads_ready;
>  
>  again:
>  #ifdef SCRAMBLE_DELAYED_REFS
>