Re: [PATCH 07/13] btrfs: kick off async delayed ref flushing if we are over time budget

[Nikolay Borisov <nborisov@suse.com>]

On 13.03.20 г. 23:23 ч., Josef Bacik wrote:
> For very large file systems we cannot rely on the space reservation
> system to provide enough pressure to flush delayed refs in a timely
> manner.  We have the infrastructure in place to keep track of how much
> theoretical time it'll take to run our outstanding delayed refs, but
> unfortunately I ripped all of that out when I added the delayed refs
> rsv.  This code originally was added to address the problem of too many
> delayed refs building up and thus causing transaction commits to take
> several minutes to finish.
> 
> Fix this by adding back the ability to flush delayed refs based on the
> time budget for them.  We want to limit to around 1 seconds worth of
> delayed refs to be pending at any given time.  In order to keep up with
> demand we will start the async flusher once we are at the 500ms mark,
> and the async flusher will attempt to keep us in this ballpark.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  fs/btrfs/ctree.h       |  4 ++++
>  fs/btrfs/disk-io.c     |  3 +++
>  fs/btrfs/extent-tree.c | 44 ++++++++++++++++++++++++++++++++++++++++++
>  fs/btrfs/transaction.c |  8 ++++++++
>  4 files changed, 59 insertions(+)
> 

<snip>

> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index 645ae95f465e..0e81990b57e0 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -2249,6 +2249,50 @@ int btrfs_run_delayed_refs(struct btrfs_trans_handle *trans,
>  	return 0;
>  }
>  
> +static void btrfs_async_run_delayed_refs(struct work_struct *work)
> +{
> +	struct btrfs_fs_info *fs_info;
> +	struct btrfs_trans_handle *trans;
> +
> +	fs_info = container_of(work, struct btrfs_fs_info,
> +			       async_delayed_ref_work);
> +
> +	while (!btrfs_fs_closing(fs_info)) {
> +		unsigned long count;
> +		int ret;
> +
> +		trans = btrfs_attach_transaction(fs_info->extent_root);
> +		if (IS_ERR(trans))
> +			break;
> +
> +		smp_rmb();

What is this barrier ordering? IMO its usage is bogus here, because in
btrfs_should_end_transaction we use a full barrier and here only an RMB.
Further more in btrfs_should_end_transaction we don't have any memory
accesses preceding the check of the flushing state. Looking at the
callers of btrfs_should_end_transaction I also don't see any ordering
guaranteed i.e I think it could be removed altogether. Or perhahps we
really want acquire/release semantics e.g. accesses to
delayed_refs.flushing should be done via
smp_load_acquire/smp_store_release functions?


> +		if (trans->transaction->delayed_refs.flushing) {
> +			btrfs_end_transaction(trans);
> +			break;
> +		}
> +
> +		/* No longer over our threshold, lets bail. */
> +		if (!btrfs_should_throttle_delayed_refs(trans, true)) {
> +			btrfs_end_transaction(trans);
> +			break;
> +		}
> +
> +		count = atomic_read(&trans->transaction->delayed_refs.num_entries);
> +		count >>= 2;
> +
> +		ret = btrfs_run_delayed_refs(trans, count);
> +		btrfs_end_transaction(trans);
> +		if (ret < 0)
> +			break;
> +	}
> +}
> +

<snip>